[HMM 00/15] HMM (Heterogeneous Memory Management) v20

[HMM 00/15] HMM (Heterogeneous Memory Management) v20

[Jérôme Glisse]

Patchset is on top of mmotm mmotm-2017-04-18 and Michal patchset
([PATCH -v3 0/13] mm: make movable onlining suck less). Branch:

https://cgit.freedesktop.org/~glisse/linux/log/?h=hmm-v20

I have included all suggestion made since v19, it is all build
fix and change in respect to memory hotplug with Michal rework.
Changes since v19:
- Included various build fix and compilation warning fix
- Limit HMM to x86-64 (easy to enable other arch as separate patch)
- Rebase on top of Michal memory hotplug rework

Heterogeneous Memory Management (HMM) (description and justification)

Today device driver expose dedicated memory allocation API through their
device file, often relying on a combination of IOCTL and mmap calls. The
device can only access and use memory allocated through this API. This
effectively split the program address space into object allocated for the
device and useable by the device and other regular memory (malloc, mmap
of a file, share memory, …) only accessible by CPU (or in a very limited
way by a device by pinning memory).

Allowing different isolated component of a program to use a device thus
require duplication of the input data structure using device memory
allocator. This is reasonable for simple data structure (array, grid,
image, …) but this get extremely complex with advance data structure
(list, tree, graph, …) that rely on a web of memory pointers. This is
becoming a serious limitation on the kind of work load that can be
offloaded to device like GPU.

New industry standard like C++, OpenCL or CUDA are pushing to remove this
barrier. This require a shared address space between GPU device and CPU so
that GPU can access any memory of a process (while still obeying memory
protection like read only). This kind of feature is also appearing in
various other operating systems.

HMM is a set of helpers to facilitate several aspects of address space
sharing and device memory management. Unlike existing sharing mechanism
that rely on pining pages use by a device, HMM relies on mmu_notifier to
propagate CPU page table update to device page table.

Duplicating CPU page table is only one aspect necessary for efficiently
using device like GPU. GPU local memory have bandwidth in the TeraBytes/
second range but they are connected to main memory through a system bus
like PCIE that is limited to 32GigaBytes/second (PCIE 4.0 16x). Thus it
is necessary to allow migration of process memory from main system memory
to device memory. Issue is that on platform that only have PCIE the device
memory is not accessible by the CPU with the same properties as main
memory (cache coherency, atomic operations, …).

To allow migration from main memory to device memory HMM provides a set
of helper to hotplug device memory as a new type of ZONE_DEVICE memory
which is un-addressable by CPU but still has struct page representing it.
This allow most of the core kernel logic that deals with a process memory
to stay oblivious of the peculiarity of device memory.

When page backing an address of a process is migrated to device memory
the CPU page table entry is set to a new specific swap entry. CPU access
to such address triggers a migration back to system memory, just like if
the page was swap on disk. HMM also blocks any one from pinning a
ZONE_DEVICE page so that it can always be migrated back to system memory
if CPU access it. Conversely HMM does not migrate to device memory any
page that is pin in system memory.

To allow efficient migration between device memory and main memory a new
migrate_vma() helpers is added with this patchset. It allows to leverage
device DMA engine to perform the copy operation.

This feature will be use by upstream driver like nouveau mlx5 and probably
other in the future (amdgpu is next suspect  in line). We are actively
working on nouveau and mlx5 support. To test this patchset we also worked
with NVidia close source driver team, they have more resources than us to
test this kind of infrastructure and also a bigger and better userspace
eco-system with various real industry workload they can be use to test and
profile HMM.

The expected workload is a program builds a data set on the CPU (from disk,
from network, from sensors, …). Program uses GPU API (OpenCL, CUDA, ...)
to give hint on memory placement for the input data and also for the output
buffer. Program call GPU API to schedule a GPU job, this happens using
device driver specific ioctl. All this is hidden from programmer point of
view in case of C++ compiler that transparently offload some part of a
program to GPU. Program can keep doing other stuff on the CPU while the
GPU is crunching numbers.

It is expected that CPU will not access the same data set as the GPU while
GPU is working on it, but this is not mandatory. In fact we expect some
small memory object to be actively access by both GPU and CPU concurrently
as synchronization channel and/or for monitoring purposes. Such object will
stay in system memory and should not be bottlenecked by system bus
bandwidth (rare write and read access from both CPU and GPU).

As we are relying on device driver API, HMM does not introduce any new
syscall nor does it modify any existing ones. It does not change any POSIX
semantics or behaviors. For instance the child after a fork of a process
that is using HMM will not be impacted in anyway, nor is there any data
hazard between child COW or parent COW of memory that was migrated to
device prior to fork.

HMM assume a numbers of hardware features. Device must allow device page
table to be updated at any time (ie device job must be preemptable). Device
page table must provides memory protection such as read only. Device must
track write access (dirty bit). Device must have a minimum granularity that
match PAGE_SIZE (ie 4k).


Reviewer (just hint):
Patch 1    add new add_pages() helper to avoid modifying each arch memory
           hot plug function
Patch 2    move the page reference decrement from put_page() to
           put_zone_device_page() Dan Williams is the best person to review
           this change
Patch 3    add a new memory type for ZONE_DEVICE and also add all the logic
           in various core mm to support this new type. Dan Williams and
           any core mm contributor are best people to review each half of
           this patchset
Patch 4    add a new migrate mode. Any one familiar with page migration is
           welcome to review.
Patch 5    introduce a new migration helper (migrate_vma()) that allow to
           migrate a range of virtual address of a process using device DMA
           engine to perform the copy. It is not limited to do copy from and
           to device but can also do copy between any kind of source and
           destination memory. Again anyone familiar with migration code
           should be able to verify the logic.
Patch 6    optimize the new migrate_vma() by unmapping pages while we are
           collecting them. This can be review by any mm folks.
Patch 7    introduce core infrastructure and definition of HMM, pretty
           small patch and easy to review
Patch 8    introduce the mirror functionality of HMM, it relies on
           mmu_notifier and thus someone familiar with that part would be
           in better position to review
Patch 9    is an helper to snapshot CPU page table while synchronizing with
           concurrent page table update. Understanding mmu_notifier makes
           review easier.
Patch 10   is mostly a wrapper around handle_mm_fault()
Patch 11   add unaddressable memory migration to helper introduced in patch
           6, this can be review by anyone familiar with migration code
Patch 12   add a feature that allow device to allocate non-present page on
           the GPU when migrating a range of address to device memory. This
           is an helper for device driver to avoid having to first allocate
           system memory before migration to device memory
Patch 13   add helper to hotplug un-addressable device memory as new type
           of ZONE_DEVICE memory (new type introducted in patch 3 of this
           serie). This is boiler plate code around memory hotplug and it
           also pick a free range of physical address for the device memory.
           Note that the physical address do not point to anything (at least
           as far as the kernel knows).
Patch 14   introduce a new hmm_device class as an helper for device driver
           that want to expose multiple device memory under a common fake
           device driver. This is usefull for multi-gpu configuration.
           Anyone familiar with device driver infrastructure can review
           this. Boiler plate code really.
Patch 15   is the documentation for everything


Previous patchset posting :
    v1 http://lwn.net/Articles/597289/
    v2 https://lkml.org/lkml/2014/6/12/559
    v3 https://lkml.org/lkml/2014/6/13/633
    v4 https://lkml.org/lkml/2014/8/29/423
    v5 https://lkml.org/lkml/2014/11/3/759
    v6 http://lwn.net/Articles/619737/
    v7 http://lwn.net/Articles/627316/
    v8 https://lwn.net/Articles/645515/
    v9 https://lwn.net/Articles/651553/
    v10 https://lwn.net/Articles/654430/
    v11 http://www.gossamer-threads.com/lists/linux/kernel/2286424
    v12 http://www.kernelhub.org/?msg=972982&p=2
    v13 https://lwn.net/Articles/706856/
    v14 https://lkml.org/lkml/2016/12/8/344
    v15 http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1304107.html
    v16 http://www.spinics.net/lists/linux-mm/msg119814.html
    v17 https://lkml.org/lkml/2017/1/27/847
    v18 https://lkml.org/lkml/2017/3/16/596
    v19 https://lkml.org/lkml/2017/4/5/831

Jérôme Glisse (14):
  mm/put_page: move ZONE_DEVICE page reference decrement v2
  mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable
    memory
  mm/migrate: new migrate mode MIGRATE_SYNC_NO_COPY
  mm/migrate: new memory migration helper for use with device memory v4
  mm/migrate: migrate_vma() unmap page from vma while collecting pages
  mm/hmm: heterogeneous memory management (HMM for short) v2
  mm/hmm/mirror: mirror process address space on device with HMM helpers
    v2
  mm/hmm/mirror: helper to snapshot CPU page table v2
  mm/hmm/mirror: device page fault handler
  mm/migrate: support un-addressable ZONE_DEVICE page in migration
  mm/migrate: allow migrate_vma() to alloc new page on empty entry v2
  mm/hmm/devmem: device memory hotplug using ZONE_DEVICE v3
  mm/hmm/devmem: dummy HMM device for ZONE_DEVICE memory v3
  hmm: heterogeneous memory management documentation

Michal Hocko (1):
  mm, memory_hotplug: introduce add_pages

 Documentation/vm/hmm.txt       |  362 ++++++++++++
 MAINTAINERS                    |    7 +
 arch/x86/Kconfig               |    4 +
 arch/x86/mm/init_64.c          |   22 +-
 fs/aio.c                       |    8 +
 fs/f2fs/data.c                 |    5 +-
 fs/hugetlbfs/inode.c           |    5 +-
 fs/proc/task_mmu.c             |    7 +
 fs/ubifs/file.c                |    5 +-
 include/linux/hmm.h            |  468 ++++++++++++++++
 include/linux/ioport.h         |    1 +
 include/linux/memory_hotplug.h |   11 +
 include/linux/memremap.h       |   82 +++
 include/linux/migrate.h        |  115 ++++
 include/linux/migrate_mode.h   |    5 +
 include/linux/mm.h             |   14 +-
 include/linux/mm_types.h       |    5 +
 include/linux/swap.h           |   24 +-
 include/linux/swapops.h        |   68 +++
 kernel/fork.c                  |    2 +
 kernel/memremap.c              |   47 ++
 mm/Kconfig                     |   47 ++
 mm/Makefile                    |    1 +
 mm/balloon_compaction.c        |    8 +
 mm/hmm.c                       | 1203 ++++++++++++++++++++++++++++++++++++++++
 mm/memory.c                    |   61 ++
 mm/memory_hotplug.c            |   10 +-
 mm/migrate.c                   |  789 +++++++++++++++++++++++++-
 mm/mprotect.c                  |   14 +
 mm/page_vma_mapped.c           |   10 +
 mm/rmap.c                      |   25 +
 mm/zsmalloc.c                  |    8 +
 32 files changed, 3412 insertions(+), 31 deletions(-)
 create mode 100644 Documentation/vm/hmm.txt
 create mode 100644 include/linux/hmm.h
 create mode 100644 mm/hmm.c

-- 
2.9.3

[HMM 01/15] mm, memory_hotplug: introduce add_pages

[Jérôme Glisse]

From: Michal Hocko <mhocko@suse.com>

There are new users of memory hotplug emerging. Some of them require
different subset of arch_add_memory. There are some which only require
allocation of struct pages without mapping those pages to the kernel
address space. We currently have __add_pages for that purpose. But this
is rather lowlevel and not very suitable for the code outside of the
memory hotplug. E.g. x86_64 wants to update max_pfn which should be
done by the caller. Introduce add_pages() which should care about those
details if they are needed. Each architecture should define its
implementation and select CONFIG_ARCH_HAS_ADD_PAGES. All others use
the currently existing __add_pages.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
---
 arch/x86/Kconfig               |  4 ++++
 arch/x86/mm/init_64.c          | 22 +++++++++++++++-------
 include/linux/memory_hotplug.h | 11 +++++++++++
 3 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c43f476..e515dc2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2263,6 +2263,10 @@ source "kernel/livepatch/Kconfig"
 
 endmenu
 
+config ARCH_HAS_ADD_PAGES
+	def_bool y
+	depends on X86_64 && ARCH_ENABLE_MEMORY_HOTPLUG
+
 config ARCH_ENABLE_MEMORY_HOTPLUG
 	def_bool y
 	depends on X86_64 || (X86_32 && HIGHMEM)
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index ffeba90..a573ebc 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -671,7 +671,7 @@ void __init paging_init(void)
  * After memory hotplug the variables max_pfn, max_low_pfn and high_memory need
  * updating.
  */
-static void  update_end_of_memory_vars(u64 start, u64 size)
+static void update_end_of_memory_vars(u64 start, u64 size)
 {
 	unsigned long end_pfn = PFN_UP(start + size);
 
@@ -682,22 +682,30 @@ static void  update_end_of_memory_vars(u64 start, u64 size)
 	}
 }
 
-int arch_add_memory(int nid, u64 start, u64 size, bool want_memblock)
+int add_pages(int nid, unsigned long start_pfn,
+	      unsigned long nr_pages, bool want_memblock)
 {
-	unsigned long start_pfn = start >> PAGE_SHIFT;
-	unsigned long nr_pages = size >> PAGE_SHIFT;
 	int ret;
 
-	init_memory_mapping(start, start + size);
-
 	ret = __add_pages(nid, start_pfn, nr_pages, want_memblock);
 	WARN_ON_ONCE(ret);
 
 	/* update max_pfn, max_low_pfn and high_memory */
-	update_end_of_memory_vars(start, size);
+	update_end_of_memory_vars(start_pfn << PAGE_SHIFT,
+				  nr_pages << PAGE_SHIFT);
 
 	return ret;
 }
+
+int arch_add_memory(int nid, u64 start, u64 size, bool want_memblock)
+{
+	unsigned long start_pfn = start >> PAGE_SHIFT;
+	unsigned long nr_pages = size >> PAGE_SHIFT;
+
+	init_memory_mapping(start, start + size);
+
+	return add_pages(nid, start_pfn, nr_pages, want_memblock);
+}
 EXPORT_SYMBOL_GPL(arch_add_memory);
 
 #define PAGE_INUSE 0xFD
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index aec8865..5ec6d64 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -126,6 +126,17 @@ extern int __remove_pages(struct zone *zone, unsigned long start_pfn,
 extern int __add_pages(int nid, unsigned long start_pfn,
 	unsigned long nr_pages, bool want_memblock);
 
+#ifndef CONFIG_ARCH_HAS_ADD_PAGES
+static inline int add_pages(int nid, unsigned long start_pfn,
+			    unsigned long nr_pages, bool want_memblock)
+{
+	return __add_pages(nid, start_pfn, nr_pages, want_memblock);
+}
+#else /* ARCH_HAS_ADD_PAGES */
+int add_pages(int nid, unsigned long start_pfn,
+	      unsigned long nr_pages, bool want_memblock);
+#endif /* ARCH_HAS_ADD_PAGES */
+
 #ifdef CONFIG_NUMA
 extern int memory_add_physaddr_to_nid(u64 start);
 #else
-- 
2.9.3

[HMM 02/15] mm/put_page: move ZONE_DEVICE page reference decrement v2

[Jérôme Glisse]

Move page reference decrement of ZONE_DEVICE from put_page()
to put_zone_device_page() this does not affect non ZONE_DEVICE
page.

Doing this allow to catch when a ZONE_DEVICE page refcount reach
1 which means the device is no longer reference by any one (unlike
page from other zone, ZONE_DEVICE page refcount never reach 0).

This patch is just a preparatory patch for HMM.

Changes since v1:
  - commit message

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
---
 include/linux/mm.h | 14 +++++++++++---
 kernel/memremap.c  |  6 ++++++
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c82e8db..022423c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -821,11 +821,19 @@ static inline void put_page(struct page *page)
 {
 	page = compound_head(page);
 
+	/*
+	 * ZONE_DEVICE pages should never have their refcount reach 0 (this
+	 * would be a bug), so call page_ref_dec() in put_zone_device_page()
+	 * to decrement page refcount and skip __put_page() here, as this
+	 * would worsen things if a ZONE_DEVICE had a refcount bug.
+	 */
+	if (unlikely(is_zone_device_page(page))) {
+		put_zone_device_page(page);
+		return;
+	}
+
 	if (put_page_testzero(page))
 		__put_page(page);
-
-	if (unlikely(is_zone_device_page(page)))
-		put_zone_device_page(page);
 }
 
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
diff --git a/kernel/memremap.c b/kernel/memremap.c
index ea714ee..97ef676 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -190,6 +190,12 @@ EXPORT_SYMBOL(get_zone_device_page);
 
 void put_zone_device_page(struct page *page)
 {
+	/*
+	 * ZONE_DEVICE page refcount should never reach 0 and never be freed
+	 * to kernel memory allocator.
+	 */
+	page_ref_dec(page);
+
 	put_dev_pagemap(page->pgmap);
 }
 EXPORT_SYMBOL(put_zone_device_page);
-- 
2.9.3

[HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable memory

[Jérôme Glisse]

HMM (heterogeneous memory management) need struct page to support migration
from system main memory to device memory.  Reasons for HMM and migration to
device memory is explained with HMM core patch.

This patch deals with device memory that is un-addressable memory (ie CPU
can not access it). Hence we do not want those struct page to be manage
like regular memory. That is why we extend ZONE_DEVICE to support different
types of memory.

A persistent memory type is define for existing user of ZONE_DEVICE and a
new device un-addressable type is added for the un-addressable memory type.
There is a clear separation between what is expected from each memory type
and existing user of ZONE_DEVICE are un-affected by new requirement and new
use of the un-addressable type. All specific code path are protect with
test against the memory type.

Because memory is un-addressable we use a new special swap type for when
a page is migrated to device memory (this reduces the number of maximum
swap file).

The main two additions beside memory type to ZONE_DEVICE is two callbacks.
First one, page_free() is call whenever page refcount reach 1 (which means
the page is free as ZONE_DEVICE page never reach a refcount of 0). This
allow device driver to manage its memory and associated struct page.

The second callback page_fault() happens when there is a CPU access to
an address that is back by a device page (which are un-addressable by the
CPU). This callback is responsible to migrate the page back to system
main memory. Device driver can not block migration back to system memory,
HMM make sure that such page can not be pin into device memory.

If device is in some error condition and can not migrate memory back then
a CPU page fault to device memory should end with SIGBUS.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
---
 fs/proc/task_mmu.c       |  7 +++++
 include/linux/ioport.h   |  1 +
 include/linux/memremap.h | 82 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/swap.h     | 24 ++++++++++++--
 include/linux/swapops.h  | 68 +++++++++++++++++++++++++++++++++++++++
 kernel/memremap.c        | 43 ++++++++++++++++++++++++-
 mm/Kconfig               | 13 ++++++++
 mm/memory.c              | 61 +++++++++++++++++++++++++++++++++++
 mm/memory_hotplug.c      | 10 ++++--
 mm/mprotect.c            | 14 +++++++++
 10 files changed, 317 insertions(+), 6 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index f0c8b33..a12ba94 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -542,6 +542,8 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
 			}
 		} else if (is_migration_entry(swpent))
 			page = migration_entry_to_page(swpent);
+		else if (is_device_entry(swpent))
+			page = device_entry_to_page(swpent);
 	} else if (unlikely(IS_ENABLED(CONFIG_SHMEM) && mss->check_shmem_swap
 							&& pte_none(*pte))) {
 		page = find_get_entry(vma->vm_file->f_mapping,
@@ -704,6 +706,8 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
 
 		if (is_migration_entry(swpent))
 			page = migration_entry_to_page(swpent);
+		else if (is_device_entry(swpent))
+			page = device_entry_to_page(swpent);
 	}
 	if (page) {
 		int mapcount = page_mapcount(page);
@@ -1196,6 +1200,9 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
 		flags |= PM_SWAP;
 		if (is_migration_entry(entry))
 			page = migration_entry_to_page(entry);
+
+		if (is_device_entry(entry))
+			page = device_entry_to_page(entry);
 	}
 
 	if (page && !PageAnon(page))
diff --git a/include/linux/ioport.h b/include/linux/ioport.h
index 6230064..ec619dc 100644
--- a/include/linux/ioport.h
+++ b/include/linux/ioport.h
@@ -130,6 +130,7 @@ enum {
 	IORES_DESC_ACPI_NV_STORAGE		= 3,
 	IORES_DESC_PERSISTENT_MEMORY		= 4,
 	IORES_DESC_PERSISTENT_MEMORY_LEGACY	= 5,
+	IORES_DESC_DEVICE_MEMORY_UNADDRESSABLE	= 6,
 };
 
 /* helpers to define resources */
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 9341619..365fb4e 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -35,24 +35,101 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
 }
 #endif
 
+/*
+ * Specialize ZONE_DEVICE memory into multiple types each having differents
+ * usage.
+ *
+ * MEMORY_DEVICE_PERSISTENT:
+ * Persistent device memory (pmem): struct page might be allocated in different
+ * memory and architecture might want to perform special actions. It is similar
+ * to regular memory, in that the CPU can access it transparently. However,
+ * it is likely to have different bandwidth and latency than regular memory.
+ * See Documentation/nvdimm/nvdimm.txt for more information.
+ *
+ * MEMORY_DEVICE_UNADDRESSABLE:
+ * Device memory that is not directly addressable by the CPU: CPU can neither
+ * read nor write _UNADDRESSABLE memory. In this case, we do still have struct
+ * pages backing the device memory. Doing so simplifies the implementation, but
+ * it is important to remember that there are certain points at which the struct
+ * page must be treated as an opaque object, rather than a "normal" struct page.
+ * A more complete discussion of unaddressable memory may be found in
+ * include/linux/hmm.h and Documentation/vm/hmm.txt.
+ */
+enum memory_type {
+	MEMORY_DEVICE_PERSISTENT = 0,
+	MEMORY_DEVICE_UNADDRESSABLE,
+};
+
+/*
+ * For MEMORY_DEVICE_UNADDRESSABLE we use ZONE_DEVICE and extend it with two
+ * callbacks:
+ *   page_fault()
+ *   page_free()
+ *
+ * Additional notes about MEMORY_DEVICE_UNADDRESSABLE may be found in
+ * include/linux/hmm.h and Documentation/vm/hmm.txt. There is also a brief
+ * explanation in include/linux/memory_hotplug.h.
+ *
+ * The page_fault() callback must migrate page back, from device memory to
+ * system memory, so that the CPU can access it. This might fail for various
+ * reasons (device issues,  device have been unplugged, ...). When such error
+ * conditions happen, the page_fault() callback must return VM_FAULT_SIGBUS and
+ * set the CPU page table entry to "poisoned".
+ *
+ * Note that because memory cgroup charges are transferred to the device memory,
+ * this should never fail due to memory restrictions. However, allocation
+ * of a regular system page might still fail because we are out of memory. If
+ * that happens, the page_fault() callback must return VM_FAULT_OOM.
+ *
+ * The page_fault() callback can also try to migrate back multiple pages in one
+ * chunk, as an optimization. It must, however, prioritize the faulting address
+ * over all the others.
+ *
+ *
+ * The page_free() callback is called once the page refcount reaches 1
+ * (ZONE_DEVICE pages never reach 0 refcount unless there is a refcount bug.
+ * This allows the device driver to implement its own memory management.)
+ */
+typedef int (*dev_page_fault_t)(struct vm_area_struct *vma,
+				unsigned long addr,
+				struct page *page,
+				unsigned int flags,
+				pmd_t *pmdp);
+typedef void (*dev_page_free_t)(struct page *page, void *data);
+
 /**
  * struct dev_pagemap - metadata for ZONE_DEVICE mappings
+ * @page_fault: callback when CPU fault on an unaddressable device page
+ * @page_free: free page callback when page refcount reaches 1
  * @altmap: pre-allocated/reserved memory for vmemmap allocations
  * @res: physical address range covered by @ref
  * @ref: reference count that pins the devm_memremap_pages() mapping
  * @dev: host device of the mapping for debug
+ * @data: private data pointer for page_free()
+ * @type: memory type: see MEMORY_* in memory_hotplug.h
  */
 struct dev_pagemap {
+	dev_page_fault_t page_fault;
+	dev_page_free_t page_free;
 	struct vmem_altmap *altmap;
 	const struct resource *res;
 	struct percpu_ref *ref;
 	struct device *dev;
+	void *data;
+	enum memory_type type;
 };
 
 #ifdef CONFIG_ZONE_DEVICE
 void *devm_memremap_pages(struct device *dev, struct resource *res,
 		struct percpu_ref *ref, struct vmem_altmap *altmap);
 struct dev_pagemap *find_dev_pagemap(resource_size_t phys);
+
+static inline bool is_device_unaddressable_page(const struct page *page)
+{
+	/* See MEMORY_DEVICE_UNADDRESSABLE in include/linux/memory_hotplug.h */
+	return ((page_zonenum(page) == ZONE_DEVICE) &&
+		(page->pgmap->type == MEMORY_DEVICE_UNADDRESSABLE));
+}
 #else
 static inline void *devm_memremap_pages(struct device *dev,
 		struct resource *res, struct percpu_ref *ref,
@@ -71,6 +148,11 @@ static inline struct dev_pagemap *find_dev_pagemap(resource_size_t phys)
 {
 	return NULL;
 }
+
+static inline bool is_device_unaddressable_page(const struct page *page)
+{
+	return false;
+}
 #endif
 
 /**
diff --git a/include/linux/swap.h b/include/linux/swap.h
index ba58824..52a137d 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -51,6 +51,23 @@ static inline int current_is_kswapd(void)
  */
 
 /*
+ * Unaddressable device memory support. See include/linux/hmm.h and
+ * Documentation/vm/hmm.txt. Short description is we need struct pages for
+ * device memory that is unaddressable (inaccessible) by CPU, so that we can
+ * migrate part of a process memory to device memory.
+ *
+ * When a page is migrated from CPU to device, we set the CPU page table entry
+ * to a special SWP_DEVICE_* entry.
+ */
+#ifdef CONFIG_DEVICE_UNADDRESSABLE
+#define SWP_DEVICE_NUM 2
+#define SWP_DEVICE_WRITE (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM)
+#define SWP_DEVICE_READ (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+1)
+#else
+#define SWP_DEVICE_NUM 0
+#endif
+
+/*
  * NUMA node memory migration support
  */
 #ifdef CONFIG_MIGRATION
@@ -72,7 +89,8 @@ static inline int current_is_kswapd(void)
 #endif
 
 #define MAX_SWAPFILES \
-	((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
+	((1 << MAX_SWAPFILES_SHIFT) - SWP_DEVICE_NUM - \
+	SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
 
 /*
  * Magic header for a swap area. The first part of the union is
@@ -432,8 +450,8 @@ static inline void show_swap_cache_info(void)
 {
 }
 
-#define free_swap_and_cache(swp)	is_migration_entry(swp)
-#define swapcache_prepare(swp)		is_migration_entry(swp)
+#define free_swap_and_cache(e) (is_migration_entry(e) || is_device_entry(e))
+#define swapcache_prepare(e) (is_migration_entry(e) || is_device_entry(e))
 
 static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
 {
diff --git a/include/linux/swapops.h b/include/linux/swapops.h
index 5c3a5f3..960eb6b 100644
--- a/include/linux/swapops.h
+++ b/include/linux/swapops.h
@@ -100,6 +100,74 @@ static inline void *swp_to_radix_entry(swp_entry_t entry)
 	return (void *)(value | RADIX_TREE_EXCEPTIONAL_ENTRY);
 }
 
+#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
+static inline swp_entry_t make_device_entry(struct page *page, bool write)
+{
+	return swp_entry(write ? SWP_DEVICE_WRITE : SWP_DEVICE_READ,
+			 page_to_pfn(page));
+}
+
+static inline bool is_device_entry(swp_entry_t entry)
+{
+	int type = swp_type(entry);
+	return type == SWP_DEVICE_READ || type == SWP_DEVICE_WRITE;
+}
+
+static inline void make_device_entry_read(swp_entry_t *entry)
+{
+	*entry = swp_entry(SWP_DEVICE_READ, swp_offset(*entry));
+}
+
+static inline bool is_write_device_entry(swp_entry_t entry)
+{
+	return unlikely(swp_type(entry) == SWP_DEVICE_WRITE);
+}
+
+static inline struct page *device_entry_to_page(swp_entry_t entry)
+{
+	return pfn_to_page(swp_offset(entry));
+}
+
+int device_entry_fault(struct vm_area_struct *vma,
+		       unsigned long addr,
+		       swp_entry_t entry,
+		       unsigned int flags,
+		       pmd_t *pmdp);
+#else /* CONFIG_DEVICE_UNADDRESSABLE */
+static inline swp_entry_t make_device_entry(struct page *page, bool write)
+{
+	return swp_entry(0, 0);
+}
+
+static inline void make_device_entry_read(swp_entry_t *entry)
+{
+}
+
+static inline bool is_device_entry(swp_entry_t entry)
+{
+	return false;
+}
+
+static inline bool is_write_device_entry(swp_entry_t entry)
+{
+	return false;
+}
+
+static inline struct page *device_entry_to_page(swp_entry_t entry)
+{
+	return NULL;
+}
+
+static inline int device_entry_fault(struct vm_area_struct *vma,
+				     unsigned long addr,
+				     swp_entry_t entry,
+				     unsigned int flags,
+				     pmd_t *pmdp)
+{
+	return VM_FAULT_SIGBUS;
+}
+#endif /* CONFIG_DEVICE_UNADDRESSABLE */
+
 #ifdef CONFIG_MIGRATION
 static inline swp_entry_t make_migration_entry(struct page *page, int write)
 {
diff --git a/kernel/memremap.c b/kernel/memremap.c
index 97ef676..dce3b29 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -18,6 +18,8 @@
 #include <linux/io.h>
 #include <linux/mm.h>
 #include <linux/memory_hotplug.h>
+#include <linux/swap.h>
+#include <linux/swapops.h>
 
 #ifndef ioremap_cache
 /* temporary while we convert existing ioremap_cache users to memremap */
@@ -194,12 +196,47 @@ void put_zone_device_page(struct page *page)
 	 * ZONE_DEVICE page refcount should never reach 0 and never be freed
 	 * to kernel memory allocator.
 	 */
-	page_ref_dec(page);
+	int count = page_ref_dec_return(page);
+
+	/*
+	 * If refcount is 1 then page is freed and refcount is stable as nobody
+	 * holds a reference on the page.
+	 */
+	if (page->pgmap->page_free && count == 1)
+		page->pgmap->page_free(page, page->pgmap->data);
 
 	put_dev_pagemap(page->pgmap);
 }
 EXPORT_SYMBOL(put_zone_device_page);
 
+#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
+int device_entry_fault(struct vm_area_struct *vma,
+		       unsigned long addr,
+		       swp_entry_t entry,
+		       unsigned int flags,
+		       pmd_t *pmdp)
+{
+	struct page *page = device_entry_to_page(entry);
+
+	/*
+	 * The page_fault() callback must migrate page back to system memory
+	 * so that CPU can access it. This might fail for various reasons
+	 * (device issue, device was unsafely unplugged, ...). When such
+	 * error conditions happen, the callback must return VM_FAULT_SIGBUS.
+	 *
+	 * Note that because memory cgroup charges are accounted to the device
+	 * memory, this should never fail because of memory restrictions (but
+	 * allocation of regular system page might still fail because we are
+	 * out of memory).
+	 *
+	 * There is a more in-depth description of what that callback can and
+	 * cannot do, in include/linux/memremap.h
+	 */
+	return page->pgmap->page_fault(vma, addr, page, flags, pmdp);
+}
+EXPORT_SYMBOL(device_entry_fault);
+#endif /* CONFIG_DEVICE_UNADDRESSABLE */
+
 static void pgmap_radix_release(struct resource *res)
 {
 	resource_size_t key, align_start, align_size, align_end;
@@ -332,6 +369,10 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
 	}
 	pgmap->ref = ref;
 	pgmap->res = &page_map->res;
+	pgmap->type = MEMORY_DEVICE_PERSISTENT;
+	pgmap->page_fault = NULL;
+	pgmap->page_free = NULL;
+	pgmap->data = NULL;
 
 	mutex_lock(&pgmap_lock);
 	error = 0;
diff --git a/mm/Kconfig b/mm/Kconfig
index c89f472..ad4bd50 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -700,6 +700,19 @@ config ZONE_DEVICE
 
 	  If FS_DAX is enabled, then say Y.
 
+config DEVICE_UNADDRESSABLE
+	bool "Unaddressable device memory (GPU memory, ...)"
+	depends on X86_64
+	depends on ZONE_DEVICE
+	depends on MEMORY_HOTPLUG
+	depends on MEMORY_HOTREMOVE
+	depends on SPARSEMEM_VMEMMAP
+
+	help
+	  Allows creation of struct pages to represent unaddressable device
+	  memory; i.e., memory that is only accessible from the device (or
+	  group of devices).
+
 config FRAME_VECTOR
 	bool
 
diff --git a/mm/memory.c b/mm/memory.c
index 6ff5d72..4116ab2 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -49,6 +49,7 @@
 #include <linux/swap.h>
 #include <linux/highmem.h>
 #include <linux/pagemap.h>
+#include <linux/memremap.h>
 #include <linux/ksm.h>
 #include <linux/rmap.h>
 #include <linux/export.h>
@@ -927,6 +928,35 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 					pte = pte_swp_mksoft_dirty(pte);
 				set_pte_at(src_mm, addr, src_pte, pte);
 			}
+		} else if (is_device_entry(entry)) {
+			page = device_entry_to_page(entry);
+
+			/*
+			 * Update rss count even for unaddressable pages, as
+			 * they should treated just like normal pages in this
+			 * respect.
+			 *
+			 * We will likely want to have some new rss counters
+			 * for unaddressable pages, at some point. But for now
+			 * keep things as they are.
+			 */
+			get_page(page);
+			rss[mm_counter(page)]++;
+			page_dup_rmap(page, false);
+
+			/*
+			 * We do not preserve soft-dirty information, because so
+			 * far, checkpoint/restore is the only feature that
+			 * requires that. And checkpoint/restore does not work
+			 * when a device driver is involved (you cannot easily
+			 * save and restore device driver state).
+			 */
+			if (is_write_device_entry(entry) &&
+			    is_cow_mapping(vm_flags)) {
+				make_device_entry_read(&entry);
+				pte = swp_entry_to_pte(entry);
+				set_pte_at(src_mm, addr, src_pte, pte);
+			}
 		}
 		goto out_set_pte;
 	}
@@ -1243,6 +1273,29 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
 			}
 			continue;
 		}
+
+		entry = pte_to_swp_entry(ptent);
+		if (non_swap_entry(entry) && is_device_entry(entry)) {
+			struct page *page = device_entry_to_page(entry);
+
+			if (unlikely(details && details->check_mapping)) {
+				/*
+				 * unmap_shared_mapping_pages() wants to
+				 * invalidate cache without truncating:
+				 * unmap shared but keep private pages.
+				 */
+				if (details->check_mapping !=
+				    page_rmapping(page))
+					continue;
+			}
+
+			pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
+			rss[mm_counter(page)]--;
+			page_remove_rmap(page, false);
+			put_page(page);
+			continue;
+		}
+
 		/* If details->check_mapping, we leave swap entries. */
 		if (unlikely(details))
 			continue;
@@ -2690,6 +2743,14 @@ int do_swap_page(struct vm_fault *vmf)
 		if (is_migration_entry(entry)) {
 			migration_entry_wait(vma->vm_mm, vmf->pmd,
 					     vmf->address);
+		} else if (is_device_entry(entry)) {
+			/*
+			 * For un-addressable device memory we call the pgmap
+			 * fault handler callback. The callback must migrate
+			 * the page back to some CPU accessible page.
+			 */
+			ret = device_entry_fault(vma, vmf->address, entry,
+						 vmf->flags, vmf->pmd);
 		} else if (is_hwpoison_entry(entry)) {
 			ret = VM_FAULT_HWPOISON;
 		} else {
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index de30822..d988fbe 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -156,7 +156,7 @@ void mem_hotplug_done(void)
 /* add this memory to iomem resource */
 static struct resource *register_memory_resource(u64 start, u64 size)
 {
-	struct resource *res;
+	struct resource *res, *conflict;
 	res = kzalloc(sizeof(struct resource), GFP_KERNEL);
 	if (!res)
 		return ERR_PTR(-ENOMEM);
@@ -165,7 +165,13 @@ static struct resource *register_memory_resource(u64 start, u64 size)
 	res->start = start;
 	res->end = start + size - 1;
 	res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
-	if (request_resource(&iomem_resource, res) < 0) {
+	conflict =  request_resource_conflict(&iomem_resource, res);
+	if (conflict) {
+		if (conflict->desc == IORES_DESC_DEVICE_MEMORY_UNADDRESSABLE) {
+			pr_debug("Device unaddressable memory block "
+				 "memory hotplug at %#010llx !\n",
+				 (unsigned long long)start);
+		}
 		pr_debug("System RAM resource %pR cannot be added\n", res);
 		kfree(res);
 		return ERR_PTR(-EEXIST);
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 8edd0d5..dadb020 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -126,6 +126,20 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
 
 				pages++;
 			}
+
+			if (is_write_device_entry(entry)) {
+				pte_t newpte;
+
+				/*
+				 * We do not preserve soft-dirtiness. See
+				 * copy_one_pte() for explanation.
+				 */
+				make_device_entry_read(&entry);
+				newpte = swp_entry_to_pte(entry);
+				set_pte_at(mm, addr, pte, newpte);
+
+				pages++;
+			}
 		}
 	} while (pte++, addr += PAGE_SIZE, addr != end);
 	arch_leave_lazy_mmu_mode();
-- 
2.9.3

[HMM 04/15] mm/migrate: new migrate mode MIGRATE_SYNC_NO_COPY

[Jérôme Glisse]

Introduce a new migration mode that allow to offload the copy to
a device DMA engine. This changes the workflow of migration and
not all address_space migratepage callback can support this. So
it needs to be tested in those cases.

This is intended to be use by migrate_vma() which itself is use
for thing like HMM (see include/linux/hmm.h).

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
---
 fs/aio.c                     |  8 +++++++
 fs/f2fs/data.c               |  5 ++++-
 fs/hugetlbfs/inode.c         |  5 ++++-
 fs/ubifs/file.c              |  5 ++++-
 include/linux/migrate.h      |  5 +++++
 include/linux/migrate_mode.h |  5 +++++
 mm/balloon_compaction.c      |  8 +++++++
 mm/migrate.c                 | 52 ++++++++++++++++++++++++++++++++++----------
 mm/zsmalloc.c                |  8 +++++++
 9 files changed, 86 insertions(+), 15 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index f52d925..e51351e 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -373,6 +373,14 @@ static int aio_migratepage(struct address_space *mapping, struct page *new,
 	pgoff_t idx;
 	int rc;
 
+	/*
+	 * We cannot support the _NO_COPY case here, because copy needs to
+	 * happen under the ctx->completion_lock. That does not work with the
+	 * migration workflow of MIGRATE_SYNC_NO_COPY.
+	 */
+	if (mode == MIGRATE_SYNC_NO_COPY)
+		return -EINVAL;
+
 	rc = 0;
 
 	/* mapping->private_lock here protects against the kioctx teardown.  */
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index e984a42..b36191f 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -2131,7 +2131,10 @@ int f2fs_migrate_page(struct address_space *mapping,
 		SetPagePrivate(newpage);
 	set_page_private(newpage, page_private(page));
 
-	migrate_page_copy(newpage, page);
+	if (mode != MIGRATE_SYNC_NO_COPY)
+		migrate_page_copy(newpage, page);
+	else
+		migrate_page_states(newpage, page);
 
 	return MIGRATEPAGE_SUCCESS;
 }
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index dde8613..c02ff56 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -846,7 +846,10 @@ static int hugetlbfs_migrate_page(struct address_space *mapping,
 	rc = migrate_huge_page_move_mapping(mapping, newpage, page);
 	if (rc != MIGRATEPAGE_SUCCESS)
 		return rc;
-	migrate_page_copy(newpage, page);
+	if (mode != MIGRATE_SYNC_NO_COPY)
+		migrate_page_copy(newpage, page);
+	else
+		migrate_page_states(newpage, page);
 
 	return MIGRATEPAGE_SUCCESS;
 }
diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 2cda3d6..b2292be 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1482,7 +1482,10 @@ static int ubifs_migrate_page(struct address_space *mapping,
 		SetPagePrivate(newpage);
 	}
 
-	migrate_page_copy(newpage, page);
+	if (mode != MIGRATE_SYNC_NO_COPY)
+		migrate_page_copy(newpage, page);
+	else
+		migrate_page_states(newpage, page);
 	return MIGRATEPAGE_SUCCESS;
 }
 #endif
diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 48e2484..78a0fdc 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -43,6 +43,7 @@ extern void putback_movable_page(struct page *page);
 
 extern int migrate_prep(void);
 extern int migrate_prep_local(void);
+extern void migrate_page_states(struct page *newpage, struct page *page);
 extern void migrate_page_copy(struct page *newpage, struct page *page);
 extern int migrate_huge_page_move_mapping(struct address_space *mapping,
 				  struct page *newpage, struct page *page);
@@ -63,6 +64,10 @@ static inline int isolate_movable_page(struct page *page, isolate_mode_t mode)
 static inline int migrate_prep(void) { return -ENOSYS; }
 static inline int migrate_prep_local(void) { return -ENOSYS; }
 
+static inline void migrate_page_states(struct page *newpage, struct page *page)
+{
+}
+
 static inline void migrate_page_copy(struct page *newpage,
 				     struct page *page) {}
 
diff --git a/include/linux/migrate_mode.h b/include/linux/migrate_mode.h
index ebf3d89..bdf66af 100644
--- a/include/linux/migrate_mode.h
+++ b/include/linux/migrate_mode.h
@@ -6,11 +6,16 @@
  *	on most operations but not ->writepage as the potential stall time
  *	is too significant
  * MIGRATE_SYNC will block when migrating pages
+ * MIGRATE_SYNC_NO_COPY will block when migrating pages but will not copy pages
+ *	with the CPU. Instead, page copy happens outside the migratepage()
+ *	callback and is likely using a DMA engine. See migrate_vma() and HMM
+ *	(mm/hmm.c) for users of this mode.
  */
 enum migrate_mode {
 	MIGRATE_ASYNC,
 	MIGRATE_SYNC_LIGHT,
 	MIGRATE_SYNC,
+	MIGRATE_SYNC_NO_COPY,
 };
 
 #endif		/* MIGRATE_MODE_H_INCLUDED */
diff --git a/mm/balloon_compaction.c b/mm/balloon_compaction.c
index da91df5..145b903 100644
--- a/mm/balloon_compaction.c
+++ b/mm/balloon_compaction.c
@@ -139,6 +139,14 @@ int balloon_page_migrate(struct address_space *mapping,
 {
 	struct balloon_dev_info *balloon = balloon_page_device(page);
 
+	/*
+	 * We can not easily support the no copy case here so ignore it as it
+	 * is unlikely to be use with ballon pages. See include/linux/hmm.h for
+	 * user of the MIGRATE_SYNC_NO_COPY mode.
+	 */
+	if (mode == MIGRATE_SYNC_NO_COPY)
+		return -EINVAL;
+
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
 
diff --git a/mm/migrate.c b/mm/migrate.c
index 410b56b..28002b9 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -602,15 +602,10 @@ static void copy_huge_page(struct page *dst, struct page *src)
 /*
  * Copy the page to its new location
  */
-void migrate_page_copy(struct page *newpage, struct page *page)
+void migrate_page_states(struct page *newpage, struct page *page)
 {
 	int cpupid;
 
-	if (PageHuge(page) || PageTransHuge(page))
-		copy_huge_page(newpage, page);
-	else
-		copy_highpage(newpage, page);
-
 	if (PageError(page))
 		SetPageError(newpage);
 	if (PageReferenced(page))
@@ -664,6 +659,17 @@ void migrate_page_copy(struct page *newpage, struct page *page)
 
 	mem_cgroup_migrate(page, newpage);
 }
+EXPORT_SYMBOL(migrate_page_states);
+
+void migrate_page_copy(struct page *newpage, struct page *page)
+{
+	if (PageHuge(page) || PageTransHuge(page))
+		copy_huge_page(newpage, page);
+	else
+		copy_highpage(newpage, page);
+
+	migrate_page_states(newpage, page);
+}
 EXPORT_SYMBOL(migrate_page_copy);
 
 /************************************************************
@@ -689,7 +695,10 @@ int migrate_page(struct address_space *mapping,
 	if (rc != MIGRATEPAGE_SUCCESS)
 		return rc;
 
-	migrate_page_copy(newpage, page);
+	if (mode != MIGRATE_SYNC_NO_COPY)
+		migrate_page_copy(newpage, page);
+	else
+		migrate_page_states(newpage, page);
 	return MIGRATEPAGE_SUCCESS;
 }
 EXPORT_SYMBOL(migrate_page);
@@ -739,12 +748,15 @@ int buffer_migrate_page(struct address_space *mapping,
 
 	SetPagePrivate(newpage);
 
-	migrate_page_copy(newpage, page);
+	if (mode != MIGRATE_SYNC_NO_COPY)
+		migrate_page_copy(newpage, page);
+	else
+		migrate_page_states(newpage, page);
 
 	bh = head;
 	do {
 		unlock_buffer(bh);
- 		put_bh(bh);
+		put_bh(bh);
 		bh = bh->b_this_page;
 
 	} while (bh != head);
@@ -803,8 +815,13 @@ static int fallback_migrate_page(struct address_space *mapping,
 {
 	if (PageDirty(page)) {
 		/* Only writeback pages in full synchronous migration */
-		if (mode != MIGRATE_SYNC)
+		switch (mode) {
+		case MIGRATE_SYNC:
+		case MIGRATE_SYNC_NO_COPY:
+			break;
+		default:
 			return -EBUSY;
+		}
 		return writeout(mapping, page);
 	}
 
@@ -941,7 +958,11 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
 		 * the retry loop is too short and in the sync-light case,
 		 * the overhead of stalling is too much
 		 */
-		if (mode != MIGRATE_SYNC) {
+		switch (mode) {
+		case MIGRATE_SYNC:
+		case MIGRATE_SYNC_NO_COPY:
+			break;
+		default:
 			rc = -EBUSY;
 			goto out_unlock;
 		}
@@ -1211,8 +1232,15 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 		return -ENOMEM;
 
 	if (!trylock_page(hpage)) {
-		if (!force || mode != MIGRATE_SYNC)
+		if (!force)
 			goto out;
+		switch (mode) {
+		case MIGRATE_SYNC:
+		case MIGRATE_SYNC_NO_COPY:
+			break;
+		default:
+			goto out;
+		}
 		lock_page(hpage);
 	}
 
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index d41edd2..aeea3a5 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1983,6 +1983,14 @@ int zs_page_migrate(struct address_space *mapping, struct page *newpage,
 	unsigned int obj_idx;
 	int ret = -EAGAIN;
 
+	/*
+	 * We cannot support the _NO_COPY case here, because copy needs to
+	 * happen under the zs lock, which does not work with
+	 * MIGRATE_SYNC_NO_COPY workflow.
+	 */
+	if (mode == MIGRATE_SYNC_NO_COPY)
+		return -EINVAL;
+
 	VM_BUG_ON_PAGE(!PageMovable(page), page);
 	VM_BUG_ON_PAGE(!PageIsolated(page), page);
 
-- 
2.9.3

[HMM 05/15] mm/migrate: new memory migration helper for use with device memory v4

[Jérôme Glisse]

This patch add a new memory migration helpers, which migrate memory
backing a range of virtual address of a process to different memory
(which can be allocated through special allocator). It differs from
numa migration by working on a range of virtual address and thus by
doing migration in chunk that can be large enough to use DMA engine
or special copy offloading engine.

Expected users are any one with heterogeneous memory where different
memory have different characteristics (latency, bandwidth, ...). As
an example IBM platform with CAPI bus can make use of this feature
to migrate between regular memory and CAPI device memory. New CPU
architecture with a pool of high performance memory not manage as
cache but presented as regular memory (while being faster and with
lower latency than DDR) will also be prime user of this patch.

Migration to private device memory will be useful for device that
have large pool of such like GPU, NVidia plans to use HMM for that.

Changes since v3:
  - Rebase

Changes since v2:
  - droped HMM prefix and HMM specific code
Changes since v1:
  - typos fix
  - split early unmap optimization for page with single mapping

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 include/linux/migrate.h | 104 ++++++++++++
 mm/migrate.c            | 444 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 548 insertions(+)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 78a0fdc..576b3f5 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -127,4 +127,108 @@ static inline int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 }
 #endif /* CONFIG_NUMA_BALANCING && CONFIG_TRANSPARENT_HUGEPAGE*/
 
+
+#ifdef CONFIG_MIGRATION
+
+#define MIGRATE_PFN_VALID	(1UL << 0)
+#define MIGRATE_PFN_MIGRATE	(1UL << 1)
+#define MIGRATE_PFN_LOCKED	(1UL << 2)
+#define MIGRATE_PFN_WRITE	(1UL << 3)
+#define MIGRATE_PFN_ERROR	(1UL << 4)
+#define MIGRATE_PFN_SHIFT	5
+
+static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
+{
+	if (!(mpfn & MIGRATE_PFN_VALID))
+		return NULL;
+	return pfn_to_page(mpfn >> MIGRATE_PFN_SHIFT);
+}
+
+static inline unsigned long migrate_pfn(unsigned long pfn)
+{
+	return (pfn << MIGRATE_PFN_SHIFT) | MIGRATE_PFN_VALID;
+}
+
+/*
+ * struct migrate_vma_ops - migrate operation callback
+ *
+ * @alloc_and_copy: alloc destination memory and copy source memory to it
+ * @finalize_and_map: allow caller to map the successfully migrated pages
+ *
+ *
+ * The alloc_and_copy() callback happens once all source pages have been locked,
+ * unmapped and checked (checked whether pinned or not). All pages that can be
+ * migrated will have an entry in the src array set with the pfn value of the
+ * page and with the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set (other
+ * flags might be set but should be ignored by the callback).
+ *
+ * The alloc_and_copy() callback can then allocate destination memory and copy
+ * source memory to it for all those entries (ie with MIGRATE_PFN_VALID and
+ * MIGRATE_PFN_MIGRATE flag set). Once these are allocated and copied, the
+ * callback must update each corresponding entry in the dst array with the pfn
+ * value of the destination page and with the MIGRATE_PFN_VALID and
+ * MIGRATE_PFN_LOCKED flags set (destination pages must have their struct pages
+ * locked, via lock_page()).
+ *
+ * At this point the alloc_and_copy() callback is done and returns.
+ *
+ * Note that the callback does not have to migrate all the pages that are
+ * marked with MIGRATE_PFN_MIGRATE flag in src array unless this is a migration
+ * from device memory to system memory (ie the MIGRATE_PFN_DEVICE flag is also
+ * set in the src array entry). If the device driver cannot migrate a device
+ * page back to system memory, then it must set the corresponding dst array
+ * entry to MIGRATE_PFN_ERROR. This will trigger a SIGBUS if CPU tries to
+ * access any of the virtual addresses originally backed by this page. Because
+ * a SIGBUS is such a severe result for the userspace process, the device
+ * driver should avoid setting MIGRATE_PFN_ERROR unless it is really in an
+ * unrecoverable state.
+ *
+ * THE alloc_and_copy() CALLBACK MUST NOT CHANGE ANY OF THE SRC ARRAY ENTRIES
+ * OR BAD THINGS WILL HAPPEN !
+ *
+ *
+ * The finalize_and_map() callback happens after struct page migration from
+ * source to destination (destination struct pages are the struct pages for the
+ * memory allocated by the alloc_and_copy() callback).  Migration can fail, and
+ * thus the finalize_and_map() allows the driver to inspect which pages were
+ * successfully migrated, and which were not. Successfully migrated pages will
+ * have the MIGRATE_PFN_MIGRATE flag set for their src array entry.
+ *
+ * It is safe to update device page table from within the finalize_and_map()
+ * callback because both destination and source page are still locked, and the
+ * mmap_sem is held in read mode (hence no one can unmap the range being
+ * migrated).
+ *
+ * Once callback is done cleaning up things and updating its page table (if it
+ * chose to do so, this is not an obligation) then it returns. At this point,
+ * the HMM core will finish up the final steps, and the migration is complete.
+ *
+ * THE finalize_and_map() CALLBACK MUST NOT CHANGE ANY OF THE SRC OR DST ARRAY
+ * ENTRIES OR BAD THINGS WILL HAPPEN !
+ */
+struct migrate_vma_ops {
+	void (*alloc_and_copy)(struct vm_area_struct *vma,
+			       const unsigned long *src,
+			       unsigned long *dst,
+			       unsigned long start,
+			       unsigned long end,
+			       void *private);
+	void (*finalize_and_map)(struct vm_area_struct *vma,
+				 const unsigned long *src,
+				 const unsigned long *dst,
+				 unsigned long start,
+				 unsigned long end,
+				 void *private);
+};
+
+int migrate_vma(const struct migrate_vma_ops *ops,
+		struct vm_area_struct *vma,
+		unsigned long start,
+		unsigned long end,
+		unsigned long *src,
+		unsigned long *dst,
+		void *private);
+
+#endif /* CONFIG_MIGRATION */
+
 #endif /* _LINUX_MIGRATE_H */
diff --git a/mm/migrate.c b/mm/migrate.c
index 28002b9..452f894 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -396,6 +396,14 @@ int migrate_page_move_mapping(struct address_space *mapping,
 	int expected_count = 1 + extra_count;
 	void **pslot;
 
+	/*
+	 * ZONE_DEVICE pages have 1 refcount always held by their device
+	 *
+	 * Note that DAX memory will never reach that point as it does not have
+	 * the MEMORY_DEVICE_ALLOW_MIGRATE flag set (see memory_hotplug.h).
+	 */
+	expected_count += is_zone_device_page(page);
+
 	if (!mapping) {
 		/* Anonymous page without mapping */
 		if (page_count(page) != expected_count)
@@ -2076,3 +2084,439 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 #endif /* CONFIG_NUMA_BALANCING */
 
 #endif /* CONFIG_NUMA */
+
+
+struct migrate_vma {
+	struct vm_area_struct	*vma;
+	unsigned long		*dst;
+	unsigned long		*src;
+	unsigned long		cpages;
+	unsigned long		npages;
+	unsigned long		start;
+	unsigned long		end;
+};
+
+static int migrate_vma_collect_hole(unsigned long start,
+				    unsigned long end,
+				    struct mm_walk *walk)
+{
+	struct migrate_vma *migrate = walk->private;
+	unsigned long addr, next;
+
+	for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE) {
+		migrate->dst[migrate->npages] = 0;
+		migrate->src[migrate->npages++] = 0;
+	}
+
+	return 0;
+}
+
+static int migrate_vma_collect_pmd(pmd_t *pmdp,
+				   unsigned long start,
+				   unsigned long end,
+				   struct mm_walk *walk)
+{
+	struct migrate_vma *migrate = walk->private;
+	struct mm_struct *mm = walk->vma->vm_mm;
+	unsigned long addr = start;
+	spinlock_t *ptl;
+	pte_t *ptep;
+
+	if (pmd_none(*pmdp) || pmd_trans_unstable(pmdp)) {
+		/* FIXME support THP */
+		return migrate_vma_collect_hole(start, end, walk);
+	}
+
+	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
+	for (; addr < end; addr += PAGE_SIZE, ptep++) {
+		unsigned long mpfn, pfn;
+		struct page *page;
+		pte_t pte;
+
+		pte = *ptep;
+		pfn = pte_pfn(pte);
+
+		if (!pte_present(pte)) {
+			mpfn = pfn = 0;
+			goto next;
+		}
+
+		/* FIXME support THP */
+		page = vm_normal_page(migrate->vma, addr, pte);
+		if (!page || !page->mapping || PageTransCompound(page)) {
+			mpfn = pfn = 0;
+			goto next;
+		}
+
+		/*
+		 * By getting a reference on the page we pin it and that blocks
+		 * any kind of migration. Side effect is that it "freezes" the
+		 * pte.
+		 *
+		 * We drop this reference after isolating the page from the lru
+		 * for non device page (device page are not on the lru and thus
+		 * can't be dropped from it).
+		 */
+		get_page(page);
+		migrate->cpages++;
+		mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
+		mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
+
+next:
+		migrate->src[migrate->npages++] = mpfn;
+	}
+	pte_unmap_unlock(ptep - 1, ptl);
+
+	return 0;
+}
+
+/*
+ * migrate_vma_collect() - collect pages over a range of virtual addresses
+ * @migrate: migrate struct containing all migration information
+ *
+ * This will walk the CPU page table. For each virtual address backed by a
+ * valid page, it updates the src array and takes a reference on the page, in
+ * order to pin the page until we lock it and unmap it.
+ */
+static void migrate_vma_collect(struct migrate_vma *migrate)
+{
+	struct mm_walk mm_walk;
+
+	mm_walk.pmd_entry = migrate_vma_collect_pmd;
+	mm_walk.pte_entry = NULL;
+	mm_walk.pte_hole = migrate_vma_collect_hole;
+	mm_walk.hugetlb_entry = NULL;
+	mm_walk.test_walk = NULL;
+	mm_walk.vma = migrate->vma;
+	mm_walk.mm = migrate->vma->vm_mm;
+	mm_walk.private = migrate;
+
+	walk_page_range(migrate->start, migrate->end, &mm_walk);
+
+	migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
+}
+
+/*
+ * migrate_vma_check_page() - check if page is pinned or not
+ * @page: struct page to check
+ *
+ * Pinned pages cannot be migrated. This is the same test as in
+ * migrate_page_move_mapping(), except that here we allow migration of a
+ * ZONE_DEVICE page.
+ */
+static bool migrate_vma_check_page(struct page *page)
+{
+	/*
+	 * One extra ref because caller holds an extra reference, either from
+	 * isolate_lru_page() for a regular page, or migrate_vma_collect() for
+	 * a device page.
+	 */
+	int extra = 1;
+
+	/*
+	 * FIXME support THP (transparent huge page), it is bit more complex to
+	 * check them than regular pages, because they can be mapped with a pmd
+	 * or with a pte (split pte mapping).
+	 */
+	if (PageCompound(page))
+		return false;
+
+	if ((page_count(page) - extra) > page_mapcount(page))
+		return false;
+
+	return true;
+}
+
+/*
+ * migrate_vma_prepare() - lock pages and isolate them from the lru
+ * @migrate: migrate struct containing all migration information
+ *
+ * This locks pages that have been collected by migrate_vma_collect(). Once each
+ * page is locked it is isolated from the lru (for non-device pages). Finally,
+ * the ref taken by migrate_vma_collect() is dropped, as locked pages cannot be
+ * migrated by concurrent kernel threads.
+ */
+static void migrate_vma_prepare(struct migrate_vma *migrate)
+{
+	const unsigned long npages = migrate->npages;
+	const unsigned long start = migrate->start;
+	unsigned long addr, i, restore = 0;
+	bool allow_drain = true;
+
+	lru_add_drain();
+
+	for (i = 0; i < npages; i++) {
+		struct page *page = migrate_pfn_to_page(migrate->src[i]);
+
+		if (!page)
+			continue;
+
+		lock_page(page);
+		migrate->src[i] |= MIGRATE_PFN_LOCKED;
+
+		if (!PageLRU(page) && allow_drain) {
+			/* Drain CPU's pagevec */
+			lru_add_drain_all();
+			allow_drain = false;
+		}
+
+		if (isolate_lru_page(page)) {
+			migrate->src[i] = 0;
+			unlock_page(page);
+			migrate->cpages--;
+			put_page(page);
+			continue;
+		}
+
+		if (!migrate_vma_check_page(page)) {
+			migrate->src[i] = 0;
+			unlock_page(page);
+			migrate->cpages--;
+
+			putback_lru_page(page);
+		}
+	}
+}
+
+/*
+ * migrate_vma_unmap() - replace page mapping with special migration pte entry
+ * @migrate: migrate struct containing all migration information
+ *
+ * Replace page mapping (CPU page table pte) with a special migration pte entry
+ * and check again if it has been pinned. Pinned pages are restored because we
+ * cannot migrate them.
+ *
+ * This is the last step before we call the device driver callback to allocate
+ * destination memory and copy contents of original page over to new page.
+ */
+static void migrate_vma_unmap(struct migrate_vma *migrate)
+{
+	int flags = TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
+	const unsigned long npages = migrate->npages;
+	const unsigned long start = migrate->start;
+	unsigned long addr, i, restore = 0;
+
+	for (i = 0; i < npages; i++) {
+		struct page *page = migrate_pfn_to_page(migrate->src[i]);
+
+		if (!page || !(migrate->src[i] & MIGRATE_PFN_MIGRATE))
+			continue;
+
+		try_to_unmap(page, flags);
+		if (page_mapped(page) || !migrate_vma_check_page(page)) {
+			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+			migrate->cpages--;
+			restore++;
+		}
+	}
+
+	for (addr = start, i = 0; i < npages && restore; addr += PAGE_SIZE, i++) {
+		struct page *page = migrate_pfn_to_page(migrate->src[i]);
+
+		if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
+			continue;
+
+		remove_migration_ptes(page, page, false);
+
+		migrate->src[i] = 0;
+		unlock_page(page);
+		restore--;
+
+		putback_lru_page(page);
+	}
+}
+
+/*
+ * migrate_vma_pages() - migrate meta-data from src page to dst page
+ * @migrate: migrate struct containing all migration information
+ *
+ * This migrates struct page meta-data from source struct page to destination
+ * struct page. This effectively finishes the migration from source page to the
+ * destination page.
+ */
+static void migrate_vma_pages(struct migrate_vma *migrate)
+{
+	const unsigned long npages = migrate->npages;
+	const unsigned long start = migrate->start;
+	unsigned long addr, i;
+
+	for (i = 0, addr = start; i < npages; addr += PAGE_SIZE, i++) {
+		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
+		struct page *page = migrate_pfn_to_page(migrate->src[i]);
+		struct address_space *mapping;
+		int r;
+
+		if (!page || !newpage)
+			continue;
+		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
+			continue;
+
+		mapping = page_mapping(page);
+
+		r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);
+		if (r != MIGRATEPAGE_SUCCESS)
+			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+	}
+}
+
+/*
+ * migrate_vma_finalize() - restore CPU page table entry
+ * @migrate: migrate struct containing all migration information
+ *
+ * This replaces the special migration pte entry with either a mapping to the
+ * new page if migration was successful for that page, or to the original page
+ * otherwise.
+ *
+ * This also unlocks the pages and puts them back on the lru, or drops the extra
+ * refcount, for device pages.
+ */
+static void migrate_vma_finalize(struct migrate_vma *migrate)
+{
+	const unsigned long npages = migrate->npages;
+	unsigned long i;
+
+	for (i = 0; i < npages; i++) {
+		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
+		struct page *page = migrate_pfn_to_page(migrate->src[i]);
+
+		if (!page)
+			continue;
+		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE) || !newpage) {
+			if (newpage) {
+				unlock_page(newpage);
+				put_page(newpage);
+			}
+			newpage = page;
+		}
+
+		remove_migration_ptes(page, newpage, false);
+		unlock_page(page);
+		migrate->cpages--;
+
+		putback_lru_page(page);
+
+		if (newpage != page) {
+			unlock_page(newpage);
+			putback_lru_page(newpage);
+		}
+	}
+}
+
+/*
+ * migrate_vma() - migrate a range of memory inside vma
+ *
+ * @ops: migration callback for allocating destination memory and copying
+ * @vma: virtual memory area containing the range to be migrated
+ * @start: start address of the range to migrate (inclusive)
+ * @end: end address of the range to migrate (exclusive)
+ * @src: array of hmm_pfn_t containing source pfns
+ * @dst: array of hmm_pfn_t containing destination pfns
+ * @private: pointer passed back to each of the callback
+ * Returns: 0 on success, error code otherwise
+ *
+ * This function tries to migrate a range of memory virtual address range, using
+ * callbacks to allocate and copy memory from source to destination. First it
+ * collects all the pages backing each virtual address in the range, saving this
+ * inside the src array. Then it locks those pages and unmaps them. Once the pages
+ * are locked and unmapped, it checks whether each page is pinned or not. Pages
+ * that aren't pinned have the MIGRATE_PFN_MIGRATE flag set (by this function)
+ * in the corresponding src array entry. It then restores any pages that are
+ * pinned, by remapping and unlocking those pages.
+ *
+ * At this point it calls the alloc_and_copy() callback. For documentation on
+ * what is expected from that callback, see struct migrate_vma_ops comments in
+ * include/linux/migrate.h
+ *
+ * After the alloc_and_copy() callback, this function goes over each entry in
+ * the src array that has the MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag
+ * set. If the corresponding entry in dst array has MIGRATE_PFN_VALID flag set,
+ * then the function tries to migrate struct page information from the source
+ * struct page to the destination struct page. If it fails to migrate the struct
+ * page information, then it clears the MIGRATE_PFN_MIGRATE flag in the src
+ * array.
+ *
+ * At this point all successfully migrated pages have an entry in the src
+ * array with MIGRATE_PFN_VALID and MIGRATE_PFN_MIGRATE flag set and the dst
+ * array entry with MIGRATE_PFN_VALID flag set.
+ *
+ * It then calls the finalize_and_map() callback. See comments for "struct
+ * migrate_vma_ops", in include/linux/migrate.h for details about
+ * finalize_and_map() behavior.
+ *
+ * After the finalize_and_map() callback, for successfully migrated pages, this
+ * function updates the CPU page table to point to new pages, otherwise it
+ * restores the CPU page table to point to the original source pages.
+ *
+ * Function returns 0 after the above steps, even if no pages were migrated
+ * (The function only returns an error if any of the arguments are invalid.)
+ *
+ * Both src and dst array must be big enough for (end - start) >> PAGE_SHIFT
+ * unsigned long entries.
+ */
+int migrate_vma(const struct migrate_vma_ops *ops,
+		struct vm_area_struct *vma,
+		unsigned long start,
+		unsigned long end,
+		unsigned long *src,
+		unsigned long *dst,
+		void *private)
+{
+	struct migrate_vma migrate;
+
+	/* Sanity check the arguments */
+	start &= PAGE_MASK;
+	end &= PAGE_MASK;
+	if (!vma || is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL))
+		return -EINVAL;
+	if (start < vma->vm_start || start >= vma->vm_end)
+		return -EINVAL;
+	if (end <= vma->vm_start || end > vma->vm_end)
+		return -EINVAL;
+	if (!ops || !src || !dst || start >= end)
+		return -EINVAL;
+
+	memset(src, 0, sizeof(*src) * ((end - start) >> PAGE_SHIFT));
+	migrate.src = src;
+	migrate.dst = dst;
+	migrate.start = start;
+	migrate.npages = 0;
+	migrate.cpages = 0;
+	migrate.end = end;
+	migrate.vma = vma;
+
+	/* Collect, and try to unmap source pages */
+	migrate_vma_collect(&migrate);
+	if (!migrate.cpages)
+		return 0;
+
+	/* Lock and isolate page */
+	migrate_vma_prepare(&migrate);
+	if (!migrate.cpages)
+		return 0;
+
+	/* Unmap pages */
+	migrate_vma_unmap(&migrate);
+	if (!migrate.cpages)
+		return 0;
+
+	/*
+	 * At this point pages are locked and unmapped, and thus they have
+	 * stable content and can safely be copied to destination memory that
+	 * is allocated by the callback.
+	 *
+	 * Note that migration can fail in migrate_vma_struct_page() for each
+	 * individual page.
+	 */
+	ops->alloc_and_copy(vma, src, dst, start, end, private);
+
+	/* This does the real migration of struct page */
+	migrate_vma_pages(&migrate);
+
+	ops->finalize_and_map(vma, src, dst, start, end, private);
+
+	/* Unlock and remap pages */
+	migrate_vma_finalize(&migrate);
+
+	return 0;
+}
+EXPORT_SYMBOL(migrate_vma);
-- 
2.9.3

[HMM 06/15] mm/migrate: migrate_vma() unmap page from vma while collecting pages

[Jérôme Glisse]

Common case for migration of virtual address range is page are map
only once inside the vma in which migration is taking place. Because
we already walk the CPU page table for that range we can directly do
the unmap there and setup special migration swap entry.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 mm/migrate.c | 114 ++++++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 98 insertions(+), 16 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 452f894..4ac2a7a 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2118,7 +2118,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 {
 	struct migrate_vma *migrate = walk->private;
 	struct mm_struct *mm = walk->vma->vm_mm;
-	unsigned long addr = start;
+	unsigned long addr = start, unmapped = 0;
 	spinlock_t *ptl;
 	pte_t *ptep;
 
@@ -2128,9 +2128,12 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 	}
 
 	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
+	arch_enter_lazy_mmu_mode();
+
 	for (; addr < end; addr += PAGE_SIZE, ptep++) {
 		unsigned long mpfn, pfn;
 		struct page *page;
+		swp_entry_t entry;
 		pte_t pte;
 
 		pte = *ptep;
@@ -2162,11 +2165,44 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
 		mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
 
+		/*
+		 * Optimize for the common case where page is only mapped once
+		 * in one process. If we can lock the page, then we can safely
+		 * set up a special migration page table entry now.
+		 */
+		if (trylock_page(page)) {
+			pte_t swp_pte;
+
+			mpfn |= MIGRATE_PFN_LOCKED;
+			ptep_get_and_clear(mm, addr, ptep);
+
+			/* Setup special migration page table entry */
+			entry = make_migration_entry(page, pte_write(pte));
+			swp_pte = swp_entry_to_pte(entry);
+			if (pte_soft_dirty(pte))
+				swp_pte = pte_swp_mksoft_dirty(swp_pte);
+			set_pte_at(mm, addr, ptep, swp_pte);
+
+			/*
+			 * This is like regular unmap: we remove the rmap and
+			 * drop page refcount. Page won't be freed, as we took
+			 * a reference just above.
+			 */
+			page_remove_rmap(page, false);
+			put_page(page);
+			unmapped++;
+		}
+
 next:
 		migrate->src[migrate->npages++] = mpfn;
 	}
+	arch_leave_lazy_mmu_mode();
 	pte_unmap_unlock(ptep - 1, ptl);
 
+	/* Only flush the TLB if we actually modified any entries */
+	if (unmapped)
+		flush_tlb_range(walk->vma, start, end);
+
 	return 0;
 }
 
@@ -2191,7 +2227,13 @@ static void migrate_vma_collect(struct migrate_vma *migrate)
 	mm_walk.mm = migrate->vma->vm_mm;
 	mm_walk.private = migrate;
 
+	mmu_notifier_invalidate_range_start(mm_walk.mm,
+					    migrate->start,
+					    migrate->end);
 	walk_page_range(migrate->start, migrate->end, &mm_walk);
+	mmu_notifier_invalidate_range_end(mm_walk.mm,
+					  migrate->start,
+					  migrate->end);
 
 	migrate->end = migrate->start + (migrate->npages << PAGE_SHIFT);
 }
@@ -2247,12 +2289,16 @@ static void migrate_vma_prepare(struct migrate_vma *migrate)
 
 	for (i = 0; i < npages; i++) {
 		struct page *page = migrate_pfn_to_page(migrate->src[i]);
+		bool remap = true;
 
 		if (!page)
 			continue;
 
-		lock_page(page);
-		migrate->src[i] |= MIGRATE_PFN_LOCKED;
+		if (!(migrate->src[i] & MIGRATE_PFN_LOCKED)) {
+			remap = false;
+			lock_page(page);
+			migrate->src[i] |= MIGRATE_PFN_LOCKED;
+		}
 
 		if (!PageLRU(page) && allow_drain) {
 			/* Drain CPU's pagevec */
@@ -2261,21 +2307,50 @@ static void migrate_vma_prepare(struct migrate_vma *migrate)
 		}
 
 		if (isolate_lru_page(page)) {
-			migrate->src[i] = 0;
-			unlock_page(page);
-			migrate->cpages--;
-			put_page(page);
+			if (remap) {
+				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+				migrate->cpages--;
+				restore++;
+			} else {
+				migrate->src[i] = 0;
+				unlock_page(page);
+				migrate->cpages--;
+				put_page(page);
+			}
 			continue;
 		}
 
 		if (!migrate_vma_check_page(page)) {
-			migrate->src[i] = 0;
-			unlock_page(page);
-			migrate->cpages--;
+			if (remap) {
+				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+				migrate->cpages--;
+				restore++;
 
-			putback_lru_page(page);
+				get_page(page);
+				putback_lru_page(page);
+			} else {
+				migrate->src[i] = 0;
+				unlock_page(page);
+				migrate->cpages--;
+
+				putback_lru_page(page);
+			}
 		}
 	}
+
+	for (i = 0, addr = start; i < npages && restore; i++, addr += PAGE_SIZE) {
+		struct page *page = migrate_pfn_to_page(migrate->src[i]);
+
+		if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
+			continue;
+
+		remove_migration_pte(page, migrate->vma, addr, page);
+
+		migrate->src[i] = 0;
+		unlock_page(page);
+		put_page(page);
+		restore--;
+	}
 }
 
 /*
@@ -2302,12 +2377,19 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
 		if (!page || !(migrate->src[i] & MIGRATE_PFN_MIGRATE))
 			continue;
 
-		try_to_unmap(page, flags);
-		if (page_mapped(page) || !migrate_vma_check_page(page)) {
-			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
-			migrate->cpages--;
-			restore++;
+		if (page_mapped(page)) {
+			try_to_unmap(page, flags);
+			if (page_mapped(page))
+				goto restore;
 		}
+
+		if (migrate_vma_check_page(page))
+			continue;
+
+restore:
+		migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+		migrate->cpages--;
+		restore++;
 	}
 
 	for (addr = start, i = 0; i < npages && restore; addr += PAGE_SIZE, i++) {
-- 
2.9.3

[HMM 07/15] mm/hmm: heterogeneous memory management (HMM for short) v2

[Jérôme Glisse]

HMM provides 3 separate types of functionality:
    - Mirroring: synchronize CPU page table and device page table
    - Device memory: allocating struct page for device memory
    - Migration: migrating regular memory to device memory

This patch introduces some common helpers and definitions to all of
those 3 functionality.

Changed since v1:
  - Kconfig logic (depend on x86-64 and use ARCH_HAS pattern)

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 MAINTAINERS              |   7 +++
 include/linux/hmm.h      | 146 +++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/mm_types.h |   5 ++
 kernel/fork.c            |   2 +
 mm/Kconfig               |  13 +++++
 mm/Makefile              |   1 +
 mm/hmm.c                 |  71 +++++++++++++++++++++++
 7 files changed, 245 insertions(+)
 create mode 100644 include/linux/hmm.h
 create mode 100644 mm/hmm.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 09341a9..6df21a9 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6052,6 +6052,13 @@ S:	Supported
 F:	drivers/scsi/hisi_sas/
 F:	Documentation/devicetree/bindings/scsi/hisilicon-sas.txt
 
+HMM - Heterogeneous Memory Management
+M:	Jérôme Glisse <jglisse@redhat.com>
+L:	linux-mm@kvack.org
+S:	Maintained
+F:	mm/hmm*
+F:	include/linux/hmm*
+
 HOST AP DRIVER
 M:	Jouni Malinen <j@w1.fi>
 L:	linux-wireless@vger.kernel.org
diff --git a/include/linux/hmm.h b/include/linux/hmm.h
new file mode 100644
index 0000000..93b363d
--- /dev/null
+++ b/include/linux/hmm.h
@@ -0,0 +1,146 @@
+/*
+ * Copyright 2013 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * Authors: Jérôme Glisse <jglisse@redhat.com>
+ */
+/*
+ * Heterogeneous Memory Management (HMM)
+ *
+ * See Documentation/vm/hmm.txt for reasons and overview of what HMM is and it
+ * is for. Here we focus on the HMM API description, with some explanation of
+ * the underlying implementation.
+ *
+ * Short description: HMM provides a set of helpers to share a virtual address
+ * space between CPU and a device, so that the device can access any valid
+ * address of the process (while still obeying memory protection). HMM also
+ * provides helpers to migrate process memory to device memory, and back. Each
+ * set of functionality (address space mirroring, and migration to and from
+ * device memory) can be used independently of the other.
+ *
+ *
+ * HMM address space mirroring API:
+ *
+ * Use HMM address space mirroring if you want to mirror range of the CPU page
+ * table of a process into a device page table. Here, "mirror" means "keep
+ * synchronized". Prerequisites: the device must provide the ability to write-
+ * protect its page tables (at PAGE_SIZE granularity), and must be able to
+ * recover from the resulting potential page faults.
+ *
+ * HMM guarantees that at any point in time, a given virtual address points to
+ * either the same memory in both CPU and device page tables (that is: CPU and
+ * device page tables each point to the same pages), or that one page table (CPU
+ * or device) points to no entry, while the other still points to the old page
+ * for the address. The latter case happens when the CPU page table update
+ * happens first, and then the update is mirrored over to the device page table.
+ * This does not cause any issue, because the CPU page table cannot start
+ * pointing to a new page until the device page table is invalidated.
+ *
+ * HMM uses mmu_notifiers to monitor the CPU page tables, and forwards any
+ * updates to each device driver that has registered a mirror. It also provides
+ * some API calls to help with taking a snapshot of the CPU page table, and to
+ * synchronize with any updates that might happen concurrently.
+ *
+ *
+ * HMM migration to and from device memory:
+ *
+ * HMM provides a set of helpers to hotplug device memory as ZONE_DEVICE, with
+ * a new MEMORY_DEVICE_UNADDRESSABLE type. This provides a struct page for
+ * each page of the device memory, and allows the device driver to manage its
+ * memory using those struct pages. Having struct pages for device memory makes
+ * migration easier. Because that memory is not addressable by the CPU it must
+ * never be pinned to the device; in other words, any CPU page fault can always
+ * cause the device memory to be migrated (copied/moved) back to regular memory.
+ *
+ * A new migrate helper (migrate_vma()) has been added (see mm/migrate.c) that
+ * allows use of a device DMA engine to perform the copy operation between
+ * regular system memory and device memory.
+ */
+#ifndef LINUX_HMM_H
+#define LINUX_HMM_H
+
+#include <linux/kconfig.h>
+
+#if IS_ENABLED(CONFIG_HMM)
+
+
+/*
+ * hmm_pfn_t - HMM uses its own pfn type to keep several flags per page
+ *
+ * Flags:
+ * HMM_PFN_VALID: pfn is valid
+ * HMM_PFN_WRITE: CPU page table has write permission set
+ */
+typedef unsigned long hmm_pfn_t;
+
+#define HMM_PFN_VALID (1 << 0)
+#define HMM_PFN_WRITE (1 << 1)
+#define HMM_PFN_SHIFT 2
+
+/*
+ * hmm_pfn_t_to_page() - return struct page pointed to by a valid hmm_pfn_t
+ * @pfn: hmm_pfn_t to convert to struct page
+ * Returns: struct page pointer if pfn is a valid hmm_pfn_t, NULL otherwise
+ *
+ * If the hmm_pfn_t is valid (ie valid flag set) then return the struct page
+ * matching the pfn value stored in the hmm_pfn_t. Otherwise return NULL.
+ */
+static inline struct page *hmm_pfn_t_to_page(hmm_pfn_t pfn)
+{
+	if (!(pfn & HMM_PFN_VALID))
+		return NULL;
+	return pfn_to_page(pfn >> HMM_PFN_SHIFT);
+}
+
+/*
+ * hmm_pfn_t_to_pfn() - return pfn value store in a hmm_pfn_t
+ * @pfn: hmm_pfn_t to extract pfn from
+ * Returns: pfn value if hmm_pfn_t is valid, -1UL otherwise
+ */
+static inline unsigned long hmm_pfn_t_to_pfn(hmm_pfn_t pfn)
+{
+	if (!(pfn & HMM_PFN_VALID))
+		return -1UL;
+	return (pfn >> HMM_PFN_SHIFT);
+}
+
+/*
+ * hmm_pfn_t_from_page() - create a valid hmm_pfn_t value from struct page
+ * @page: struct page pointer for which to create the hmm_pfn_t
+ * Returns: valid hmm_pfn_t for the page
+ */
+static inline hmm_pfn_t hmm_pfn_t_from_page(struct page *page)
+{
+	return (page_to_pfn(page) << HMM_PFN_SHIFT) | HMM_PFN_VALID;
+}
+
+/*
+ * hmm_pfn_t_from_pfn() - create a valid hmm_pfn_t value from pfn
+ * @pfn: pfn value for which to create the hmm_pfn_t
+ * Returns: valid hmm_pfn_t for the pfn
+ */
+static inline hmm_pfn_t hmm_pfn_t_from_pfn(unsigned long pfn)
+{
+	return (pfn << HMM_PFN_SHIFT) | HMM_PFN_VALID;
+}
+
+
+/* Below are for HMM internal use only! Not to be used by device driver! */
+void hmm_mm_destroy(struct mm_struct *mm);
+
+#else /* IS_ENABLED(CONFIG_HMM) */
+
+/* Below are for HMM internal use only! Not to be used by device driver! */
+static inline void hmm_mm_destroy(struct mm_struct *mm) {}
+
+#endif /* IS_ENABLED(CONFIG_HMM) */
+#endif /* LINUX_HMM_H */
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 45cdb27..5ddeacc7 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -23,6 +23,7 @@
 
 struct address_space;
 struct mem_cgroup;
+struct hmm;
 
 /*
  * Each physical page in the system has a struct page associated with
@@ -500,6 +501,10 @@ struct mm_struct {
 	atomic_long_t hugetlb_usage;
 #endif
 	struct work_struct async_put_work;
+#if IS_ENABLED(CONFIG_HMM)
+	/* HMM needs to track a few things per mm */
+	struct hmm *hmm;
+#endif
 };
 
 extern struct mm_struct init_mm;
diff --git a/kernel/fork.c b/kernel/fork.c
index 81347bd..65a3e94f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -37,6 +37,7 @@
 #include <linux/binfmts.h>
 #include <linux/mman.h>
 #include <linux/mmu_notifier.h>
+#include <linux/hmm.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/vmacache.h>
@@ -891,6 +892,7 @@ void __mmdrop(struct mm_struct *mm)
 	BUG_ON(mm == &init_mm);
 	mm_free_pgd(mm);
 	destroy_context(mm);
+	hmm_mm_destroy(mm);
 	mmu_notifier_mm_destroy(mm);
 	check_mm(mm);
 	put_user_ns(mm->user_ns);
diff --git a/mm/Kconfig b/mm/Kconfig
index ad4bd50..90025b3 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -289,6 +289,19 @@ config MIGRATION
 config ARCH_ENABLE_HUGEPAGE_MIGRATION
 	bool
 
+config ARCH_HAS_HMM
+	bool
+	default y
+	depends on X86_64
+	depends on ZONE_DEVICE
+	depends on MMU && 64BIT
+	depends on MEMORY_HOTPLUG
+	depends on MEMORY_HOTREMOVE
+	depends on SPARSEMEM_VMEMMAP
+
+config HMM
+	bool
+
 config PHYS_ADDR_T_64BIT
 	def_bool 64BIT || ARCH_PHYS_ADDR_T_64BIT
 
diff --git a/mm/Makefile b/mm/Makefile
index 026f6a8..9eb4121 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -75,6 +75,7 @@ obj-$(CONFIG_FAILSLAB) += failslab.o
 obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
 obj-$(CONFIG_MEMTEST)		+= memtest.o
 obj-$(CONFIG_MIGRATION) += migrate.o
+obj-$(CONFIG_HMM) += hmm.o
 obj-$(CONFIG_QUICKLIST) += quicklist.o
 obj-$(CONFIG_TRANSPARENT_HUGEPAGE) += huge_memory.o khugepaged.o
 obj-$(CONFIG_PAGE_COUNTER) += page_counter.o
diff --git a/mm/hmm.c b/mm/hmm.c
new file mode 100644
index 0000000..acadb49
--- /dev/null
+++ b/mm/hmm.c
@@ -0,0 +1,71 @@
+/*
+ * Copyright 2013 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * Authors: Jérôme Glisse <jglisse@redhat.com>
+ */
+/*
+ * Refer to include/linux/hmm.h for information about heterogeneous memory
+ * management or HMM for short.
+ */
+#include <linux/mm.h>
+#include <linux/hmm.h>
+#include <linux/slab.h>
+#include <linux/sched.h>
+
+/*
+ * struct hmm - HMM per mm struct
+ *
+ * @mm: mm struct this HMM struct is bound to
+ */
+struct hmm {
+	struct mm_struct	*mm;
+};
+
+/*
+ * hmm_register - register HMM against an mm (HMM internal)
+ *
+ * @mm: mm struct to attach to
+ *
+ * This is not intended to be used directly by device drivers. It allocates an
+ * HMM struct if mm does not have one, and initializes it.
+ */
+static struct hmm *hmm_register(struct mm_struct *mm)
+{
+	if (!mm->hmm) {
+		struct hmm *hmm = NULL;
+
+		hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
+		if (!hmm)
+			return NULL;
+		hmm->mm = mm;
+
+		spin_lock(&mm->page_table_lock);
+		if (!mm->hmm)
+			mm->hmm = hmm;
+		else
+			kfree(hmm);
+		spin_unlock(&mm->page_table_lock);
+	}
+
+	/*
+	 * The hmm struct can only be freed once the mm_struct goes away,
+	 * hence we should always have pre-allocated an new hmm struct
+	 * above.
+	 */
+	return mm->hmm;
+}
+
+void hmm_mm_destroy(struct mm_struct *mm)
+{
+	kfree(mm->hmm);
+}
-- 
2.9.3

[HMM 08/15] mm/hmm/mirror: mirror process address space on device with HMM helpers v2

[Jérôme Glisse]

This is a heterogeneous memory management (HMM) process address space
mirroring. In a nutshell this provide an API to mirror process address
space on a device. This boils down to keeping CPU and device page table
synchronize (we assume that both device and CPU are cache coherent like
PCIe device can be).

This patch provide a simple API for device driver to achieve address
space mirroring thus avoiding each device driver to grow its own CPU
page table walker and its own CPU page table synchronization mechanism.

This is useful for NVidia GPU >= Pascal, Mellanox IB >= mlx5 and more
hardware in the future.

Changed since v1:
  - Kconfig logic (depend on x86-64 and use ARCH_HAS pattern)

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 include/linux/hmm.h | 110 ++++++++++++++++++++++++++++++++++
 mm/Kconfig          |  12 ++++
 mm/hmm.c            | 170 +++++++++++++++++++++++++++++++++++++++++++++++-----
 3 files changed, 277 insertions(+), 15 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 93b363d..6668a1b 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -72,6 +72,7 @@
 
 #if IS_ENABLED(CONFIG_HMM)
 
+struct hmm;
 
 /*
  * hmm_pfn_t - HMM uses its own pfn type to keep several flags per page
@@ -134,6 +135,115 @@ static inline hmm_pfn_t hmm_pfn_t_from_pfn(unsigned long pfn)
 }
 
 
+#if IS_ENABLED(CONFIG_HMM_MIRROR)
+/*
+ * Mirroring: how to synchronize device page table with CPU page table.
+ *
+ * A device driver that is participating in HMM mirroring must always
+ * synchronize with CPU page table updates. For this, device drivers can either
+ * directly use mmu_notifier APIs or they can use the hmm_mirror API. Device
+ * drivers can decide to register one mirror per device per process, or just
+ * one mirror per process for a group of devices. The pattern is:
+ *
+ *      int device_bind_address_space(..., struct mm_struct *mm, ...)
+ *      {
+ *          struct device_address_space *das;
+ *
+ *          // Device driver specific initialization, and allocation of das
+ *          // which contains an hmm_mirror struct as one of its fields.
+ *          ...
+ *
+ *          ret = hmm_mirror_register(&das->mirror, mm, &device_mirror_ops);
+ *          if (ret) {
+ *              // Cleanup on error
+ *              return ret;
+ *          }
+ *
+ *          // Other device driver specific initialization
+ *          ...
+ *      }
+ *
+ * Once an hmm_mirror is registered for an address space, the device driver
+ * will get callbacks through sync_cpu_device_pagetables() operation (see
+ * hmm_mirror_ops struct).
+ *
+ * Device driver must not free the struct containing the hmm_mirror struct
+ * before calling hmm_mirror_unregister(). The expected usage is to do that when
+ * the device driver is unbinding from an address space.
+ *
+ *
+ *      void device_unbind_address_space(struct device_address_space *das)
+ *      {
+ *          // Device driver specific cleanup
+ *          ...
+ *
+ *          hmm_mirror_unregister(&das->mirror);
+ *
+ *          // Other device driver specific cleanup, and now das can be freed
+ *          ...
+ *      }
+ */
+
+struct hmm_mirror;
+
+/*
+ * enum hmm_update_type - type of update
+ * @HMM_UPDATE_INVALIDATE: invalidate range (no indication as to why)
+ */
+enum hmm_update_type {
+	HMM_UPDATE_INVALIDATE,
+};
+
+/*
+ * struct hmm_mirror_ops - HMM mirror device operations callback
+ *
+ * @update: callback to update range on a device
+ */
+struct hmm_mirror_ops {
+	/* sync_cpu_device_pagetables() - synchronize page tables
+	 *
+	 * @mirror: pointer to struct hmm_mirror
+	 * @update_type: type of update that occurred to the CPU page table
+	 * @start: virtual start address of the range to update
+	 * @end: virtual end address of the range to update
+	 *
+	 * This callback ultimately originates from mmu_notifiers when the CPU
+	 * page table is updated. The device driver must update its page table
+	 * in response to this callback. The update argument tells what action
+	 * to perform.
+	 *
+	 * The device driver must not return from this callback until the device
+	 * page tables are completely updated (TLBs flushed, etc); this is a
+	 * synchronous call.
+	 */
+	void (*sync_cpu_device_pagetables)(struct hmm_mirror *mirror,
+					   enum hmm_update_type update_type,
+					   unsigned long start,
+					   unsigned long end);
+};
+
+/*
+ * struct hmm_mirror - mirror struct for a device driver
+ *
+ * @hmm: pointer to struct hmm (which is unique per mm_struct)
+ * @ops: device driver callback for HMM mirror operations
+ * @list: for list of mirrors of a given mm
+ *
+ * Each address space (mm_struct) being mirrored by a device must register one
+ * instance of an hmm_mirror struct with HMM. HMM will track the list of all
+ * mirrors for each mm_struct.
+ */
+struct hmm_mirror {
+	struct hmm			*hmm;
+	const struct hmm_mirror_ops	*ops;
+	struct list_head		list;
+};
+
+int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm);
+void hmm_mirror_unregister(struct hmm_mirror *mirror);
+#endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
+
+
 /* Below are for HMM internal use only! Not to be used by device driver! */
 void hmm_mm_destroy(struct mm_struct *mm);
 
diff --git a/mm/Kconfig b/mm/Kconfig
index 90025b3..3c1ffb1 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -302,6 +302,18 @@ config ARCH_HAS_HMM
 config HMM
 	bool
 
+config HMM_MIRROR
+	bool "HMM mirror CPU page table into a device page table"
+	depends on ARCH_HAS_HMM
+	select MMU_NOTIFIER
+	select HMM
+	help
+	  Select HMM_MIRROR if you want to mirror range of the CPU page table of a
+	  process into a device page table. Here, mirror means "keep synchronized".
+	  Prerequisites: the device must provide the ability to write-protect its
+	  page tables (at PAGE_SIZE granularity), and must be able to recover from
+	  the resulting potential page faults.
+
 config PHYS_ADDR_T_64BIT
 	def_bool 64BIT || ARCH_PHYS_ADDR_T_64BIT
 
diff --git a/mm/hmm.c b/mm/hmm.c
index acadb49..7ed4b4c 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -21,14 +21,26 @@
 #include <linux/hmm.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
+#include <linux/mmu_notifier.h>
+
+static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
+
 
 /*
  * struct hmm - HMM per mm struct
  *
  * @mm: mm struct this HMM struct is bound to
+ * @sequence: we track updates to the CPU page table with a sequence number
+ * @mirrors: list of mirrors for this mm
+ * @mmu_notifier: mmu notifier to track updates to CPU page table
+ * @mirrors_sem: read/write semaphore protecting the mirrors list
  */
 struct hmm {
 	struct mm_struct	*mm;
+	atomic_t		sequence;
+	struct list_head	mirrors;
+	struct mmu_notifier	mmu_notifier;
+	struct rw_semaphore	mirrors_sem;
 };
 
 /*
@@ -41,27 +53,48 @@ struct hmm {
  */
 static struct hmm *hmm_register(struct mm_struct *mm)
 {
-	if (!mm->hmm) {
-		struct hmm *hmm = NULL;
-
-		hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
-		if (!hmm)
-			return NULL;
-		hmm->mm = mm;
-
-		spin_lock(&mm->page_table_lock);
-		if (!mm->hmm)
-			mm->hmm = hmm;
-		else
-			kfree(hmm);
-		spin_unlock(&mm->page_table_lock);
-	}
+	struct hmm *hmm = READ_ONCE(mm->hmm);
+	bool cleanup = false;
 
 	/*
 	 * The hmm struct can only be freed once the mm_struct goes away,
 	 * hence we should always have pre-allocated an new hmm struct
 	 * above.
 	 */
+	if (hmm)
+		return hmm;
+
+	hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
+	if (!hmm)
+		return NULL;
+	INIT_LIST_HEAD(&hmm->mirrors);
+	init_rwsem(&hmm->mirrors_sem);
+	atomic_set(&hmm->sequence, 0);
+	hmm->mmu_notifier.ops = NULL;
+	hmm->mm = mm;
+
+	/*
+	 * We should only get here if hold the mmap_sem in write mode ie on
+	 * registration of first mirror through hmm_mirror_register()
+	 */
+	hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
+	if (__mmu_notifier_register(&hmm->mmu_notifier, mm)) {
+		kfree(hmm);
+		return NULL;
+	}
+
+	spin_lock(&mm->page_table_lock);
+	if (!mm->hmm)
+		mm->hmm = hmm;
+	else
+		cleanup = true;
+	spin_unlock(&mm->page_table_lock);
+
+	if (cleanup) {
+		mmu_notifier_unregister(&hmm->mmu_notifier, mm);
+		kfree(hmm);
+	}
+
 	return mm->hmm;
 }
 
@@ -69,3 +102,110 @@ void hmm_mm_destroy(struct mm_struct *mm)
 {
 	kfree(mm->hmm);
 }
+
+
+#if IS_ENABLED(CONFIG_HMM_MIRROR)
+static void hmm_invalidate_range(struct hmm *hmm,
+				 enum hmm_update_type action,
+				 unsigned long start,
+				 unsigned long end)
+{
+	struct hmm_mirror *mirror;
+
+	down_read(&hmm->mirrors_sem);
+	list_for_each_entry(mirror, &hmm->mirrors, list)
+		mirror->ops->sync_cpu_device_pagetables(mirror, action,
+							start, end);
+	up_read(&hmm->mirrors_sem);
+}
+
+static void hmm_invalidate_page(struct mmu_notifier *mn,
+				struct mm_struct *mm,
+				unsigned long addr)
+{
+	unsigned long start = addr & PAGE_MASK;
+	unsigned long end = start + PAGE_SIZE;
+	struct hmm *hmm = mm->hmm;
+
+	VM_BUG_ON(!hmm);
+
+	atomic_inc(&hmm->sequence);
+	hmm_invalidate_range(mm->hmm, HMM_UPDATE_INVALIDATE, start, end);
+}
+
+static void hmm_invalidate_range_start(struct mmu_notifier *mn,
+				       struct mm_struct *mm,
+				       unsigned long start,
+				       unsigned long end)
+{
+	struct hmm *hmm = mm->hmm;
+
+	VM_BUG_ON(!hmm);
+
+	atomic_inc(&hmm->sequence);
+}
+
+static void hmm_invalidate_range_end(struct mmu_notifier *mn,
+				     struct mm_struct *mm,
+				     unsigned long start,
+				     unsigned long end)
+{
+	struct hmm *hmm = mm->hmm;
+
+	VM_BUG_ON(!hmm);
+
+	hmm_invalidate_range(mm->hmm, HMM_UPDATE_INVALIDATE, start, end);
+}
+
+static const struct mmu_notifier_ops hmm_mmu_notifier_ops = {
+	.invalidate_page	= hmm_invalidate_page,
+	.invalidate_range_start	= hmm_invalidate_range_start,
+	.invalidate_range_end	= hmm_invalidate_range_end,
+};
+
+/*
+ * hmm_mirror_register() - register a mirror against an mm
+ *
+ * @mirror: new mirror struct to register
+ * @mm: mm to register against
+ *
+ * To start mirroring a process address space, the device driver must register
+ * an HMM mirror struct.
+ *
+ * THE mm->mmap_sem MUST BE HELD IN WRITE MODE !
+ */
+int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
+{
+	/* Sanity check */
+	if (!mm || !mirror || !mirror->ops)
+		return -EINVAL;
+
+	mirror->hmm = hmm_register(mm);
+	if (!mirror->hmm)
+		return -ENOMEM;
+
+	down_write(&mirror->hmm->mirrors_sem);
+	list_add(&mirror->list, &mirror->hmm->mirrors);
+	up_write(&mirror->hmm->mirrors_sem);
+
+	return 0;
+}
+EXPORT_SYMBOL(hmm_mirror_register);
+
+/*
+ * hmm_mirror_unregister() - unregister a mirror
+ *
+ * @mirror: new mirror struct to register
+ *
+ * Stop mirroring a process address space, and cleanup.
+ */
+void hmm_mirror_unregister(struct hmm_mirror *mirror)
+{
+	struct hmm *hmm = mirror->hmm;
+
+	down_write(&hmm->mirrors_sem);
+	list_del(&mirror->list);
+	up_write(&hmm->mirrors_sem);
+}
+EXPORT_SYMBOL(hmm_mirror_unregister);
+#endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
-- 
2.9.3

[HMM 09/15] mm/hmm/mirror: helper to snapshot CPU page table v2

[Jérôme Glisse]

This does not use existing page table walker because we want to share
same code for our page fault handler.

Changes since v1:
  - Use spinlock instead of rcu synchronized list traversal

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 include/linux/hmm.h |  55 +++++++++-
 mm/hmm.c            | 285 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 338 insertions(+), 2 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 6668a1b..defa7cd 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -79,13 +79,26 @@ struct hmm;
  *
  * Flags:
  * HMM_PFN_VALID: pfn is valid
+ * HMM_PFN_READ:  CPU page table has read permission set
  * HMM_PFN_WRITE: CPU page table has write permission set
+ * HMM_PFN_ERROR: corresponding CPU page table entry points to poisoned memory
+ * HMM_PFN_EMPTY: corresponding CPU page table entry is pte_none()
+ * HMM_PFN_SPECIAL: corresponding CPU page table entry is special; i.e., the
+ *      result of vm_insert_pfn() or vm_insert_page(). Therefore, it should not
+ *      be mirrored by a device, because the entry will never have HMM_PFN_VALID
+ *      set and the pfn value is undefined.
+ * HMM_PFN_DEVICE_UNADDRESSABLE: unaddressable device memory (ZONE_DEVICE)
  */
 typedef unsigned long hmm_pfn_t;
 
 #define HMM_PFN_VALID (1 << 0)
-#define HMM_PFN_WRITE (1 << 1)
-#define HMM_PFN_SHIFT 2
+#define HMM_PFN_READ (1 << 1)
+#define HMM_PFN_WRITE (1 << 2)
+#define HMM_PFN_ERROR (1 << 3)
+#define HMM_PFN_EMPTY (1 << 4)
+#define HMM_PFN_SPECIAL (1 << 5)
+#define HMM_PFN_DEVICE_UNADDRESSABLE (1 << 6)
+#define HMM_PFN_SHIFT 7
 
 /*
  * hmm_pfn_t_to_page() - return struct page pointed to by a valid hmm_pfn_t
@@ -241,6 +254,44 @@ struct hmm_mirror {
 
 int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm);
 void hmm_mirror_unregister(struct hmm_mirror *mirror);
+
+
+/*
+ * struct hmm_range - track invalidation lock on virtual address range
+ *
+ * @list: all range lock are on a list
+ * @start: range virtual start address (inclusive)
+ * @end: range virtual end address (exclusive)
+ * @pfns: array of pfns (big enough for the range)
+ * @valid: pfns array did not change since it has been fill by an HMM function
+ */
+struct hmm_range {
+	struct list_head	list;
+	unsigned long		start;
+	unsigned long		end;
+	hmm_pfn_t		*pfns;
+	bool			valid;
+};
+
+/*
+ * To snapshot the CPU page table, call hmm_vma_get_pfns(), then take a device
+ * driver lock that serializes device page table updates, then call
+ * hmm_vma_range_done(), to check if the snapshot is still valid. The same
+ * device driver page table update lock must also be used in the
+ * hmm_mirror_ops.sync_cpu_device_pagetables() callback, so that CPU page
+ * table invalidation serializes on it.
+ *
+ * YOU MUST CALL hmm_vma_range_done() ONCE AND ONLY ONCE EACH TIME YOU CALL
+ * hmm_vma_get_pfns() WITHOUT ERROR !
+ *
+ * IF YOU DO NOT FOLLOW THE ABOVE RULE THE SNAPSHOT CONTENT MIGHT BE INVALID !
+ */
+int hmm_vma_get_pfns(struct vm_area_struct *vma,
+		     struct hmm_range *range,
+		     unsigned long start,
+		     unsigned long end,
+		     hmm_pfn_t *pfns);
+bool hmm_vma_range_done(struct vm_area_struct *vma, struct hmm_range *range);
 #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
 
 
diff --git a/mm/hmm.c b/mm/hmm.c
index 7ed4b4c..4828b97 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -19,8 +19,12 @@
  */
 #include <linux/mm.h>
 #include <linux/hmm.h>
+#include <linux/rmap.h>
+#include <linux/swap.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
+#include <linux/swapops.h>
+#include <linux/hugetlb.h>
 #include <linux/mmu_notifier.h>
 
 static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
@@ -30,14 +34,18 @@ static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
  * struct hmm - HMM per mm struct
  *
  * @mm: mm struct this HMM struct is bound to
+ * @lock: lock protecting ranges list
  * @sequence: we track updates to the CPU page table with a sequence number
+ * @ranges: list of range being snapshotted
  * @mirrors: list of mirrors for this mm
  * @mmu_notifier: mmu notifier to track updates to CPU page table
  * @mirrors_sem: read/write semaphore protecting the mirrors list
  */
 struct hmm {
 	struct mm_struct	*mm;
+	spinlock_t		lock;
 	atomic_t		sequence;
+	struct list_head	ranges;
 	struct list_head	mirrors;
 	struct mmu_notifier	mmu_notifier;
 	struct rw_semaphore	mirrors_sem;
@@ -71,6 +79,8 @@ static struct hmm *hmm_register(struct mm_struct *mm)
 	init_rwsem(&hmm->mirrors_sem);
 	atomic_set(&hmm->sequence, 0);
 	hmm->mmu_notifier.ops = NULL;
+	INIT_LIST_HEAD(&hmm->ranges);
+	spin_lock_init(&hmm->lock);
 	hmm->mm = mm;
 
 	/*
@@ -111,6 +121,22 @@ static void hmm_invalidate_range(struct hmm *hmm,
 				 unsigned long end)
 {
 	struct hmm_mirror *mirror;
+	struct hmm_range *range;
+
+	spin_lock(&hmm->lock);
+	list_for_each_entry(range, &hmm->ranges, list) {
+		unsigned long addr, idx, npages;
+
+		if (end < range->start || start >= range->end)
+			continue;
+
+		range->valid = false;
+		addr = max(start, range->start);
+		idx = (addr - range->start) >> PAGE_SHIFT;
+		npages = (min(range->end, end) - addr) >> PAGE_SHIFT;
+		memset(&range->pfns[idx], 0, sizeof(*range->pfns) * npages);
+	}
+	spin_unlock(&hmm->lock);
 
 	down_read(&hmm->mirrors_sem);
 	list_for_each_entry(mirror, &hmm->mirrors, list)
@@ -208,4 +234,263 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
 	up_write(&hmm->mirrors_sem);
 }
 EXPORT_SYMBOL(hmm_mirror_unregister);
+
+static void hmm_pfns_special(hmm_pfn_t *pfns,
+			     unsigned long addr,
+			     unsigned long end)
+{
+	for (; addr < end; addr += PAGE_SIZE, pfns++)
+		*pfns = HMM_PFN_SPECIAL;
+}
+
+static int hmm_vma_walk_hole(unsigned long addr,
+			     unsigned long end,
+			     struct mm_walk *walk)
+{
+	struct hmm_range *range = walk->private;
+	hmm_pfn_t *pfns = range->pfns;
+	unsigned long i;
+
+	i = (addr - range->start) >> PAGE_SHIFT;
+	for (; addr < end; addr += PAGE_SIZE, i++)
+		pfns[i] = HMM_PFN_EMPTY;
+
+	return 0;
+}
+
+static int hmm_vma_walk_clear(unsigned long addr,
+			      unsigned long end,
+			      struct mm_walk *walk)
+{
+	struct hmm_range *range = walk->private;
+	hmm_pfn_t *pfns = range->pfns;
+	unsigned long i;
+
+	i = (addr - range->start) >> PAGE_SHIFT;
+	for (; addr < end; addr += PAGE_SIZE, i++)
+		pfns[i] = 0;
+
+	return 0;
+}
+
+static int hmm_vma_walk_pmd(pmd_t *pmdp,
+			    unsigned long start,
+			    unsigned long end,
+			    struct mm_walk *walk)
+{
+	struct hmm_range *range = walk->private;
+	struct vm_area_struct *vma = walk->vma;
+	hmm_pfn_t *pfns = range->pfns;
+	unsigned long addr = start, i;
+	hmm_pfn_t flag;
+	pte_t *ptep;
+
+	i = (addr - range->start) >> PAGE_SHIFT;
+	flag = vma->vm_flags & VM_READ ? HMM_PFN_READ : 0;
+
+	if (pmd_none(*pmdp) || pmd_trans_unstable(pmdp)) {
+		pmd_t pmd;
+
+		pmd = pmd_read_atomic(pmdp);
+		barrier();
+		if (pmd_none(pmd))
+			return hmm_vma_walk_hole(start, end, walk);
+
+		if (pmd_bad(pmd) || pmd_protnone(pmd))
+			return hmm_vma_walk_clear(start, end, walk);
+
+		if (pmd_trans_huge(pmd) || pmd_devmap(pmd)) {
+			unsigned long pfn = pmd_pfn(pmd) + pte_index(addr);
+
+			flag |= pmd_write(pmd) ? HMM_PFN_WRITE : 0;
+			for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
+				pfns[i] = hmm_pfn_t_from_pfn(pfn) | flag;
+			return 0;
+		} else {
+			/*
+			 * Something unusual is going on. Better to have the
+			 * driver assume there is nothing for this range and
+			 * let the fault code path sort out proper pages for the
+			 * range.
+			 */
+			return hmm_vma_walk_clear(start, end, walk);
+		}
+	}
+
+	ptep = pte_offset_map(pmdp, addr);
+	for (; addr < end; addr += PAGE_SIZE, ptep++, i++) {
+		pte_t pte = *ptep;
+
+		pfns[i] = 0;
+
+		if (pte_none(pte)) {
+			pfns[i] = HMM_PFN_EMPTY;
+			continue;
+		}
+
+		if (!pte_present(pte)) {
+			swp_entry_t entry;
+
+			if (!non_swap_entry(entry))
+				continue;
+			entry = pte_to_swp_entry(pte);
+
+			/*
+			 * This is a special swap entry, ignore migration, use
+			 * device and report anything else as error.
+			 */
+			if (is_device_entry(entry)) {
+				pfns[i] = hmm_pfn_t_from_pfn(swp_offset(entry));
+				if (is_write_device_entry(entry))
+					pfns[i] |= HMM_PFN_WRITE;
+				pfns[i] |= HMM_PFN_DEVICE_UNADDRESSABLE;
+				pfns[i] |= flag;
+			} else if (!is_migration_entry(entry)) {
+				pfns[i] = HMM_PFN_ERROR;
+			}
+			continue;
+		}
+
+		pfns[i] = hmm_pfn_t_from_pfn(pte_pfn(pte)) | flag;
+		pfns[i] |= pte_write(pte) ? HMM_PFN_WRITE : 0;
+	}
+	pte_unmap(ptep - 1);
+
+	return 0;
+}
+
+/*
+ * hmm_vma_get_pfns() - snapshot CPU page table for a range of virtual addresses
+ * @vma: virtual memory area containing the virtual address range
+ * @range: used to track snapshot validity
+ * @start: range virtual start address (inclusive)
+ * @end: range virtual end address (exclusive)
+ * @entries: array of hmm_pfn_t: provided by the caller, filled in by function
+ * Returns: -EINVAL if invalid argument, -ENOMEM out of memory, 0 success
+ *
+ * This snapshots the CPU page table for a range of virtual addresses. Snapshot
+ * validity is tracked by range struct. See hmm_vma_range_done() for further
+ * information.
+ *
+ * The range struct is initialized here. It tracks the CPU page table, but only
+ * if the function returns success (0), in which case the caller must then call
+ * hmm_vma_range_done() to stop CPU page table update tracking on this range.
+ *
+ * NOT CALLING hmm_vma_range_done() IF FUNCTION RETURNS 0 WILL LEAD TO SERIOUS
+ * MEMORY CORRUPTION ! YOU HAVE BEEN WARNED !
+ */
+int hmm_vma_get_pfns(struct vm_area_struct *vma,
+		     struct hmm_range *range,
+		     unsigned long start,
+		     unsigned long end,
+		     hmm_pfn_t *pfns)
+{
+	struct mm_walk mm_walk;
+	struct hmm *hmm;
+
+	/* FIXME support hugetlb fs */
+	if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL)) {
+		hmm_pfns_special(pfns, start, end);
+		return -EINVAL;
+	}
+
+	/* Sanity check, this really should not happen ! */
+	if (start < vma->vm_start || start >= vma->vm_end)
+		return -EINVAL;
+	if (end < vma->vm_start || end > vma->vm_end)
+		return -EINVAL;
+
+	hmm = hmm_register(vma->vm_mm);
+	if (!hmm)
+		return -ENOMEM;
+	/* Caller must have registered a mirror, via hmm_mirror_register() ! */
+	if (!hmm->mmu_notifier.ops)
+		return -EINVAL;
+
+	/* Initialize range to track CPU page table update */
+	range->start = start;
+	range->pfns = pfns;
+	range->end = end;
+	spin_lock(&hmm->lock);
+	range->valid = true;
+	list_add_rcu(&range->list, &hmm->ranges);
+	spin_unlock(&hmm->lock);
+
+	mm_walk.vma = vma;
+	mm_walk.mm = vma->vm_mm;
+	mm_walk.private = range;
+	mm_walk.pte_entry = NULL;
+	mm_walk.test_walk = NULL;
+	mm_walk.hugetlb_entry = NULL;
+	mm_walk.pmd_entry = hmm_vma_walk_pmd;
+	mm_walk.pte_hole = hmm_vma_walk_hole;
+
+	walk_page_range(start, end, &mm_walk);
+
+	return 0;
+}
+EXPORT_SYMBOL(hmm_vma_get_pfns);
+
+/*
+ * hmm_vma_range_done() - stop tracking change to CPU page table over a range
+ * @vma: virtual memory area containing the virtual address range
+ * @range: range being tracked
+ * Returns: false if range data has been invalidated, true otherwise
+ *
+ * Range struct is used to track updates to the CPU page table after a call to
+ * either hmm_vma_get_pfns() or hmm_vma_fault(). Once the device driver is done
+ * using the data,  or wants to lock updates to the data it got from those
+ * functions, it must call the hmm_vma_range_done() function, which will then
+ * stop tracking CPU page table updates.
+ *
+ * Note that device driver must still implement general CPU page table update
+ * tracking either by using hmm_mirror (see hmm_mirror_register()) or by using
+ * the mmu_notifier API directly.
+ *
+ * CPU page table update tracking done through hmm_range is only temporary and
+ * to be used while trying to duplicate CPU page table contents for a range of
+ * virtual addresses.
+ *
+ * There are two ways to use this :
+ * again:
+ *   hmm_vma_get_pfns(vma, range, start, end, pfns);
+ *   trans = device_build_page_table_update_transaction(pfns);
+ *   device_page_table_lock();
+ *   if (!hmm_vma_range_done(vma, range)) {
+ *     device_page_table_unlock();
+ *     goto again;
+ *   }
+ *   device_commit_transaction(trans);
+ *   device_page_table_unlock();
+ *
+ * Or:
+ *   hmm_vma_get_pfns(vma, range, start, end, pfns);
+ *   device_page_table_lock();
+ *   hmm_vma_range_done(vma, range);
+ *   device_update_page_table(pfns);
+ *   device_page_table_unlock();
+ */
+bool hmm_vma_range_done(struct vm_area_struct *vma, struct hmm_range *range)
+{
+	unsigned long npages = (range->end - range->start) >> PAGE_SHIFT;
+	struct hmm *hmm;
+
+	if (range->end <= range->start) {
+		BUG();
+		return false;
+	}
+
+	hmm = hmm_register(vma->vm_mm);
+	if (!hmm) {
+		memset(range->pfns, 0, sizeof(*range->pfns) * npages);
+		return false;
+	}
+
+	spin_lock(&hmm->lock);
+	list_del_rcu(&range->list);
+	spin_unlock(&hmm->lock);
+
+	return range->valid;
+}
+EXPORT_SYMBOL(hmm_vma_range_done);
 #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
-- 
2.9.3

[HMM 10/15] mm/hmm/mirror: device page fault handler

[Jérôme Glisse]

This handle page fault on behalf of device driver, unlike handle_mm_fault()
it does not trigger migration back to system memory for device memory.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 include/linux/hmm.h |  27 ++++++
 mm/hmm.c            | 243 +++++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 256 insertions(+), 14 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index defa7cd..d267989 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -292,6 +292,33 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
 		     unsigned long end,
 		     hmm_pfn_t *pfns);
 bool hmm_vma_range_done(struct vm_area_struct *vma, struct hmm_range *range);
+
+
+/*
+ * Fault memory on behalf of device driver. Unlike handle_mm_fault(), this will
+ * not migrate any device memory back to system memory. The hmm_pfn_t array will
+ * be updated with the fault result and current snapshot of the CPU page table
+ * for the range.
+ *
+ * The mmap_sem must be taken in read mode before entering and it might be
+ * dropped by the function if the block argument is false. In that case, the
+ * function returns -EAGAIN.
+ *
+ * Return value does not reflect if the fault was successful for every single
+ * address or not. Therefore, the caller must to inspect the hmm_pfn_t array to
+ * determine fault status for each address.
+ *
+ * Trying to fault inside an invalid vma will result in -EINVAL.
+ *
+ * See the function description in mm/hmm.c for further documentation.
+ */
+int hmm_vma_fault(struct vm_area_struct *vma,
+		  struct hmm_range *range,
+		  unsigned long start,
+		  unsigned long end,
+		  hmm_pfn_t *pfns,
+		  bool write,
+		  bool block);
 #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
 
 
diff --git a/mm/hmm.c b/mm/hmm.c
index 4828b97..be88807 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -235,6 +235,36 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror)
 }
 EXPORT_SYMBOL(hmm_mirror_unregister);
 
+struct hmm_vma_walk {
+	struct hmm_range	*range;
+	unsigned long		last;
+	bool			fault;
+	bool			block;
+	bool			write;
+};
+
+static int hmm_vma_do_fault(struct mm_walk *walk,
+			    unsigned long addr,
+			    hmm_pfn_t *pfn)
+{
+	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_REMOTE;
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct vm_area_struct *vma = walk->vma;
+	int r;
+
+	flags |= hmm_vma_walk->block ? 0 : FAULT_FLAG_ALLOW_RETRY;
+	flags |= hmm_vma_walk->write ? FAULT_FLAG_WRITE : 0;
+	r = handle_mm_fault(vma, addr, flags);
+	if (r & VM_FAULT_RETRY)
+		return -EBUSY;
+	if (r & VM_FAULT_ERROR) {
+		*pfn = HMM_PFN_ERROR;
+		return -EFAULT;
+	}
+
+	return -EAGAIN;
+}
+
 static void hmm_pfns_special(hmm_pfn_t *pfns,
 			     unsigned long addr,
 			     unsigned long end)
@@ -243,34 +273,62 @@ static void hmm_pfns_special(hmm_pfn_t *pfns,
 		*pfns = HMM_PFN_SPECIAL;
 }
 
+static void hmm_pfns_clear(hmm_pfn_t *pfns,
+			   unsigned long addr,
+			   unsigned long end)
+{
+	for (; addr < end; addr += PAGE_SIZE, pfns++)
+		*pfns = 0;
+}
+
 static int hmm_vma_walk_hole(unsigned long addr,
 			     unsigned long end,
 			     struct mm_walk *walk)
 {
-	struct hmm_range *range = walk->private;
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
 	hmm_pfn_t *pfns = range->pfns;
 	unsigned long i;
 
+	hmm_vma_walk->last = addr;
 	i = (addr - range->start) >> PAGE_SHIFT;
-	for (; addr < end; addr += PAGE_SIZE, i++)
+	for (; addr < end; addr += PAGE_SIZE, i++) {
 		pfns[i] = HMM_PFN_EMPTY;
+		if (hmm_vma_walk->fault) {
+			int ret;
 
-	return 0;
+			ret = hmm_vma_do_fault(walk, addr, &pfns[i]);
+			if (ret != -EAGAIN)
+				return ret;
+		}
+	}
+
+	return hmm_vma_walk->fault ? -EAGAIN : 0;
 }
 
 static int hmm_vma_walk_clear(unsigned long addr,
 			      unsigned long end,
 			      struct mm_walk *walk)
 {
-	struct hmm_range *range = walk->private;
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
 	hmm_pfn_t *pfns = range->pfns;
 	unsigned long i;
 
+	hmm_vma_walk->last = addr;
 	i = (addr - range->start) >> PAGE_SHIFT;
-	for (; addr < end; addr += PAGE_SIZE, i++)
+	for (; addr < end; addr += PAGE_SIZE, i++) {
 		pfns[i] = 0;
+		if (hmm_vma_walk->fault) {
+			int ret;
 
-	return 0;
+			ret = hmm_vma_do_fault(walk, addr, &pfns[i]);
+			if (ret != -EAGAIN)
+				return ret;
+		}
+	}
+
+	return hmm_vma_walk->fault ? -EAGAIN : 0;
 }
 
 static int hmm_vma_walk_pmd(pmd_t *pmdp,
@@ -278,15 +336,18 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 			    unsigned long end,
 			    struct mm_walk *walk)
 {
-	struct hmm_range *range = walk->private;
+	struct hmm_vma_walk *hmm_vma_walk = walk->private;
+	struct hmm_range *range = hmm_vma_walk->range;
 	struct vm_area_struct *vma = walk->vma;
 	hmm_pfn_t *pfns = range->pfns;
 	unsigned long addr = start, i;
+	bool write_fault;
 	hmm_pfn_t flag;
 	pte_t *ptep;
 
 	i = (addr - range->start) >> PAGE_SHIFT;
 	flag = vma->vm_flags & VM_READ ? HMM_PFN_READ : 0;
+	write_fault = hmm_vma_walk->fault & hmm_vma_walk->write;
 
 	if (pmd_none(*pmdp) || pmd_trans_unstable(pmdp)) {
 		pmd_t pmd;
@@ -302,6 +363,9 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 		if (pmd_trans_huge(pmd) || pmd_devmap(pmd)) {
 			unsigned long pfn = pmd_pfn(pmd) + pte_index(addr);
 
+			if (write_fault && !pmd_write(pmd))
+				return hmm_vma_walk_clear(start, end, walk);
+
 			flag |= pmd_write(pmd) ? HMM_PFN_WRITE : 0;
 			for (; addr < end; addr += PAGE_SIZE, i++, pfn++)
 				pfns[i] = hmm_pfn_t_from_pfn(pfn) | flag;
@@ -325,14 +389,19 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 
 		if (pte_none(pte)) {
 			pfns[i] = HMM_PFN_EMPTY;
+			if (hmm_vma_walk->fault)
+				goto fault;
 			continue;
 		}
 
 		if (!pte_present(pte)) {
 			swp_entry_t entry;
 
-			if (!non_swap_entry(entry))
+			if (!non_swap_entry(entry)) {
+				if (hmm_vma_walk->fault)
+					goto fault;
 				continue;
+			}
 			entry = pte_to_swp_entry(pte);
 
 			/*
@@ -341,18 +410,38 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
 			 */
 			if (is_device_entry(entry)) {
 				pfns[i] = hmm_pfn_t_from_pfn(swp_offset(entry));
-				if (is_write_device_entry(entry))
+				if (is_write_device_entry(entry)) {
 					pfns[i] |= HMM_PFN_WRITE;
+				} else if (write_fault)
+					goto fault;
 				pfns[i] |= HMM_PFN_DEVICE_UNADDRESSABLE;
 				pfns[i] |= flag;
-			} else if (!is_migration_entry(entry)) {
+			} else if (is_migration_entry(entry)) {
+				if (hmm_vma_walk->fault) {
+					pte_unmap(ptep - 1);
+					hmm_vma_walk->last = addr;
+					migration_entry_wait(vma->vm_mm,
+							     pmdp, addr);
+					return -EAGAIN;
+				}
+				continue;
+			} else {
+				/* Report error for everything else */
 				pfns[i] = HMM_PFN_ERROR;
 			}
 			continue;
 		}
 
+		if (write_fault && !pte_write(pte))
+			goto fault;
 		pfns[i] = hmm_pfn_t_from_pfn(pte_pfn(pte)) | flag;
 		pfns[i] |= pte_write(pte) ? HMM_PFN_WRITE : 0;
+		continue;
+
+fault:
+		pte_unmap(ptep - 1);
+		/* Fault all pages in range */
+		return hmm_vma_walk_clear(start, end, walk);
 	}
 	pte_unmap(ptep - 1);
 
@@ -385,6 +474,7 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
 		     unsigned long end,
 		     hmm_pfn_t *pfns)
 {
+	struct hmm_vma_walk hmm_vma_walk;
 	struct mm_walk mm_walk;
 	struct hmm *hmm;
 
@@ -416,9 +506,12 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
 	list_add_rcu(&range->list, &hmm->ranges);
 	spin_unlock(&hmm->lock);
 
+	hmm_vma_walk.fault = false;
+	hmm_vma_walk.range = range;
+	mm_walk.private = &hmm_vma_walk;
+
 	mm_walk.vma = vma;
 	mm_walk.mm = vma->vm_mm;
-	mm_walk.private = range;
 	mm_walk.pte_entry = NULL;
 	mm_walk.test_walk = NULL;
 	mm_walk.hugetlb_entry = NULL;
@@ -426,7 +519,6 @@ int hmm_vma_get_pfns(struct vm_area_struct *vma,
 	mm_walk.pte_hole = hmm_vma_walk_hole;
 
 	walk_page_range(start, end, &mm_walk);
-
 	return 0;
 }
 EXPORT_SYMBOL(hmm_vma_get_pfns);
@@ -453,7 +545,7 @@ EXPORT_SYMBOL(hmm_vma_get_pfns);
  *
  * There are two ways to use this :
  * again:
- *   hmm_vma_get_pfns(vma, range, start, end, pfns);
+ *   hmm_vma_get_pfns(vma, range, start, end, pfns); or hmm_vma_fault(...);
  *   trans = device_build_page_table_update_transaction(pfns);
  *   device_page_table_lock();
  *   if (!hmm_vma_range_done(vma, range)) {
@@ -464,7 +556,7 @@ EXPORT_SYMBOL(hmm_vma_get_pfns);
  *   device_page_table_unlock();
  *
  * Or:
- *   hmm_vma_get_pfns(vma, range, start, end, pfns);
+ *   hmm_vma_get_pfns(vma, range, start, end, pfns); or hmm_vma_fault(...);
  *   device_page_table_lock();
  *   hmm_vma_range_done(vma, range);
  *   device_update_page_table(pfns);
@@ -493,4 +585,127 @@ bool hmm_vma_range_done(struct vm_area_struct *vma, struct hmm_range *range)
 	return range->valid;
 }
 EXPORT_SYMBOL(hmm_vma_range_done);
+
+/*
+ * hmm_vma_fault() - try to fault some address in a virtual address range
+ * @vma: virtual memory area containing the virtual address range
+ * @range: use to track pfns array content validity
+ * @start: fault range virtual start address (inclusive)
+ * @end: fault range virtual end address (exclusive)
+ * @pfns: array of hmm_pfn_t, only entry with fault flag set will be faulted
+ * @write: is it a write fault
+ * @block: allow blocking on fault (if true it sleeps and do not drop mmap_sem)
+ * Returns: 0 success, error otherwise (-EAGAIN means mmap_sem have been drop)
+ *
+ * This is similar to a regular CPU page fault except that it will not trigger
+ * any memory migration if the memory being faulted is not accessible by CPUs.
+ *
+ * On error, for one virtual address in the range, the function will set the
+ * hmm_pfn_t error flag for the corresponding pfn entry.
+ *
+ * Expected use pattern:
+ * retry:
+ *   down_read(&mm->mmap_sem);
+ *   // Find vma and address device wants to fault, initialize hmm_pfn_t
+ *   // array accordingly
+ *   ret = hmm_vma_fault(vma, start, end, pfns, allow_retry);
+ *   switch (ret) {
+ *   case -EAGAIN:
+ *     hmm_vma_range_done(vma, range);
+ *     // You might want to rate limit or yield to play nicely, you may
+ *     // also commit any valid pfn in the array assuming that you are
+ *     // getting true from hmm_vma_range_monitor_end()
+ *     goto retry;
+ *   case 0:
+ *     break;
+ *   default:
+ *     // Handle error !
+ *     up_read(&mm->mmap_sem)
+ *     return;
+ *   }
+ *   // Take device driver lock that serialize device page table update
+ *   driver_lock_device_page_table_update();
+ *   hmm_vma_range_done(vma, range);
+ *   // Commit pfns we got from hmm_vma_fault()
+ *   driver_unlock_device_page_table_update();
+ *   up_read(&mm->mmap_sem)
+ *
+ * YOU MUST CALL hmm_vma_range_done() AFTER THIS FUNCTION RETURN SUCCESS (0)
+ * BEFORE FREEING THE range struct OR YOU WILL HAVE SERIOUS MEMORY CORRUPTION !
+ *
+ * YOU HAVE BEEN WARNED !
+ */
+int hmm_vma_fault(struct vm_area_struct *vma,
+		  struct hmm_range *range,
+		  unsigned long start,
+		  unsigned long end,
+		  hmm_pfn_t *pfns,
+		  bool write,
+		  bool block)
+{
+	struct hmm_vma_walk hmm_vma_walk;
+	struct mm_walk mm_walk;
+	struct hmm *hmm;
+	int ret;
+
+	/* Sanity check, this really should not happen ! */
+	if (start < vma->vm_start || start >= vma->vm_end)
+		return -EINVAL;
+	if (end < vma->vm_start || end > vma->vm_end)
+		return -EINVAL;
+
+	hmm = hmm_register(vma->vm_mm);
+	if (!hmm) {
+		hmm_pfns_clear(pfns, start, end);
+		return -ENOMEM;
+	}
+	/* Caller must have registered a mirror using hmm_mirror_register() */
+	if (!hmm->mmu_notifier.ops)
+		return -EINVAL;
+
+	/* Initialize range to track CPU page table update */
+	range->start = start;
+	range->pfns = pfns;
+	range->end = end;
+	spin_lock(&hmm->lock);
+	range->valid = true;
+	list_add_rcu(&range->list, &hmm->ranges);
+	spin_unlock(&hmm->lock);
+
+	/* FIXME support hugetlb fs */
+	if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL)) {
+		hmm_pfns_special(pfns, start, end);
+		return 0;
+	}
+
+	hmm_vma_walk.fault = true;
+	hmm_vma_walk.write = write;
+	hmm_vma_walk.block = block;
+	hmm_vma_walk.range = range;
+	mm_walk.private = &hmm_vma_walk;
+	hmm_vma_walk.last = range->start;
+
+	mm_walk.vma = vma;
+	mm_walk.mm = vma->vm_mm;
+	mm_walk.pte_entry = NULL;
+	mm_walk.test_walk = NULL;
+	mm_walk.hugetlb_entry = NULL;
+	mm_walk.pmd_entry = hmm_vma_walk_pmd;
+	mm_walk.pte_hole = hmm_vma_walk_hole;
+
+	do {
+		ret = walk_page_range(start, end, &mm_walk);
+		start = hmm_vma_walk.last;
+	} while (ret == -EAGAIN);
+
+	if (ret) {
+		unsigned long i;
+
+		i = (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
+		hmm_pfns_clear(&pfns[i], hmm_vma_walk.last, end);
+		hmm_vma_range_done(vma, range);
+	}
+	return ret;
+}
+EXPORT_SYMBOL(hmm_vma_fault);
 #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
-- 
2.9.3

[HMM 11/15] mm/migrate: support un-addressable ZONE_DEVICE page in migration

[Jérôme Glisse]

Allow to unmap and restore special swap entry of un-addressable
ZONE_DEVICE memory.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 include/linux/migrate.h |  10 +++-
 mm/migrate.c            | 136 ++++++++++++++++++++++++++++++++++++++----------
 mm/page_vma_mapped.c    |  10 ++++
 mm/rmap.c               |  25 +++++++++
 4 files changed, 152 insertions(+), 29 deletions(-)

diff --git a/include/linux/migrate.h b/include/linux/migrate.h
index 576b3f5..7dd875a 100644
--- a/include/linux/migrate.h
+++ b/include/linux/migrate.h
@@ -130,12 +130,18 @@ static inline int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 
 #ifdef CONFIG_MIGRATION
 
+/*
+ * Watch out for PAE architecture, which has an unsigned long, and might not
+ * have enough bits to store all physical address and flags. So far we have
+ * enough room for all our flags.
+ */
 #define MIGRATE_PFN_VALID	(1UL << 0)
 #define MIGRATE_PFN_MIGRATE	(1UL << 1)
 #define MIGRATE_PFN_LOCKED	(1UL << 2)
 #define MIGRATE_PFN_WRITE	(1UL << 3)
-#define MIGRATE_PFN_ERROR	(1UL << 4)
-#define MIGRATE_PFN_SHIFT	5
+#define MIGRATE_PFN_DEVICE	(1UL << 4)
+#define MIGRATE_PFN_ERROR	(1UL << 5)
+#define MIGRATE_PFN_SHIFT	6
 
 static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
 {
diff --git a/mm/migrate.c b/mm/migrate.c
index 4ac2a7a..62ad41c 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -36,6 +36,7 @@
 #include <linux/hugetlb.h>
 #include <linux/hugetlb_cgroup.h>
 #include <linux/gfp.h>
+#include <linux/memremap.h>
 #include <linux/balloon_compaction.h>
 #include <linux/mmu_notifier.h>
 #include <linux/page_idle.h>
@@ -233,7 +234,15 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
 			pte = arch_make_huge_pte(pte, vma, new, 0);
 		}
 #endif
-		flush_dcache_page(new);
+
+		if (unlikely(is_zone_device_page(new)) &&
+		    is_device_unaddressable_page(new)) {
+			entry = make_device_entry(new, pte_write(pte));
+			pte = swp_entry_to_pte(entry);
+			if (pte_swp_soft_dirty(*pvmw.pte))
+				pte = pte_mksoft_dirty(pte);
+		} else
+			flush_dcache_page(new);
 		set_pte_at(vma->vm_mm, pvmw.address, pvmw.pte, pte);
 
 		if (PageHuge(new)) {
@@ -305,6 +314,8 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
 	 */
 	if (!get_page_unless_zero(page))
 		goto out;
+	if (is_zone_device_page(page))
+		get_zone_device_page(page);
 	pte_unmap_unlock(ptep, ptl);
 	wait_on_page_locked(page);
 	put_page(page);
@@ -2139,17 +2150,40 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		pte = *ptep;
 		pfn = pte_pfn(pte);
 
-		if (!pte_present(pte)) {
+		if (pte_none(pte)) {
 			mpfn = pfn = 0;
 			goto next;
 		}
 
+		if (!pte_present(pte)) {
+			mpfn = pfn = 0;
+
+			/*
+			 * Only care about unaddressable device page special
+			 * page table entry. Other special swap entries are not
+			 * migratable, and we ignore regular swapped page.
+			 */
+			entry = pte_to_swp_entry(pte);
+			if (!is_device_entry(entry))
+				goto next;
+
+			page = device_entry_to_page(entry);
+			mpfn = migrate_pfn(page_to_pfn(page))|
+				MIGRATE_PFN_DEVICE | MIGRATE_PFN_MIGRATE;
+			if (is_write_device_entry(entry))
+				mpfn |= MIGRATE_PFN_WRITE;
+		} else {
+			page = vm_normal_page(migrate->vma, addr, pte);
+			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
+			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
+		}
+
 		/* FIXME support THP */
-		page = vm_normal_page(migrate->vma, addr, pte);
 		if (!page || !page->mapping || PageTransCompound(page)) {
 			mpfn = pfn = 0;
 			goto next;
 		}
+		pfn = page_to_pfn(page);
 
 		/*
 		 * By getting a reference on the page we pin it and that blocks
@@ -2162,8 +2196,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		 */
 		get_page(page);
 		migrate->cpages++;
-		mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
-		mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
 
 		/*
 		 * Optimize for the common case where page is only mapped once
@@ -2194,6 +2226,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		}
 
 next:
+		migrate->dst[migrate->npages] = 0;
 		migrate->src[migrate->npages++] = mpfn;
 	}
 	arch_leave_lazy_mmu_mode();
@@ -2263,6 +2296,15 @@ static bool migrate_vma_check_page(struct page *page)
 	if (PageCompound(page))
 		return false;
 
+	/* Page from ZONE_DEVICE have one extra reference */
+	if (is_zone_device_page(page)) {
+		if (is_device_unaddressable_page(page)) {
+			extra++;
+		} else
+			/* Other ZONE_DEVICE memory type are not supported */
+			return false;
+	}
+
 	if ((page_count(page) - extra) > page_mapcount(page))
 		return false;
 
@@ -2300,24 +2342,30 @@ static void migrate_vma_prepare(struct migrate_vma *migrate)
 			migrate->src[i] |= MIGRATE_PFN_LOCKED;
 		}
 
-		if (!PageLRU(page) && allow_drain) {
-			/* Drain CPU's pagevec */
-			lru_add_drain_all();
-			allow_drain = false;
-		}
+		/* ZONE_DEVICE pages are not on LRU */
+		if (!is_zone_device_page(page)) {
+			if (!PageLRU(page) && allow_drain) {
+				/* Drain CPU's pagevec */
+				lru_add_drain_all();
+				allow_drain = false;
+			}
 
-		if (isolate_lru_page(page)) {
-			if (remap) {
-				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
-				migrate->cpages--;
-				restore++;
-			} else {
-				migrate->src[i] = 0;
-				unlock_page(page);
-				migrate->cpages--;
-				put_page(page);
+			if (isolate_lru_page(page)) {
+				if (remap) {
+					migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+					migrate->cpages--;
+					restore++;
+				} else {
+					migrate->src[i] = 0;
+					unlock_page(page);
+					migrate->cpages--;
+					put_page(page);
+				}
+				continue;
 			}
-			continue;
+
+			/* Drop the reference we took in collect */
+			put_page(page);
 		}
 
 		if (!migrate_vma_check_page(page)) {
@@ -2326,14 +2374,19 @@ static void migrate_vma_prepare(struct migrate_vma *migrate)
 				migrate->cpages--;
 				restore++;
 
-				get_page(page);
-				putback_lru_page(page);
+				if (!is_zone_device_page(page)) {
+					get_page(page);
+					putback_lru_page(page);
+				}
 			} else {
 				migrate->src[i] = 0;
 				unlock_page(page);
 				migrate->cpages--;
 
-				putback_lru_page(page);
+				if (!is_zone_device_page(page))
+					putback_lru_page(page);
+				else
+					put_page(page);
 			}
 		}
 	}
@@ -2404,7 +2457,10 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
 		unlock_page(page);
 		restore--;
 
-		putback_lru_page(page);
+		if (is_zone_device_page(page))
+			put_page(page);
+		else
+			putback_lru_page(page);
 	}
 }
 
@@ -2435,6 +2491,26 @@ static void migrate_vma_pages(struct migrate_vma *migrate)
 
 		mapping = page_mapping(page);
 
+		if (is_zone_device_page(newpage)) {
+			if (is_device_unaddressable_page(newpage)) {
+				/*
+				 * For now only support private anonymous when
+				 * migrating to un-addressable device memory.
+				 */
+				if (mapping) {
+					migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+					continue;
+				}
+			} else {
+				/*
+				 * Other types of ZONE_DEVICE page are not
+				 * supported.
+				 */
+				migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
+				continue;
+			}
+		}
+
 		r = migrate_page(mapping, newpage, page, MIGRATE_SYNC_NO_COPY);
 		if (r != MIGRATEPAGE_SUCCESS)
 			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
@@ -2475,11 +2551,17 @@ static void migrate_vma_finalize(struct migrate_vma *migrate)
 		unlock_page(page);
 		migrate->cpages--;
 
-		putback_lru_page(page);
+		if (is_zone_device_page(page))
+			put_page(page);
+		else
+			putback_lru_page(page);
 
 		if (newpage != page) {
 			unlock_page(newpage);
-			putback_lru_page(newpage);
+			if (is_zone_device_page(newpage))
+				put_page(newpage);
+			else
+				putback_lru_page(newpage);
 		}
 	}
 }
diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index de9c40d..12b8c1a 100644
--- a/mm/page_vma_mapped.c
+++ b/mm/page_vma_mapped.c
@@ -48,6 +48,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
 		if (!is_swap_pte(*pvmw->pte))
 			return false;
 		entry = pte_to_swp_entry(*pvmw->pte);
+
 		if (!is_migration_entry(entry))
 			return false;
 		if (migration_entry_to_page(entry) - pvmw->page >=
@@ -60,6 +61,15 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
 		WARN_ON_ONCE(1);
 #endif
 	} else {
+		if (is_swap_pte(*pvmw->pte)) {
+			swp_entry_t entry;
+
+			entry = pte_to_swp_entry(*pvmw->pte);
+			if (is_device_entry(entry) &&
+			    device_entry_to_page(entry) == pvmw->page)
+				return true;
+		}
+
 		if (!pte_present(*pvmw->pte))
 			return false;
 
diff --git a/mm/rmap.c b/mm/rmap.c
index d405f0e..72fadad 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -63,6 +63,7 @@
 #include <linux/hugetlb.h>
 #include <linux/backing-dev.h>
 #include <linux/page_idle.h>
+#include <linux/memremap.h>
 
 #include <asm/tlbflush.h>
 
@@ -1308,6 +1309,10 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 	if ((flags & TTU_MUNLOCK) && !(vma->vm_flags & VM_LOCKED))
 		return true;
 
+	if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
+	    is_zone_device_page(page) && !is_device_unaddressable_page(page))
+		return true;
+
 	if (flags & TTU_SPLIT_HUGE_PMD) {
 		split_huge_pmd_address(vma, address,
 				flags & TTU_MIGRATION, page);
@@ -1343,6 +1348,26 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 		subpage = page - page_to_pfn(page) + pte_pfn(*pvmw.pte);
 		address = pvmw.address;
 
+		if (IS_ENABLED(CONFIG_MIGRATION) &&
+		    (flags & TTU_MIGRATION) &&
+		    is_zone_device_page(page)) {
+			swp_entry_t entry;
+			pte_t swp_pte;
+
+			pteval = ptep_get_and_clear(mm, address, pvmw.pte);
+
+			/*
+			 * Store the pfn of the page in a special migration
+			 * pte. do_swap_page() will wait until the migration
+			 * pte is removed and then restart fault handling.
+			 */
+			entry = make_migration_entry(page, 0);
+			swp_pte = swp_entry_to_pte(entry);
+			if (pte_soft_dirty(pteval))
+				swp_pte = pte_swp_mksoft_dirty(swp_pte);
+			set_pte_at(mm, address, pvmw.pte, swp_pte);
+			goto discard;
+		}
 
 		if (!(flags & TTU_IGNORE_ACCESS)) {
 			if (ptep_clear_flush_young_notify(vma, address,
-- 
2.9.3

[HMM 12/15] mm/migrate: allow migrate_vma() to alloc new page on empty entry v2

[Jérôme Glisse]

This allow caller of migrate_vma() to allocate new page for empty CPU
page table entry. It only support anoymous memory and it won't allow
new page to be instance if userfaultfd is armed.

This is useful to device driver that want to migrate a range of virtual
address and would rather allocate new memory than having to fault later
on.

Changed since v1:
  - 5 level page table fix

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
---
 mm/migrate.c | 135 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 131 insertions(+), 4 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 62ad41c..c4c3f93 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -37,6 +37,7 @@
 #include <linux/hugetlb_cgroup.h>
 #include <linux/gfp.h>
 #include <linux/memremap.h>
+#include <linux/userfaultfd_k.h>
 #include <linux/balloon_compaction.h>
 #include <linux/mmu_notifier.h>
 #include <linux/page_idle.h>
@@ -2112,9 +2113,10 @@ static int migrate_vma_collect_hole(unsigned long start,
 				    struct mm_walk *walk)
 {
 	struct migrate_vma *migrate = walk->private;
-	unsigned long addr, next;
+	unsigned long addr;
 
 	for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE) {
+		migrate->cpages++;
 		migrate->dst[migrate->npages] = 0;
 		migrate->src[migrate->npages++] = 0;
 	}
@@ -2151,6 +2153,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
 		pfn = pte_pfn(pte);
 
 		if (pte_none(pte)) {
+			migrate->cpages++;
 			mpfn = pfn = 0;
 			goto next;
 		}
@@ -2464,6 +2467,118 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
 	}
 }
 
+static void migrate_vma_insert_page(struct migrate_vma *migrate,
+				    unsigned long addr,
+				    struct page *page,
+				    unsigned long *src,
+				    unsigned long *dst)
+{
+	struct vm_area_struct *vma = migrate->vma;
+	struct mm_struct *mm = vma->vm_mm;
+	struct mem_cgroup *memcg;
+	spinlock_t *ptl;
+	pgd_t *pgdp;
+	p4d_t *p4dp;
+	pud_t *pudp;
+	pmd_t *pmdp;
+	pte_t *ptep;
+	pte_t entry;
+
+	/* Only allow populating anonymous memory */
+	if (!vma_is_anonymous(vma))
+		goto abort;
+
+	pgdp = pgd_offset(mm, addr);
+	p4dp = p4d_alloc(mm, pgdp, addr);
+	if (!p4dp)
+		goto abort;
+	pudp = pud_alloc(mm, p4dp, addr);
+	if (!pudp)
+		goto abort;
+	pmdp = pmd_alloc(mm, pudp, addr);
+	if (!pmdp)
+		goto abort;
+
+	if (pmd_trans_unstable(pmdp) || pmd_devmap(*pmdp))
+		goto abort;
+
+	/*
+	 * Use pte_alloc() instead of pte_alloc_map().  We can't run
+	 * pte_offset_map() on pmds where a huge pmd might be created
+	 * from a different thread.
+	 *
+	 * pte_alloc_map() is safe to use under down_write(mmap_sem) or when
+	 * parallel threads are excluded by other means.
+	 *
+	 * Here we only have down_read(mmap_sem).
+	 */
+	if (pte_alloc(mm, pmdp, addr))
+		goto abort;
+
+	/* See the comment in pte_alloc_one_map() */
+	if (unlikely(pmd_trans_unstable(pmdp)))
+		goto abort;
+
+	if (unlikely(anon_vma_prepare(vma)))
+		goto abort;
+	if (mem_cgroup_try_charge(page, vma->vm_mm, GFP_KERNEL, &memcg, false))
+		goto abort;
+
+	/*
+	 * The memory barrier inside __SetPageUptodate makes sure that
+	 * preceding stores to the page contents become visible before
+	 * the set_pte_at() write.
+	 */
+	__SetPageUptodate(page);
+
+	if (is_zone_device_page(page) && is_device_unaddressable_page(page)) {
+		swp_entry_t swp_entry;
+
+		swp_entry = make_device_entry(page, vma->vm_flags & VM_WRITE);
+		entry = swp_entry_to_pte(swp_entry);
+	} else {
+		entry = mk_pte(page, vma->vm_page_prot);
+		if (vma->vm_flags & VM_WRITE)
+			entry = pte_mkwrite(pte_mkdirty(entry));
+	}
+
+	ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
+	if (!pte_none(*ptep)) {
+		pte_unmap_unlock(ptep, ptl);
+		mem_cgroup_cancel_charge(page, memcg, false);
+		goto abort;
+	}
+
+	/*
+	 * Check for usefaultfd but do not deliver the fault. Instead,
+	 * just back off.
+	 */
+	if (userfaultfd_missing(vma)) {
+		pte_unmap_unlock(ptep, ptl);
+		mem_cgroup_cancel_charge(page, memcg, false);
+		goto abort;
+	}
+
+	inc_mm_counter(mm, MM_ANONPAGES);
+	page_add_new_anon_rmap(page, vma, addr, false);
+	mem_cgroup_commit_charge(page, memcg, false, false);
+	if (!is_zone_device_page(page))
+		lru_cache_add_active_or_unevictable(page, vma);
+	set_pte_at(mm, addr, ptep, entry);
+
+	/* Take a reference on the page */
+	get_page(page);
+
+	/* No need to invalidate - it was non-present before */
+	update_mmu_cache(vma, addr, ptep);
+	pte_unmap_unlock(ptep, ptl);
+	*src = MIGRATE_PFN_MIGRATE;
+	return;
+
+abort:
+	*src &= ~MIGRATE_PFN_MIGRATE;
+}
+
 /*
  * migrate_vma_pages() - migrate meta-data from src page to dst page
  * @migrate: migrate struct containing all migration information
@@ -2484,10 +2599,16 @@ static void migrate_vma_pages(struct migrate_vma *migrate)
 		struct address_space *mapping;
 		int r;
 
-		if (!page || !newpage)
+		if (!newpage) {
+			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
 			continue;
-		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE))
+		} else if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE)) {
+			if (!page)
+				migrate_vma_insert_page(migrate, addr, newpage,
+							&migrate->src[i],
+							&migrate->dst[i]);
 			continue;
+		}
 
 		mapping = page_mapping(page);
 
@@ -2537,8 +2658,14 @@ static void migrate_vma_finalize(struct migrate_vma *migrate)
 		struct page *newpage = migrate_pfn_to_page(migrate->dst[i]);
 		struct page *page = migrate_pfn_to_page(migrate->src[i]);
 
-		if (!page)
+		if (!page) {
+			if (newpage) {
+				unlock_page(newpage);
+				put_page(newpage);
+			}
 			continue;
+		}
+
 		if (!(migrate->src[i] & MIGRATE_PFN_MIGRATE) || !newpage) {
 			if (newpage) {
 				unlock_page(newpage);
-- 
2.9.3

[HMM 13/15] mm/hmm/devmem: device memory hotplug using ZONE_DEVICE v3

[Jérôme Glisse]

This introduce a simple struct and associated helpers for device driver
to use when hotpluging un-addressable device memory as ZONE_DEVICE. It
will find a unuse physical address range and trigger memory hotplug for
it which allocates and initialize struct page for the device memory.

Changed since v2:
  - s/SECTION_SIZE/PA_SECTION_SIZE
Changed since v1:
  - change to adapt to new add_pages() helper
  - make this x86-64 only for now

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 include/linux/hmm.h | 114 +++++++++++++++
 mm/Kconfig          |   9 ++
 mm/hmm.c            | 404 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 527 insertions(+)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index d267989..50a1115 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -72,6 +72,11 @@
 
 #if IS_ENABLED(CONFIG_HMM)
 
+#include <linux/migrate.h>
+#include <linux/memremap.h>
+#include <linux/completion.h>
+
+
 struct hmm;
 
 /*
@@ -322,6 +327,115 @@ int hmm_vma_fault(struct vm_area_struct *vma,
 #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
 
 
+#if IS_ENABLED(CONFIG_HMM_DEVMEM)
+struct hmm_devmem;
+
+struct page *hmm_vma_alloc_locked_page(struct vm_area_struct *vma,
+				       unsigned long addr);
+
+/*
+ * struct hmm_devmem_ops - callback for ZONE_DEVICE memory events
+ *
+ * @free: call when refcount on page reach 1 and thus is no longer use
+ * @fault: call when there is a page fault to unaddressable memory
+ */
+struct hmm_devmem_ops {
+	void (*free)(struct hmm_devmem *devmem, struct page *page);
+	int (*fault)(struct hmm_devmem *devmem,
+		     struct vm_area_struct *vma,
+		     unsigned long addr,
+		     struct page *page,
+		     unsigned int flags,
+		     pmd_t *pmdp);
+};
+
+/*
+ * struct hmm_devmem - track device memory
+ *
+ * @completion: completion object for device memory
+ * @pfn_first: first pfn for this resource (set by hmm_devmem_add())
+ * @pfn_last: last pfn for this resource (set by hmm_devmem_add())
+ * @resource: IO resource reserved for this chunk of memory
+ * @pagemap: device page map for that chunk
+ * @device: device to bind resource to
+ * @ops: memory operations callback
+ * @ref: per CPU refcount
+ *
+ * This an helper structure for device drivers that do not wish to implement
+ * the gory details related to hotplugging new memoy and allocating struct
+ * pages.
+ *
+ * Device drivers can directly use ZONE_DEVICE memory on their own if they
+ * wish to do so.
+ */
+struct hmm_devmem {
+	struct completion		completion;
+	unsigned long			pfn_first;
+	unsigned long			pfn_last;
+	struct resource			*resource;
+	struct device			*device;
+	struct dev_pagemap		pagemap;
+	const struct hmm_devmem_ops	*ops;
+	struct percpu_ref		ref;
+};
+
+/*
+ * To add (hotplug) device memory, HMM assumes that there is no real resource
+ * that reserves a range in the physical address space (this is intended to be
+ * use by unaddressable device memory). It will reserve a physical range big
+ * enough and allocate struct page for it.
+ *
+ * The device driver can wrap the hmm_devmem struct inside a private device
+ * driver struct. The device driver must call hmm_devmem_remove() before the
+ * device goes away and before freeing the hmm_devmem struct memory.
+ */
+struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
+				  struct device *device,
+				  unsigned long size);
+void hmm_devmem_remove(struct hmm_devmem *devmem);
+
+int hmm_devmem_fault_range(struct hmm_devmem *devmem,
+			   struct vm_area_struct *vma,
+			   const struct migrate_vma_ops *ops,
+			   unsigned long *src,
+			   unsigned long *dst,
+			   unsigned long start,
+			   unsigned long addr,
+			   unsigned long end,
+			   void *private);
+
+/*
+ * hmm_devmem_page_set_drvdata - set per-page driver data field
+ *
+ * @page: pointer to struct page
+ * @data: driver data value to set
+ *
+ * Because page can not be on lru we have an unsigned long that driver can use
+ * to store a per page field. This just a simple helper to do that.
+ */
+static inline void hmm_devmem_page_set_drvdata(struct page *page,
+					       unsigned long data)
+{
+	unsigned long *drvdata = (unsigned long *)&page->pgmap;
+
+	drvdata[1] = data;
+}
+
+/*
+ * hmm_devmem_page_get_drvdata - get per page driver data field
+ *
+ * @page: pointer to struct page
+ * Return: driver data value
+ */
+static inline unsigned long hmm_devmem_page_get_drvdata(struct page *page)
+{
+	unsigned long *drvdata = (unsigned long *)&page->pgmap;
+
+	return drvdata[1];
+}
+#endif /* IS_ENABLED(CONFIG_HMM_DEVMEM) */
+
+
 /* Below are for HMM internal use only! Not to be used by device driver! */
 void hmm_mm_destroy(struct mm_struct *mm);
 
diff --git a/mm/Kconfig b/mm/Kconfig
index 3c1ffb1..ba5745f 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -314,6 +314,15 @@ config HMM_MIRROR
 	  page tables (at PAGE_SIZE granularity), and must be able to recover from
 	  the resulting potential page faults.
 
+config HMM_DEVMEM
+	bool "HMM device memory helpers (to leverage ZONE_DEVICE)"
+	depends on ARCH_HAS_HMM
+	select HMM
+	help
+	  HMM devmem is a set of helper routines to leverage the ZONE_DEVICE
+	  feature. This is just to avoid having device drivers to replicating a lot
+	  of boiler plate code.  See Documentation/vm/hmm.txt.
+
 config PHYS_ADDR_T_64BIT
 	def_bool 64BIT || ARCH_PHYS_ADDR_T_64BIT
 
diff --git a/mm/hmm.c b/mm/hmm.c
index be88807..5d882e6 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -23,9 +23,15 @@
 #include <linux/swap.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
+#include <linux/mmzone.h>
+#include <linux/pagemap.h>
 #include <linux/swapops.h>
 #include <linux/hugetlb.h>
+#include <linux/memremap.h>
 #include <linux/mmu_notifier.h>
+#include <linux/memory_hotplug.h>
+
+#define PA_SECTION_SIZE (1UL << PA_SECTION_SHIFT)
 
 static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
 
@@ -709,3 +715,401 @@ int hmm_vma_fault(struct vm_area_struct *vma,
 }
 EXPORT_SYMBOL(hmm_vma_fault);
 #endif /* IS_ENABLED(CONFIG_HMM_MIRROR) */
+
+
+#if IS_ENABLED(CONFIG_HMM_DEVMEM)
+struct page *hmm_vma_alloc_locked_page(struct vm_area_struct *vma,
+				       unsigned long addr)
+{
+	struct page *page;
+
+	page = alloc_page_vma(GFP_HIGHUSER, vma, addr);
+	if (!page)
+		return NULL;
+	lock_page(page);
+	return page;
+}
+EXPORT_SYMBOL(hmm_vma_alloc_locked_page);
+
+
+static void hmm_devmem_ref_release(struct percpu_ref *ref)
+{
+	struct hmm_devmem *devmem;
+
+	devmem = container_of(ref, struct hmm_devmem, ref);
+	complete(&devmem->completion);
+}
+
+static void hmm_devmem_ref_exit(void *data)
+{
+	struct percpu_ref *ref = data;
+	struct hmm_devmem *devmem;
+
+	devmem = container_of(ref, struct hmm_devmem, ref);
+	percpu_ref_exit(ref);
+	devm_remove_action(devmem->device, &hmm_devmem_ref_exit, data);
+}
+
+static void hmm_devmem_ref_kill(void *data)
+{
+	struct percpu_ref *ref = data;
+	struct hmm_devmem *devmem;
+
+	devmem = container_of(ref, struct hmm_devmem, ref);
+	percpu_ref_kill(ref);
+	wait_for_completion(&devmem->completion);
+	devm_remove_action(devmem->device, &hmm_devmem_ref_kill, data);
+}
+
+static int hmm_devmem_fault(struct vm_area_struct *vma,
+			    unsigned long addr,
+			    struct page *page,
+			    unsigned int flags,
+			    pmd_t *pmdp)
+{
+	struct hmm_devmem *devmem = page->pgmap->data;
+
+	return devmem->ops->fault(devmem, vma, addr, page, flags, pmdp);
+}
+
+static void hmm_devmem_free(struct page *page, void *data)
+{
+	struct hmm_devmem *devmem = data;
+
+	devmem->ops->free(devmem, page);
+}
+
+static DEFINE_MUTEX(hmm_devmem_lock);
+static RADIX_TREE(hmm_devmem_radix, GFP_KERNEL);
+
+static void hmm_devmem_radix_release(struct resource *resource)
+{
+	resource_size_t key, align_start, align_size, align_end;
+
+	align_start = resource->start & ~(PA_SECTION_SIZE - 1);
+	align_size = ALIGN(resource_size(resource), PA_SECTION_SIZE);
+	align_end = align_start + align_size - 1;
+
+	mutex_lock(&hmm_devmem_lock);
+	for (key = resource->start;
+	     key <= resource->end;
+	     key += PA_SECTION_SIZE)
+		radix_tree_delete(&hmm_devmem_radix, key >> PA_SECTION_SHIFT);
+	mutex_unlock(&hmm_devmem_lock);
+}
+
+static void hmm_devmem_release(struct device *dev, void *data)
+{
+	struct hmm_devmem *devmem = data;
+	struct resource *resource = devmem->resource;
+	unsigned long start_pfn, npages;
+	struct zone *zone;
+	struct page *page;
+
+	if (percpu_ref_tryget_live(&devmem->ref)) {
+		dev_WARN(dev, "%s: page mapping is still live!\n", __func__);
+		percpu_ref_put(&devmem->ref);
+	}
+
+	/* pages are dead and unused, undo the arch mapping */
+	start_pfn = (resource->start & ~(PA_SECTION_SIZE - 1)) >> PAGE_SHIFT;
+	npages = ALIGN(resource_size(resource), PA_SECTION_SIZE) >> PAGE_SHIFT;
+
+	page = pfn_to_page(start_pfn);
+	zone = page_zone(page);
+
+	mem_hotplug_begin();
+	__remove_pages(zone, start_pfn, npages);
+	mem_hotplug_done();
+
+	hmm_devmem_radix_release(resource);
+}
+
+static struct hmm_devmem *hmm_devmem_find(resource_size_t phys)
+{
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
+	return radix_tree_lookup(&hmm_devmem_radix, phys >> PA_SECTION_SHIFT);
+}
+
+static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
+{
+	resource_size_t key, align_start, align_size, align_end;
+	struct device *device = devmem->device;
+	int ret, nid, is_ram;
+	unsigned long pfn;
+
+	align_start = devmem->resource->start & ~(PA_SECTION_SIZE - 1);
+	align_size = ALIGN(devmem->resource->start +
+			   resource_size(devmem->resource),
+			   PA_SECTION_SIZE) - align_start;
+
+	is_ram = region_intersects(align_start, align_size,
+				   IORESOURCE_SYSTEM_RAM,
+				   IORES_DESC_NONE);
+	if (is_ram == REGION_MIXED) {
+		WARN_ONCE(1, "%s attempted on mixed region %pr\n",
+				__func__, devmem->resource);
+		return -ENXIO;
+	}
+	if (is_ram == REGION_INTERSECTS)
+		return -ENXIO;
+
+	devmem->pagemap.type = MEMORY_DEVICE_UNADDRESSABLE;
+	devmem->pagemap.res = devmem->resource;
+	devmem->pagemap.page_fault = hmm_devmem_fault;
+	devmem->pagemap.page_free = hmm_devmem_free;
+	devmem->pagemap.dev = devmem->device;
+	devmem->pagemap.ref = &devmem->ref;
+	devmem->pagemap.data = devmem;
+
+	mutex_lock(&hmm_devmem_lock);
+	align_end = align_start + align_size - 1;
+	for (key = align_start; key <= align_end; key += PA_SECTION_SIZE) {
+		struct hmm_devmem *dup;
+
+		rcu_read_lock();
+		dup = hmm_devmem_find(key);
+		rcu_read_unlock();
+		if (dup) {
+			dev_err(device, "%s: collides with mapping for %s\n",
+				__func__, dev_name(dup->device));
+			mutex_unlock(&hmm_devmem_lock);
+			ret = -EBUSY;
+			goto error;
+		}
+		ret = radix_tree_insert(&hmm_devmem_radix,
+					key >> PA_SECTION_SHIFT,
+					devmem);
+		if (ret) {
+			dev_err(device, "%s: failed: %d\n", __func__, ret);
+			mutex_unlock(&hmm_devmem_lock);
+			goto error_radix;
+		}
+	}
+	mutex_unlock(&hmm_devmem_lock);
+
+	nid = dev_to_node(device);
+	if (nid < 0)
+		nid = numa_mem_id();
+
+	mem_hotplug_begin();
+	ret = add_pages(nid, align_start >> PAGE_SHIFT,
+			align_size >> PAGE_SHIFT, false);
+	if (ret) {
+		mem_hotplug_done();
+		goto error_add_memory;
+	}
+	move_pfn_range_to_zone(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
+				align_start >> PAGE_SHIFT,
+				align_size >> PAGE_SHIFT);
+	mem_hotplug_done();
+
+	for (pfn = devmem->pfn_first; pfn < devmem->pfn_last; pfn++) {
+		struct page *page = pfn_to_page(pfn);
+
+		/*
+		 * ZONE_DEVICE pages union ->lru with a ->pgmap back
+		 * pointer.  It is a bug if a ZONE_DEVICE page is ever
+		 * freed or placed on a driver-private list. Therefore,
+		 * seed the storage with LIST_POISON* values.
+		 */
+		list_del(&page->lru);
+		page->pgmap = &devmem->pagemap;
+	}
+	return 0;
+
+error_add_memory:
+	untrack_pfn(NULL, PHYS_PFN(align_start), align_size);
+error_radix:
+	hmm_devmem_radix_release(devmem->resource);
+error:
+	return ret;
+}
+
+static int hmm_devmem_match(struct device *dev, void *data, void *match_data)
+{
+	struct hmm_devmem *devmem = data;
+
+	return devmem->resource == match_data;
+}
+
+static void hmm_devmem_pages_remove(struct hmm_devmem *devmem)
+{
+	devres_release(devmem->device, &hmm_devmem_release,
+		       &hmm_devmem_match, devmem->resource);
+}
+
+/*
+ * hmm_devmem_add() - hotplug ZONE_DEVICE memory for device memory
+ *
+ * @ops: memory event device driver callback (see struct hmm_devmem_ops)
+ * @device: device struct to bind the resource too
+ * @size: size in bytes of the device memory to add
+ * Returns: pointer to new hmm_devmem struct ERR_PTR otherwise
+ *
+ * This first finds an empty range of physical address big enough to contain the
+ * new resource, and then hotplugs it as ZONE_DEVICE memory, which in turn
+ * allocates struct pages. It does not do anything beyond that; all events
+ * affecting the memory will go through the various callbacks provided by
+ * hmm_devmem_ops struct.
+ */
+struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
+				  struct device *device,
+				  unsigned long size)
+{
+	struct hmm_devmem *devmem;
+	resource_size_t addr;
+	int ret;
+
+	devmem = devres_alloc_node(&hmm_devmem_release, sizeof(*devmem),
+				   GFP_KERNEL, dev_to_node(device));
+	if (!devmem)
+		return ERR_PTR(-ENOMEM);
+
+	init_completion(&devmem->completion);
+	devmem->pfn_first = -1UL;
+	devmem->pfn_last = -1UL;
+	devmem->resource = NULL;
+	devmem->device = device;
+	devmem->ops = ops;
+
+	ret = percpu_ref_init(&devmem->ref, &hmm_devmem_ref_release,
+			      0, GFP_KERNEL);
+	if (ret)
+		goto error_percpu_ref;
+
+	ret = devm_add_action(device, hmm_devmem_ref_exit, &devmem->ref);
+	if (ret)
+		goto error_devm_add_action;
+
+	size = ALIGN(size, PA_SECTION_SIZE);
+	addr = min((unsigned long)iomem_resource.end, 1UL << MAX_PHYSMEM_BITS);
+	addr = addr - size + 1UL;
+
+	/*
+	 * FIXME add a new helper to quickly walk resource tree and find free
+	 * range
+	 *
+	 * FIXME what about ioport_resource resource ?
+	 */
+	for (; addr > size && addr >= iomem_resource.start; addr -= size) {
+		ret = region_intersects(addr, size, 0, IORES_DESC_NONE);
+		if (ret != REGION_DISJOINT)
+			continue;
+
+		devmem->resource = devm_request_mem_region(device, addr, size,
+							   dev_name(device));
+		if (!devmem->resource) {
+			ret = -ENOMEM;
+			goto error_no_resource;
+		}
+		break;
+	}
+	if (!devmem->resource) {
+		ret = -ERANGE;
+		goto error_no_resource;
+	}
+
+	devmem->resource->desc = IORES_DESC_DEVICE_MEMORY_UNADDRESSABLE;
+	devmem->pfn_first = devmem->resource->start >> PAGE_SHIFT;
+	devmem->pfn_last = devmem->pfn_first +
+			   (resource_size(devmem->resource) >> PAGE_SHIFT);
+
+	ret = hmm_devmem_pages_create(devmem);
+	if (ret)
+		goto error_pages;
+
+	devres_add(device, devmem);
+
+	ret = devm_add_action(device, hmm_devmem_ref_kill, &devmem->ref);
+	if (ret) {
+		hmm_devmem_remove(devmem);
+		return ERR_PTR(ret);
+	}
+
+	return devmem;
+
+error_pages:
+	devm_release_mem_region(device, devmem->resource->start,
+				resource_size(devmem->resource));
+error_no_resource:
+error_devm_add_action:
+	hmm_devmem_ref_kill(&devmem->ref);
+	hmm_devmem_ref_exit(&devmem->ref);
+error_percpu_ref:
+	devres_free(devmem);
+	return ERR_PTR(ret);
+}
+EXPORT_SYMBOL(hmm_devmem_add);
+
+/*
+ * hmm_devmem_remove() - remove device memory (kill and free ZONE_DEVICE)
+ *
+ * @devmem: hmm_devmem struct use to track and manage the ZONE_DEVICE memory
+ *
+ * This will hot-unplug memory that was hotplugged by hmm_devmem_add on behalf
+ * of the device driver. It will free struct page and remove the resource that
+ * reserved the physical address range for this device memory.
+ */
+void hmm_devmem_remove(struct hmm_devmem *devmem)
+{
+	resource_size_t start, size;
+	struct device *device;
+
+	if (!devmem)
+		return;
+
+	device = devmem->device;
+	start = devmem->resource->start;
+	size = resource_size(devmem->resource);
+
+	hmm_devmem_ref_kill(&devmem->ref);
+	hmm_devmem_ref_exit(&devmem->ref);
+	hmm_devmem_pages_remove(devmem);
+
+	devm_release_mem_region(device, start, size);
+}
+EXPORT_SYMBOL(hmm_devmem_remove);
+
+/*
+ * hmm_devmem_fault_range() - migrate back a virtual range of memory
+ *
+ * @devmem: hmm_devmem struct use to track and manage the ZONE_DEVICE memory
+ * @vma: virtual memory area containing the range to be migrated
+ * @ops: migration callback for allocating destination memory and copying
+ * @src: array of unsigned long containing source pfns
+ * @dst: array of unsigned long containing destination pfns
+ * @start: start address of the range to migrate (inclusive)
+ * @addr: fault address (must be inside the range)
+ * @end: end address of the range to migrate (exclusive)
+ * @private: pointer passed back to each of the callback
+ * Returns: 0 on success, VM_FAULT_SIGBUS on error
+ *
+ * This is a wrapper around migrate_vma() which checks the migration status
+ * for a given fault address and returns the corresponding page fault handler
+ * status. That will be 0 on success, or VM_FAULT_SIGBUS if migration failed
+ * for the faulting address.
+ *
+ * This is a helper intendend to be used by the ZONE_DEVICE fault handler.
+ */
+int hmm_devmem_fault_range(struct hmm_devmem *devmem,
+			   struct vm_area_struct *vma,
+			   const struct migrate_vma_ops *ops,
+			   unsigned long *src,
+			   unsigned long *dst,
+			   unsigned long start,
+			   unsigned long addr,
+			   unsigned long end,
+			   void *private)
+{
+	if (migrate_vma(ops, vma, start, end, src, dst, private))
+		return VM_FAULT_SIGBUS;
+
+	if (dst[(addr - start) >> PAGE_SHIFT] & MIGRATE_PFN_ERROR)
+		return VM_FAULT_SIGBUS;
+
+	return 0;
+}
+EXPORT_SYMBOL(hmm_devmem_fault_range);
+#endif /* IS_ENABLED(CONFIG_HMM_DEVMEM) */
-- 
2.9.3

[HMM 14/15] mm/hmm/devmem: dummy HMM device for ZONE_DEVICE memory v3

[Jérôme Glisse]

This introduce a dummy HMM device class so device driver can use it to
create hmm_device for the sole purpose of registering device memory.
It is useful to device driver that want to manage multiple physical
device memory under same struct device umbrella.

Changed since v2:
  - use device_initcall() and drop everything that is module specific
Changed since v1:
  - Improve commit message
  - Add drvdata parameter to set on struct device

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Evgeny Baskakov <ebaskakov@nvidia.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Mark Hairgrove <mhairgrove@nvidia.com>
Signed-off-by: Sherry Cheung <SCheung@nvidia.com>
Signed-off-by: Subhash Gutti <sgutti@nvidia.com>
---
 include/linux/hmm.h | 22 +++++++++++++-
 mm/hmm.c            | 88 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 109 insertions(+), 1 deletion(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 50a1115..374e5fd 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -72,11 +72,11 @@
 
 #if IS_ENABLED(CONFIG_HMM)
 
+#include <linux/device.h>
 #include <linux/migrate.h>
 #include <linux/memremap.h>
 #include <linux/completion.h>
 
-
 struct hmm;
 
 /*
@@ -433,6 +433,26 @@ static inline unsigned long hmm_devmem_page_get_drvdata(struct page *page)
 
 	return drvdata[1];
 }
+
+
+/*
+ * struct hmm_device - fake device to hang device memory onto
+ *
+ * @device: device struct
+ * @minor: device minor number
+ */
+struct hmm_device {
+	struct device		device;
+	unsigned int		minor;
+};
+
+/*
+ * A device driver that wants to handle multiple devices memory through a
+ * single fake device can use hmm_device to do so. This is purely a helper and
+ * it is not strictly needed, in order to make use of any HMM functionality.
+ */
+struct hmm_device *hmm_device_new(void *drvdata);
+void hmm_device_put(struct hmm_device *hmm_device);
 #endif /* IS_ENABLED(CONFIG_HMM_DEVMEM) */
 
 
diff --git a/mm/hmm.c b/mm/hmm.c
index 5d882e6..8b6b4c6 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -19,6 +19,7 @@
  */
 #include <linux/mm.h>
 #include <linux/hmm.h>
+#include <linux/init.h>
 #include <linux/rmap.h>
 #include <linux/swap.h>
 #include <linux/slab.h>
@@ -1112,4 +1113,91 @@ int hmm_devmem_fault_range(struct hmm_devmem *devmem,
 	return 0;
 }
 EXPORT_SYMBOL(hmm_devmem_fault_range);
+
+/*
+ * A device driver that wants to handle multiple devices memory through a
+ * single fake device can use hmm_device to do so. This is purely a helper
+ * and it is not needed to make use of any HMM functionality.
+ */
+#define HMM_DEVICE_MAX 256
+
+static DECLARE_BITMAP(hmm_device_mask, HMM_DEVICE_MAX);
+static DEFINE_SPINLOCK(hmm_device_lock);
+static struct class *hmm_device_class;
+static dev_t hmm_device_devt;
+
+static void hmm_device_release(struct device *device)
+{
+	struct hmm_device *hmm_device;
+
+	hmm_device = container_of(device, struct hmm_device, device);
+	spin_lock(&hmm_device_lock);
+	clear_bit(hmm_device->minor, hmm_device_mask);
+	spin_unlock(&hmm_device_lock);
+
+	kfree(hmm_device);
+}
+
+struct hmm_device *hmm_device_new(void *drvdata)
+{
+	struct hmm_device *hmm_device;
+	int ret;
+
+	hmm_device = kzalloc(sizeof(*hmm_device), GFP_KERNEL);
+	if (!hmm_device)
+		return ERR_PTR(-ENOMEM);
+
+	ret = alloc_chrdev_region(&hmm_device->device.devt, 0, 1, "hmm_device");
+	if (ret < 0) {
+		kfree(hmm_device);
+		return NULL;
+	}
+
+	spin_lock(&hmm_device_lock);
+	hmm_device->minor = find_first_zero_bit(hmm_device_mask, HMM_DEVICE_MAX);
+	if (hmm_device->minor >= HMM_DEVICE_MAX) {
+		spin_unlock(&hmm_device_lock);
+		kfree(hmm_device);
+		return NULL;
+	}
+	set_bit(hmm_device->minor, hmm_device_mask);
+	spin_unlock(&hmm_device_lock);
+
+	dev_set_name(&hmm_device->device, "hmm_device%d", hmm_device->minor);
+	hmm_device->device.devt = MKDEV(MAJOR(hmm_device_devt),
+					hmm_device->minor);
+	hmm_device->device.release = hmm_device_release;
+	dev_set_drvdata(&hmm_device->device, drvdata);
+	hmm_device->device.class = hmm_device_class;
+	device_initialize(&hmm_device->device);
+
+	return hmm_device;
+}
+EXPORT_SYMBOL(hmm_device_new);
+
+void hmm_device_put(struct hmm_device *hmm_device)
+{
+	put_device(&hmm_device->device);
+}
+EXPORT_SYMBOL(hmm_device_put);
+
+static int __init hmm_init(void)
+{
+	int ret;
+
+	ret = alloc_chrdev_region(&hmm_device_devt, 0,
+				  HMM_DEVICE_MAX,
+				  "hmm_device");
+	if (ret)
+		return ret;
+
+	hmm_device_class = class_create(THIS_MODULE, "hmm_device");
+	if (IS_ERR(hmm_device_class)) {
+		unregister_chrdev_region(hmm_device_devt, HMM_DEVICE_MAX);
+		return PTR_ERR(hmm_device_class);
+	}
+	return 0;
+}
+
+device_initcall(hmm_init);
 #endif /* IS_ENABLED(CONFIG_HMM_DEVMEM) */
-- 
2.9.3

[HMM 15/15] hmm: heterogeneous memory management documentation

[Jérôme Glisse]

This add documentation for HMM (Heterogeneous Memory Management). It
presents the motivation behind it, the features necessary for it to
be useful and and gives an overview of how this is implemented.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
---
 Documentation/vm/hmm.txt | 362 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 362 insertions(+)
 create mode 100644 Documentation/vm/hmm.txt

diff --git a/Documentation/vm/hmm.txt b/Documentation/vm/hmm.txt
new file mode 100644
index 0000000..a18ffc0
--- /dev/null
+++ b/Documentation/vm/hmm.txt
@@ -0,0 +1,362 @@
+Heterogeneous Memory Management (HMM)
+
+Transparently allow any component of a program to use any memory region of said
+program with a device without using device specific memory allocator. This is
+becoming a requirement to simplify the use of advance heterogeneous computing
+where GPU, DSP or FPGA are use to perform various computations.
+
+This document is divided as follow, in the first section i expose the problems
+related to the use of a device specific allocator. The second section i expose
+the hardware limitations that are inherent to many platforms. The third section
+gives an overview of HMM designs. The fourth section explains how CPU page-
+table mirroring works and what is HMM purpose in this context. Fifth section
+deals with how device memory is represented inside the kernel. Finaly the last
+section present the new migration helper that allow to leverage the device DMA
+engine.
+
+
+-------------------------------------------------------------------------------
+
+1) Problems of using device specific memory allocator:
+
+Device with large amount of on board memory (several giga bytes) like GPU have
+historically manage their memory through dedicated driver specific API. This
+creates a disconnect between memory allocated and managed by device driver and
+regular application memory (private anonymous, share memory or regular file
+back memory). From here on i will refer to this aspect as split address space.
+I use share address space to refer to the opposite situation ie one in which
+any memory region can be use by device transparently.
+
+Split address space because device can only access memory allocated through the
+device specific API. This imply that all memory object in a program are not
+equal from device point of view which complicate large program that rely on a
+wide set of libraries.
+
+Concretly this means that code that wants to leverage device like GPU need to
+copy object between genericly allocated memory (malloc, mmap private/share/)
+and memory allocated through the device driver API (this still end up with an
+mmap but of the device file).
+
+For flat dataset (array, grid, image, ...) this isn't too hard to achieve but
+complex data-set (list, tree, ...) are hard to get right. Duplicating a complex
+data-set need to re-map all the pointer relations between each of its elements.
+This is error prone and program gets harder to debug because of the duplicate
+data-set.
+
+Split address space also means that library can not transparently use data they
+are getting from core program or other library and thus each library might have
+to duplicate its input data-set using specific memory allocator. Large project
+suffer from this and waste resources because of the various memory copy.
+
+Duplicating each library API to accept as input or output memory allocted by
+each device specific allocator is not a viable option. It would lead to a
+combinatorial explosions in the library entry points.
+
+Finaly with the advance of high level language constructs (in C++ but in other
+language too) it is now possible for compiler to leverage GPU or other devices
+without even the programmer knowledge. Some of compiler identified patterns are
+only do-able with a share address. It is as well more reasonable to use a share
+address space for all the other patterns.
+
+
+-------------------------------------------------------------------------------
+
+2) System bus, device memory characteristics
+
+System bus cripple share address due to few limitations. Most system bus only
+allow basic memory access from device to main memory, even cache coherency is
+often optional. Access to device memory from CPU is even more limited, most
+often than not it is not cache coherent.
+
+If we only consider the PCIE bus than device can access main memory (often
+through an IOMMU) and be cache coherent with the CPUs. However it only allows
+a limited set of atomic operation from device on main memory. This is worse
+in the other direction the CPUs can only access a limited range of the device
+memory and can not perform atomic operations on it. Thus device memory can not
+be consider like regular memory from kernel point of view.
+
+Another crippling factor is the limited bandwidth (~32GBytes/s with PCIE 4.0
+and 16 lanes). This is 33 times less that fastest GPU memory (1 TBytes/s).
+The final limitation is latency, access to main memory from the device has an
+order of magnitude higher latency than when the device access its own memory.
+
+Some platform are developing new system bus or additions/modifications to PCIE
+to address some of those limitations (OpenCAPI, CCIX). They mainly allow two
+way cache coherency between CPU and device and allow all atomic operations the
+architecture supports. Saddly not all platform are following this trends and
+some major architecture are left without hardware solutions to those problems.
+
+So for share address space to make sense not only we must allow device to
+access any memory memory but we must also permit any memory to be migrated to
+device memory while device is using it (blocking CPU access while it happens).
+
+
+-------------------------------------------------------------------------------
+
+3) Share address space and migration
+
+HMM intends to provide two main features. First one is to share the address
+space by duplication the CPU page table into the device page table so same
+address point to same memory and this for any valid main memory address in
+the process address space.
+
+To achieve this, HMM offer a set of helpers to populate the device page table
+while keeping track of CPU page table updates. Device page table updates are
+not as easy as CPU page table updates. To update the device page table you must
+allow a buffer (or use a pool of pre-allocated buffer) and write GPU specifics
+commands in it to perform the update (unmap, cache invalidations and flush,
+...). This can not be done through common code for all device. Hence why HMM
+provides helpers to factor out everything that can be while leaving the gory
+details to the device driver.
+
+The second mechanism HMM provide is a new kind of ZONE_DEVICE memory that does
+allow to allocate a struct page for each page of the device memory. Those page
+are special because the CPU can not map them. They however allow to migrate
+main memory to device memory using exhisting migration mechanism and everything
+looks like if page was swap out to disk from CPU point of view. Using a struct
+page gives the easiest and cleanest integration with existing mm mechanisms.
+Again here HMM only provide helpers, first to hotplug new ZONE_DEVICE memory
+for the device memory and second to perform migration. Policy decision of what
+and when to migrate things is left to the device driver.
+
+Note that any CPU access to a device page trigger a page fault and a migration
+back to main memory ie when a page backing an given address A is migrated from
+a main memory page to a device page then any CPU access to address A trigger a
+page fault and initiate a migration back to main memory.
+
+
+With this two features, HMM not only allow a device to mirror a process address
+space and keeps both CPU and device page table synchronize, but also allow to
+leverage device memory by migrating part of data-set that is actively use by a
+device.
+
+
+-------------------------------------------------------------------------------
+
+4) Address space mirroring implementation and API
+
+Address space mirroring main objective is to allow to duplicate range of CPU
+page table into a device page table and HMM helps keeping both synchronize. A
+device driver that want to mirror a process address space must start with the
+registration of an hmm_mirror struct:
+
+ int hmm_mirror_register(struct hmm_mirror *mirror,
+                         struct mm_struct *mm);
+ int hmm_mirror_register_locked(struct hmm_mirror *mirror,
+                                struct mm_struct *mm);
+
+The locked variant is to be use when the driver is already holding the mmap_sem
+of the mm in write mode. The mirror struct has a set of callback that are use
+to propagate CPU page table:
+
+ struct hmm_mirror_ops {
+     /* sync_cpu_device_pagetables() - synchronize page tables
+      *
+      * @mirror: pointer to struct hmm_mirror
+      * @update_type: type of update that occurred to the CPU page table
+      * @start: virtual start address of the range to update
+      * @end: virtual end address of the range to update
+      *
+      * This callback ultimately originates from mmu_notifiers when the CPU
+      * page table is updated. The device driver must update its page table
+      * in response to this callback. The update argument tells what action
+      * to perform.
+      *
+      * The device driver must not return from this callback until the device
+      * page tables are completely updated (TLBs flushed, etc); this is a
+      * synchronous call.
+      */
+      void (*update)(struct hmm_mirror *mirror,
+                     enum hmm_update action,
+                     unsigned long start,
+                     unsigned long end);
+ };
+
+Device driver must perform update to the range following action (turn range
+read only, or fully unmap, ...). Once driver callback returns the device must
+be done with the update.
+
+
+When device driver wants to populate a range of virtual address it can use
+either:
+ int hmm_vma_get_pfns(struct vm_area_struct *vma,
+                      struct hmm_range *range,
+                      unsigned long start,
+                      unsigned long end,
+                      hmm_pfn_t *pfns);
+ int hmm_vma_fault(struct vm_area_struct *vma,
+                   struct hmm_range *range,
+                   unsigned long start,
+                   unsigned long end,
+                   hmm_pfn_t *pfns,
+                   bool write,
+                   bool block);
+
+First one (hmm_vma_get_pfns()) will only fetch present CPU page table entry and
+will not trigger a page fault on missing or non present entry. The second one
+do trigger page fault on missing or read only entry if write parameter is true.
+Page fault use the generic mm page fault code path just like a CPU page fault.
+
+Both function copy CPU page table into their pfns array argument. Each entry in
+that array correspond to an address in the virtual range. HMM provide a set of
+flags to help driver identify special CPU page table entries.
+
+Locking with the update() callback is the most important aspect the driver must
+respect in order to keep things properly synchronize. The usage pattern is :
+
+ int driver_populate_range(...)
+ {
+      struct hmm_range range;
+      ...
+ again:
+      ret = hmm_vma_get_pfns(vma, &range, start, end, pfns);
+      if (ret)
+          return ret;
+      take_lock(driver->update);
+      if (!hmm_vma_range_done(vma, &range)) {
+          release_lock(driver->update);
+          goto again;
+      }
+
+      // Use pfns array content to update device page table
+
+      release_lock(driver->update);
+      return 0;
+ }
+
+The driver->update lock is the same lock that driver takes inside its update()
+callback. That lock must be call before hmm_vma_range_done() to avoid any race
+with a concurrent CPU page table update.
+
+HMM implements all this on top of the mmu_notifier API because we wanted to a
+simpler API and also to be able to perform optimization latter own like doing
+concurrent device update in multi-devices scenario.
+
+HMM also serve as an impedence missmatch between how CPU page table update are
+done (by CPU write to the page table and TLB flushes) from how device update
+their own page table. Device update is a multi-step process, first appropriate
+commands are write to a buffer, then this buffer is schedule for execution on
+the device. It is only once the device has executed commands in the buffer that
+the update is done. Creating and scheduling update command buffer can happen
+concurrently for multiple devices. Waiting for each device to report commands
+as executed is serialize (there is no point in doing this concurrently).
+
+
+-------------------------------------------------------------------------------
+
+5) Represent and manage device memory from core kernel point of view
+
+Several differents design were try to support device memory. First one use
+device specific data structure to keep information about migrated memory and
+HMM hooked itself in various place of mm code to handle any access to address
+that were back by device memory. It turns out that this ended up replicating
+most of the fields of struct page and also needed many kernel code path to be
+updated to understand this new kind of memory.
+
+Thing is most kernel code path never try to access the memory behind a page
+but only care about struct page contents. Because of this HMM switchted to
+directly using struct page for device memory which left most kernel code path
+un-aware of the difference. We only need to make sure that no one ever try to
+map those page from the CPU side.
+
+HMM provide a set of helpers to register and hotplug device memory as a new
+region needing struct page. This is offer through a very simple API:
+
+ struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
+                                   struct device *device,
+                                   unsigned long size);
+ void hmm_devmem_remove(struct hmm_devmem *devmem);
+
+The hmm_devmem_ops is where most of the important things are:
+
+ struct hmm_devmem_ops {
+     void (*free)(struct hmm_devmem *devmem, struct page *page);
+     int (*fault)(struct hmm_devmem *devmem,
+                  struct vm_area_struct *vma,
+                  unsigned long addr,
+                  struct page *page,
+                  unsigned flags,
+                  pmd_t *pmdp);
+ };
+
+The first callback (free()) happens when the last reference on a device page is
+drop. This means the device page is now free and no longer use by anyone. The
+second callback happens whenever CPU try to access a device page which it can
+not do. This second callback must trigger a migration back to system memory,
+HMM provides an helper to do just that:
+
+ int hmm_devmem_fault_range(struct hmm_devmem *devmem,
+                            struct vm_area_struct *vma,
+                            const struct migrate_vma_ops *ops,
+                            unsigned long mentry,
+                            unsigned long *src,
+                            unsigned long *dst,
+                            unsigned long start,
+                            unsigned long addr,
+                            unsigned long end,
+                            void *private);
+
+It relies on new migrate_vma() helper which is a generic page migration helper
+that work on range of virtual address instead of working on individual pages,
+it also allow to leverage device DMA engine to perform the copy from device to
+main memory (or in the other direction). The next section goes over this new
+helper.
+
+
+-------------------------------------------------------------------------------
+
+6) Migrate to and from device memory
+
+Because CPU can not access device memory, migration must use device DMA engine
+to perform copy from and to device memory. For this we need a new migration
+helper:
+
+ int migrate_vma(const struct migrate_vma_ops *ops,
+                 struct vm_area_struct *vma,
+                 unsigned long mentries,
+                 unsigned long start,
+                 unsigned long end,
+                 unsigned long *src,
+                 unsigned long *dst,
+                 void *private);
+
+Unlike other migration function it works on a range of virtual address, there
+is two reasons for that. First device DMA copy has a high setup overhead cost
+and thus batching multiple pages is needed as otherwise the migration overhead
+make the whole excersie pointless. The second reason is because driver trigger
+such migration base on range of address the device is actively accessing.
+
+The migrate_vma_ops struct define two callbacks. First one (alloc_and_copy())
+control destination memory allocation and copy operation. Second one is there
+to allow device driver to perform cleanup operation after migration.
+
+ struct migrate_vma_ops {
+     void (*alloc_and_copy)(struct vm_area_struct *vma,
+                            const unsigned long *src,
+                            unsigned long *dst,
+                            unsigned long start,
+                            unsigned long end,
+                            void *private);
+     void (*finalize_and_map)(struct vm_area_struct *vma,
+                              const unsigned long *src,
+                              const unsigned long *dst,
+                              unsigned long start,
+                              unsigned long end,
+                              void *private);
+ };
+
+It is important to stress that this migration helpers allow for hole in the
+virtual address range. Some pages in the range might not be migrated for all
+the usual reasons (page is pin, page is lock, ...). This helper does not fail
+but just skip over those pages.
+
+The alloc_and_copy() might as well decide to not migrate all pages in the
+range (for reasons under the callback control). For those the callback just
+have to leave the corresponding dst entry empty.
+
+Finaly the migration of the struct page might fails (for file back page) for
+various reasons (failure to freeze reference, or update page cache, ...). If
+that happens then the finalize_and_map() can catch any pages that was not
+migrated. Note those page were still copied to new page and thus we wasted
+bandwidth but this is considered as a rare event and a price that we are
+willing to pay to keep all the code simpler.
-- 
2.9.3

Re: [HMM 02/15] mm/put_page: move ZONE_DEVICE page reference decrement v2

[Dan Williams]

On Fri, Apr 21, 2017 at 8:30 PM, Jérôme Glisse <jglisse@redhat.com> wrote:
> Move page reference decrement of ZONE_DEVICE from put_page()
> to put_zone_device_page() this does not affect non ZONE_DEVICE
> page.
>
> Doing this allow to catch when a ZONE_DEVICE page refcount reach
> 1 which means the device is no longer reference by any one (unlike
> page from other zone, ZONE_DEVICE page refcount never reach 0).
>
> This patch is just a preparatory patch for HMM.
>
> Changes since v1:
>   - commit message
>
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable memory

[Dan Williams]

On Fri, Apr 21, 2017 at 8:30 PM, Jérôme Glisse <jglisse@redhat.com> wrote:
> HMM (heterogeneous memory management) need struct page to support migration
> from system main memory to device memory.  Reasons for HMM and migration to
> device memory is explained with HMM core patch.
>
> This patch deals with device memory that is un-addressable memory (ie CPU
> can not access it). Hence we do not want those struct page to be manage
> like regular memory. That is why we extend ZONE_DEVICE to support different
> types of memory.
>
> A persistent memory type is define for existing user of ZONE_DEVICE and a
> new device un-addressable type is added for the un-addressable memory type.
> There is a clear separation between what is expected from each memory type
> and existing user of ZONE_DEVICE are un-affected by new requirement and new
> use of the un-addressable type. All specific code path are protect with
> test against the memory type.
>
> Because memory is un-addressable we use a new special swap type for when
> a page is migrated to device memory (this reduces the number of maximum
> swap file).
>
> The main two additions beside memory type to ZONE_DEVICE is two callbacks.
> First one, page_free() is call whenever page refcount reach 1 (which means
> the page is free as ZONE_DEVICE page never reach a refcount of 0). This
> allow device driver to manage its memory and associated struct page.
>
> The second callback page_fault() happens when there is a CPU access to
> an address that is back by a device page (which are un-addressable by the
> CPU). This callback is responsible to migrate the page back to system
> main memory. Device driver can not block migration back to system memory,
> HMM make sure that such page can not be pin into device memory.
>
> If device is in some error condition and can not migrate memory back then
> a CPU page fault to device memory should end with SIGBUS.
>
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
> ---
>  fs/proc/task_mmu.c       |  7 +++++
>  include/linux/ioport.h   |  1 +
>  include/linux/memremap.h | 82 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/swap.h     | 24 ++++++++++++--
>  include/linux/swapops.h  | 68 +++++++++++++++++++++++++++++++++++++++
>  kernel/memremap.c        | 43 ++++++++++++++++++++++++-
>  mm/Kconfig               | 13 ++++++++
>  mm/memory.c              | 61 +++++++++++++++++++++++++++++++++++
>  mm/memory_hotplug.c      | 10 ++++--
>  mm/mprotect.c            | 14 +++++++++
>  10 files changed, 317 insertions(+), 6 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index f0c8b33..a12ba94 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -542,6 +542,8 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>                         }
>                 } else if (is_migration_entry(swpent))
>                         page = migration_entry_to_page(swpent);
> +               else if (is_device_entry(swpent))
> +                       page = device_entry_to_page(swpent);
>         } else if (unlikely(IS_ENABLED(CONFIG_SHMEM) && mss->check_shmem_swap
>                                                         && pte_none(*pte))) {
>                 page = find_get_entry(vma->vm_file->f_mapping,
> @@ -704,6 +706,8 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
>
>                 if (is_migration_entry(swpent))
>                         page = migration_entry_to_page(swpent);
> +               else if (is_device_entry(swpent))
> +                       page = device_entry_to_page(swpent);
>         }
>         if (page) {
>                 int mapcount = page_mapcount(page);
> @@ -1196,6 +1200,9 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>                 flags |= PM_SWAP;
>                 if (is_migration_entry(entry))
>                         page = migration_entry_to_page(entry);
> +
> +               if (is_device_entry(entry))
> +                       page = device_entry_to_page(entry);
>         }
>
>         if (page && !PageAnon(page))
> diff --git a/include/linux/ioport.h b/include/linux/ioport.h
> index 6230064..ec619dc 100644
> --- a/include/linux/ioport.h
> +++ b/include/linux/ioport.h
> @@ -130,6 +130,7 @@ enum {
>         IORES_DESC_ACPI_NV_STORAGE              = 3,
>         IORES_DESC_PERSISTENT_MEMORY            = 4,
>         IORES_DESC_PERSISTENT_MEMORY_LEGACY     = 5,
> +       IORES_DESC_DEVICE_MEMORY_UNADDRESSABLE  = 6,
>  };
>
>  /Fput* helpers to define resources */
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 9341619..365fb4e 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -35,24 +35,101 @@ static inline struct vmem_altmap *to_vmem_altmap(unsigned long memmap_start)
>  }
>  #endif
>
> +/*
> + * Specialize ZONE_DEVICE memory into multiple types each having differents
> + * usage.
> + *
> + * MEMORY_DEVICE_PERSISTENT:
> + * Persistent device memory (pmem): struct page might be allocated in different
> + * memory and architecture might want to perform special actions. It is similar
> + * to regular memory, in that the CPU can access it transparently. However,
> + * it is likely to have different bandwidth and latency than regular memory.
> + * See Documentation/nvdimm/nvdimm.txt for more information.
> + *
> + * MEMORY_DEVICE_UNADDRESSABLE:
> + * Device memory that is not directly addressable by the CPU: CPU can neither
> + * read nor write _UNADDRESSABLE memory. In this case, we do still have struct
> + * pages backing the device memory. Doing so simplifies the implementation, but
> + * it is important to remember that there are certain points at which the struct
> + * page must be treated as an opaque object, rather than a "normal" struct page.
> + * A more complete discussion of unaddressable memory may be found in
> + * include/linux/hmm.h and Documentation/vm/hmm.txt.
> + */
> +enum memory_type {
> +       MEMORY_DEVICE_PERSISTENT = 0,
> +       MEMORY_DEVICE_UNADDRESSABLE,
> +};

Ok, this is a bikeshed, but I think it is important. I think these
should be called MEMORY_DEVICE_PUBLIC and MEMORY_DEVICE_PRIVATE. The
reason is that persistence has nothing to do with the code paths that
deal with the pmem use case of ZONE_DEVICE. The only property the mm
cares about is that the address range behaves the same as host memory
for dma and cpu accesses. The "unaddressable" designation always
confuses me because a memory range isn't memory if it's
"unaddressable". It is addressable, it's just "private" to the device.

> +
> +/*
> + * For MEMORY_DEVICE_UNADDRESSABLE we use ZONE_DEVICE and extend it with two
> + * callbacks:
> + *   page_fault()
> + *   page_free()
> + *
> + * Additional notes about MEMORY_DEVICE_UNADDRESSABLE may be found in
> + * include/linux/hmm.h and Documentation/vm/hmm.txt. There is also a brief
> + * explanation in include/linux/memory_hotplug.h.
> + *
> + * The page_fault() callback must migrate page back, from device memory to
> + * system memory, so that the CPU can access it. This might fail for various
> + * reasons (device issues,  device have been unplugged, ...). When such error
> + * conditions happen, the page_fault() callback must return VM_FAULT_SIGBUS and
> + * set the CPU page table entry to "poisoned".
> + *
> + * Note that because memory cgroup charges are transferred to the device memory,
> + * this should never fail due to memory restrictions. However, allocation
> + * of a regular system page might still fail because we are out of memory. If
> + * that happens, the page_fault() callback must return VM_FAULT_OOM.
> + *
> + * The page_fault() callback can also try to migrate back multiple pages in one
> + * chunk, as an optimization. It must, however, prioritize the faulting address
> + * over all the others.
> + *
> + *
> + * The page_free() callback is called once the page refcount reaches 1
> + * (ZONE_DEVICE pages never reach 0 refcount unless there is a refcount bug.
> + * This allows the device driver to implement its own memory management.)
> + */
> +typedef int (*dev_page_fault_t)(struct vm_area_struct *vma,
> +                               unsigned long addr,
> +                               struct page *page,
> +                               unsigned int flags,
> +                               pmd_t *pmdp);
> +typedef void (*dev_page_free_t)(struct page *page, void *data);
> +
>  /**
>   * struct dev_pagemap - metadata for ZONE_DEVICE mappings
> + * @page_fault: callback when CPU fault on an unaddressable device page
> + * @page_free: free page callback when page refcount reaches 1
>   * @altmap: pre-allocated/reserved memory for vmemmap allocations
>   * @res: physical address range covered by @ref
>   * @ref: reference count that pins the devm_memremap_pages() mapping
>   * @dev: host device of the mapping for debug
> + * @data: private data pointer for page_free()
> + * @type: memory type: see MEMORY_* in memory_hotplug.h
>   */
>  struct dev_pagemap {
> +       dev_page_fault_t page_fault;
> +       dev_page_free_t page_free;
>         struct vmem_altmap *altmap;
>         const struct resource *res;
>         struct percpu_ref *ref;
>         struct device *dev;
> +       void *data;
> +       enum memory_type type;
>  };

I think the only attribute that belongs here is @type, the rest are
specific to the device_private case.

struct dev_private_pagemap {
    dev_page_fault_t page_fault;
    dev_page_free_t page_free;
    void *data;
    struct dev_pagemap pgmap;
};

As Logan pointed out we can kill the internal struct page_map in
kernel/memremap.c and let devm_memremap_pages() take a "struct
dev_pagemap *" pointer. That way when future dev_pagemap users come
along they can wrap the core structure with whatever specific data
they need for their use case.

>
>  #ifdef CONFIG_ZONE_DEVICE
>  void *devm_memremap_pages(struct device *dev, struct resource *res,
>                 struct percpu_ref *ref, struct vmem_altmap *altmap);
>  struct dev_pagemap *find_dev_pagemap(resource_size_t phys);
> +
> +static inline bool is_device_unaddressable_page(const struct page *page)
> +{
> +       /* See MEMORY_DEVICE_UNADDRESSABLE in include/linux/memory_hotplug.h */
> +       return ((page_zonenum(page) == ZONE_DEVICE) &&
> +               (page->pgmap->type == MEMORY_DEVICE_UNADDRESSABLE));
> +}
>  #else
>  static inline void *devm_memremap_pages(struct device *dev,
>                 struct resource *res, struct percpu_ref *ref,
> @@ -71,6 +148,11 @@ static inline struct dev_pagemap *find_dev_pagemap(resource_size_t phys)
>  {
>         return NULL;
>  }
> +
> +static inline bool is_device_unaddressable_page(const struct page *page)
> +{
> +       return false;
> +}
>  #endif
>
>  /**
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index ba58824..52a137d 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -51,6 +51,23 @@ static inline int current_is_kswapd(void)
>   */
>
>  /*
> + * Unaddressable device memory support. See include/linux/hmm.h and
> + * Documentation/vm/hmm.txt. Short description is we need struct pages for
> + * device memory that is unaddressable (inaccessible) by CPU, so that we can
> + * migrate part of a process memory to device memory.
> + *
> + * When a page is migrated from CPU to device, we set the CPU page table entry
> + * to a special SWP_DEVICE_* entry.
> + */
> +#ifdef CONFIG_DEVICE_UNADDRESSABLE
> +#define SWP_DEVICE_NUM 2
> +#define SWP_DEVICE_WRITE (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM)
> +#define SWP_DEVICE_READ (MAX_SWAPFILES+SWP_HWPOISON_NUM+SWP_MIGRATION_NUM+1)
> +#else
> +#define SWP_DEVICE_NUM 0
> +#endif
> +
> +/*
>   * NUMA node memory migration support
>   */
>  #ifdef CONFIG_MIGRATION
> @@ -72,7 +89,8 @@ static inline int current_is_kswapd(void)
>  #endif
>
>  #define MAX_SWAPFILES \
> -       ((1 << MAX_SWAPFILES_SHIFT) - SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
> +       ((1 << MAX_SWAPFILES_SHIFT) - SWP_DEVICE_NUM - \
> +       SWP_MIGRATION_NUM - SWP_HWPOISON_NUM)
>
>  /*
>   * Magic header for a swap area. The first part of the union is
> @@ -432,8 +450,8 @@ static inline void show_swap_cache_info(void)
>  {
>  }
>
> -#define free_swap_and_cache(swp)       is_migration_entry(swp)
> -#define swapcache_prepare(swp)         is_migration_entry(swp)
> +#define free_swap_and_cache(e) (is_migration_entry(e) || is_device_entry(e))
> +#define swapcache_prepare(e) (is_migration_entry(e) || is_device_entry(e))
>
>  static inline int add_swap_count_continuation(swp_entry_t swp, gfp_t gfp_mask)
>  {
> diff --git a/include/linux/swapops.h b/include/linux/swapops.h
> index 5c3a5f3..960eb6b 100644
> --- a/include/linux/swapops.h
> +++ b/include/linux/swapops.h
> @@ -100,6 +100,74 @@ static inline void *swp_to_radix_entry(swp_entry_t entry)
>         return (void *)(value | RADIX_TREE_EXCEPTIONAL_ENTRY);
>  }
>
> +#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
> +static inline swp_entry_t make_device_entry(struct page *page, bool write)
> +{
> +       return swp_entry(write ? SWP_DEVICE_WRITE : SWP_DEVICE_READ,
> +                        page_to_pfn(page));
> +}
> +
> +static inline bool is_device_entry(swp_entry_t entry)
> +{
> +       int type = swp_type(entry);
> +       return type == SWP_DEVICE_READ || type == SWP_DEVICE_WRITE;
> +}
> +
> +static inline void make_device_entry_read(swp_entry_t *entry)
> +{
> +       *entry = swp_entry(SWP_DEVICE_READ, swp_offset(*entry));
> +}
> +
> +static inline bool is_write_device_entry(swp_entry_t entry)
> +{
> +       return unlikely(swp_type(entry) == SWP_DEVICE_WRITE);
> +}
> +
> +static inline struct page *device_entry_to_page(swp_entry_t entry)
> +{
> +       return pfn_to_page(swp_offset(entry));
> +}
> +
> +int device_entry_fault(struct vm_area_struct *vma,
> +                      unsigned long addr,
> +                      swp_entry_t entry,
> +                      unsigned int flags,
> +                      pmd_t *pmdp);


The use of the "device" term is ambiguous. Since these changes are
specific to the MEMORY_DEVICE_PRIVATE can we reflect "private" in the
name? Change "device" to "devpriv" or "device_private"?


> +#else /* CONFIG_DEVICE_UNADDRESSABLE */
> +static inline swp_entry_t make_device_entry(struct page *page, bool write)
> +{
> +       return swp_entry(0, 0);
> +}
> +
> +static inline void make_device_entry_read(swp_entry_t *entry)
> +{
> +}
> +
> +static inline bool is_device_entry(swp_entry_t entry)
> +{
> +       return false;
> +}
> +
> +static inline bool is_write_device_entry(swp_entry_t entry)
> +{
> +       return false;
> +}
> +
> +static inline struct page *device_entry_to_page(swp_entry_t entry)
> +{
> +       return NULL;
> +}
> +
> +static inline int device_entry_fault(struct vm_area_struct *vma,
> +                                    unsigned long addr,
> +                                    swp_entry_t entry,
> +                                    unsigned int flags,
> +                                    pmd_t *pmdp)
> +{
> +       return VM_FAULT_SIGBUS;
> +}
> +#endif /* CONFIG_DEVICE_UNADDRESSABLE */
> +
>  #ifdef CONFIG_MIGRATION
>  static inline swp_entry_t make_migration_entry(struct page *page, int write)
>  {
> diff --git a/kernel/memremap.c b/kernel/memremap.c
> index 97ef676..dce3b29 100644
> --- a/kernel/memremap.c
> +++ b/kernel/memremap.c
> @@ -18,6 +18,8 @@
>  #include <linux/io.h>
>  #include <linux/mm.h>
>  #include <linux/memory_hotplug.h>
> +#include <linux/swap.h>
> +#include <linux/swapops.h>
>
>  #ifndef ioremap_cache
>  /* temporary while we convert existing ioremap_cache users to memremap */
> @@ -194,12 +196,47 @@ void put_zone_device_page(struct page *page)
>          * ZONE_DEVICE page refcount should never reach 0 and never be freed
>          * to kernel memory allocator.
>          */
> -       page_ref_dec(page);
> +       int count = page_ref_dec_return(page);
> +
> +       /*
> +        * If refcount is 1 then page is freed and refcount is stable as nobody
> +        * holds a reference on the page.
> +        */
> +       if (page->pgmap->page_free && count == 1)
> +               page->pgmap->page_free(page, page->pgmap->data);


Reading this it isn't clear whether we are dealing with the public or
private case. I think this should be:

if (count == 1)
        zone_device_page_free(page);

...where zone_device_page_free() is a helper that is explicit about
the expectations of which ZONE_DEVICE types need to take action on a
'free' event.

static void zone_device_page_free(struct page *page)
{
        struct dev_private_pagemap *priv_pgmap;

        if (page->pgmap->type == MEMORY_DEVICE_PUBLIC)
                return;

        priv_pgmap = container_of(page->pgmap, typeof(*priv_pgmap), pgmap);
        priv_pgmap->page_free(page);
}

>
>         put_dev_pagemap(page->pgmap);
>  }
>  EXPORT_SYMBOL(put_zone_device_page);
>
> +#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
> +int device_entry_fault(struct vm_area_struct *vma,
> +                      unsigned long addr,
> +                      swp_entry_t entry,
> +                      unsigned int flags,
> +                      pmd_t *pmdp)
> +{
> +       struct page *page = device_entry_to_page(entry);
> +
> +       /*
> +        * The page_fault() callback must migrate page back to system memory
> +        * so that CPU can access it. This might fail for various reasons
> +        * (device issue, device was unsafely unplugged, ...). When such
> +        * error conditions happen, the callback must return VM_FAULT_SIGBUS.
> +        *
> +        * Note that because memory cgroup charges are accounted to the device
> +        * memory, this should never fail because of memory restrictions (but
> +        * allocation of regular system page might still fail because we are
> +        * out of memory).
> +        *
> +        * There is a more in-depth description of what that callback can and
> +        * cannot do, in include/linux/memremap.h
> +        */
> +       return page->pgmap->page_fault(vma, addr, page, flags, pmdp);
> +}
> +EXPORT_SYMBOL(device_entry_fault);
> +#endif /* CONFIG_DEVICE_UNADDRESSABLE */
> +
>  static void pgmap_radix_release(struct resource *res)
>  {
>         resource_size_t key, align_start, align_size, align_end;
> @@ -332,6 +369,10 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
>         }
>         pgmap->ref = ref;
>         pgmap->res = &page_map->res;
> +       pgmap->type = MEMORY_DEVICE_PERSISTENT;
> +       pgmap->page_fault = NULL;
> +       pgmap->page_free = NULL;
> +       pgmap->data = NULL;


This goes away if we convert to passing in the dev_pagemap, and I
think that is a useful cleanup.


>         mutex_lock(&pgmap_lock);
>         error = 0;
> diff --git a/mm/Kconfig b/mm/Kconfig
> index c89f472..ad4bd50 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -700,6 +700,19 @@ config ZONE_DEVICE
>
>           If FS_DAX is enabled, then say Y.
>
> +config DEVICE_UNADDRESSABLE
> +       bool "Unaddressable device memory (GPU memory, ...)"
> +       depends on X86_64
> +       depends on ZONE_DEVICE
> +       depends on MEMORY_HOTPLUG
> +       depends on MEMORY_HOTREMOVE
> +       depends on SPARSEMEM_VMEMMAP
> +
> +       help
> +         Allows creation of struct pages to represent unaddressable device
> +         memory; i.e., memory that is only accessible from the device (or
> +         group of devices).
> +
>  config FRAME_VECTOR
>         bool
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 6ff5d72..4116ab2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -49,6 +49,7 @@
>  #include <linux/swap.h>
>  #include <linux/highmem.h>
>  #include <linux/pagemap.h>
> +#include <linux/memremap.h>
>  #include <linux/ksm.h>
>  #include <linux/rmap.h>
>  #include <linux/export.h>
> @@ -927,6 +928,35 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>                                         pte = pte_swp_mksoft_dirty(pte);
>                                 set_pte_at(src_mm, addr, src_pte, pte);
>                         }
> +               } else if (is_device_entry(entry)) {
> +                       page = device_entry_to_page(entry);
> +
> +                       /*
> +                        * Update rss count even for unaddressable pages, as
> +                        * they should treated just like normal pages in this
> +                        * respect.
> +                        *
> +                        * We will likely want to have some new rss counters
> +                        * for unaddressable pages, at some point. But for now
> +                        * keep things as they are.
> +                        */
> +                       get_page(page);
> +                       rss[mm_counter(page)]++;
> +                       page_dup_rmap(page, false);
> +
> +                       /*
> +                        * We do not preserve soft-dirty information, because so
> +                        * far, checkpoint/restore is the only feature that
> +                        * requires that. And checkpoint/restore does not work
> +                        * when a device driver is involved (you cannot easily
> +                        * save and restore device driver state).
> +                        */
> +                       if (is_write_device_entry(entry) &&
> +                           is_cow_mapping(vm_flags)) {
> +                               make_device_entry_read(&entry);
> +                               pte = swp_entry_to_pte(entry);
> +                               set_pte_at(src_mm, addr, src_pte, pte);
> +                       }
>                 }
>                 goto out_set_pte;
>         }
> @@ -1243,6 +1273,29 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>                         }
>                         continue;
>                 }
> +
> +               entry = pte_to_swp_entry(ptent);
> +               if (non_swap_entry(entry) && is_device_entry(entry)) {
> +                       struct page *page = device_entry_to_page(entry);
> +
> +                       if (unlikely(details && details->check_mapping)) {
> +                               /*
> +                                * unmap_shared_mapping_pages() wants to
> +                                * invalidate cache without truncating:
> +                                * unmap shared but keep private pages.
> +                                */
> +                               if (details->check_mapping !=
> +                                   page_rmapping(page))
> +                                       continue;
> +                       }
> +
> +                       pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> +                       rss[mm_counter(page)]--;
> +                       page_remove_rmap(page, false);
> +                       put_page(page);
> +                       continue;
> +               }
> +
>                 /* If details->check_mapping, we leave swap entries. */
>                 if (unlikely(details))
>                         continue;
> @@ -2690,6 +2743,14 @@ int do_swap_page(struct vm_fault *vmf)
>                 if (is_migration_entry(entry)) {
>                         migration_entry_wait(vma->vm_mm, vmf->pmd,
>                                              vmf->address);
> +               } else if (is_device_entry(entry)) {
> +                       /*
> +                        * For un-addressable device memory we call the pgmap
> +                        * fault handler callback. The callback must migrate
> +                        * the page back to some CPU accessible page.
> +                        */
> +                       ret = device_entry_fault(vma, vmf->address, entry,
> +                                                vmf->flags, vmf->pmd);
>                 } else if (is_hwpoison_entry(entry)) {
>                         ret = VM_FAULT_HWPOISON;
>                 } else {
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index de30822..d988fbe 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -156,7 +156,7 @@ void mem_hotplug_done(void)
>  /* add this memory to iomem resource */
>  static struct resource *register_memory_resource(u64 start, u64 size)
>  {
> -       struct resource *res;
> +       struct resource *res, *conflict;
>         res = kzalloc(sizeof(struct resource), GFP_KERNEL);
>         if (!res)
>                 return ERR_PTR(-ENOMEM);
> @@ -165,7 +165,13 @@ static struct resource *register_memory_resource(u64 start, u64 size)
>         res->start = start;
>         res->end = start + size - 1;
>         res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;
> -       if (request_resource(&iomem_resource, res) < 0) {
> +       conflict =  request_resource_conflict(&iomem_resource, res);
> +       if (conflict) {
> +               if (conflict->desc == IORES_DESC_DEVICE_MEMORY_UNADDRESSABLE) {
> +                       pr_debug("Device unaddressable memory block "
> +                                "memory hotplug at %#010llx !\n",
> +                                (unsigned long long)start);
> +               }
>                 pr_debug("System RAM resource %pR cannot be added\n", res);
>                 kfree(res);
>                 return ERR_PTR(-EEXIST);
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 8edd0d5..dadb020 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -126,6 +126,20 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>
>                                 pages++;
>                         }
> +
> +                       if (is_write_device_entry(entry)) {
> +                               pte_t newpte;
> +
> +                               /*
> +                                * We do not preserve soft-dirtiness. See
> +                                * copy_one_pte() for explanation.
> +                                */
> +                               make_device_entry_read(&entry);
> +                               newpte = swp_entry_to_pte(entry);
> +                               set_pte_at(mm, addr, pte, newpte);
> +
> +                               pages++;
> +                       }
>                 }
>         } while (pte++, addr += PAGE_SIZE, addr != end);
>         arch_leave_lazy_mmu_mode();

I think this is a good example of how the usage of the generic
"device" term makes the new "private" code paths harder to spot /
discern from the current "public" requirements.

Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable memory

[Jerome Glisse]

On Fri, Apr 21, 2017 at 10:30:01PM -0700, Dan Williams wrote:
> On Fri, Apr 21, 2017 at 8:30 PM, Jérôme Glisse <jglisse@redhat.com> wrote:

[...]

> > +/*
> > + * Specialize ZONE_DEVICE memory into multiple types each having differents
> > + * usage.
> > + *
> > + * MEMORY_DEVICE_PERSISTENT:
> > + * Persistent device memory (pmem): struct page might be allocated in different
> > + * memory and architecture might want to perform special actions. It is similar
> > + * to regular memory, in that the CPU can access it transparently. However,
> > + * it is likely to have different bandwidth and latency than regular memory.
> > + * See Documentation/nvdimm/nvdimm.txt for more information.
> > + *
> > + * MEMORY_DEVICE_UNADDRESSABLE:
> > + * Device memory that is not directly addressable by the CPU: CPU can neither
> > + * read nor write _UNADDRESSABLE memory. In this case, we do still have struct
> > + * pages backing the device memory. Doing so simplifies the implementation, but
> > + * it is important to remember that there are certain points at which the struct
> > + * page must be treated as an opaque object, rather than a "normal" struct page.
> > + * A more complete discussion of unaddressable memory may be found in
> > + * include/linux/hmm.h and Documentation/vm/hmm.txt.
> > + */
> > +enum memory_type {
> > +       MEMORY_DEVICE_PERSISTENT = 0,
> > +       MEMORY_DEVICE_UNADDRESSABLE,
> > +};
> 
> Ok, this is a bikeshed, but I think it is important. I think these
> should be called MEMORY_DEVICE_PUBLIC and MEMORY_DEVICE_PRIVATE. The
> reason is that persistence has nothing to do with the code paths that
> deal with the pmem use case of ZONE_DEVICE. The only property the mm
> cares about is that the address range behaves the same as host memory
> for dma and cpu accesses. The "unaddressable" designation always
> confuses me because a memory range isn't memory if it's
> "unaddressable". It is addressable, it's just "private" to the device.

I can change the name but the memory is truely unaddressable, the CPU
can not access it whatsoever (well it can access a small window but
even that is not guaranteed).


> > +/*
> > + * For MEMORY_DEVICE_UNADDRESSABLE we use ZONE_DEVICE and extend it with two
> > + * callbacks:
> > + *   page_fault()
> > + *   page_free()
> > + *
> > + * Additional notes about MEMORY_DEVICE_UNADDRESSABLE may be found in
> > + * include/linux/hmm.h and Documentation/vm/hmm.txt. There is also a brief
> > + * explanation in include/linux/memory_hotplug.h.
> > + *
> > + * The page_fault() callback must migrate page back, from device memory to
> > + * system memory, so that the CPU can access it. This might fail for various
> > + * reasons (device issues,  device have been unplugged, ...). When such error
> > + * conditions happen, the page_fault() callback must return VM_FAULT_SIGBUS and
> > + * set the CPU page table entry to "poisoned".
> > + *
> > + * Note that because memory cgroup charges are transferred to the device memory,
> > + * this should never fail due to memory restrictions. However, allocation
> > + * of a regular system page might still fail because we are out of memory. If
> > + * that happens, the page_fault() callback must return VM_FAULT_OOM.
> > + *
> > + * The page_fault() callback can also try to migrate back multiple pages in one
> > + * chunk, as an optimization. It must, however, prioritize the faulting address
> > + * over all the others.
> > + *
> > + *
> > + * The page_free() callback is called once the page refcount reaches 1
> > + * (ZONE_DEVICE pages never reach 0 refcount unless there is a refcount bug.
> > + * This allows the device driver to implement its own memory management.)
> > + */
> > +typedef int (*dev_page_fault_t)(struct vm_area_struct *vma,
> > +                               unsigned long addr,
> > +                               struct page *page,
> > +                               unsigned int flags,
> > +                               pmd_t *pmdp);
> > +typedef void (*dev_page_free_t)(struct page *page, void *data);
> > +
> >  /**
> >   * struct dev_pagemap - metadata for ZONE_DEVICE mappings
> > + * @page_fault: callback when CPU fault on an unaddressable device page
> > + * @page_free: free page callback when page refcount reaches 1
> >   * @altmap: pre-allocated/reserved memory for vmemmap allocations
> >   * @res: physical address range covered by @ref
> >   * @ref: reference count that pins the devm_memremap_pages() mapping
> >   * @dev: host device of the mapping for debug
> > + * @data: private data pointer for page_free()
> > + * @type: memory type: see MEMORY_* in memory_hotplug.h
> >   */
> >  struct dev_pagemap {
> > +       dev_page_fault_t page_fault;
> > +       dev_page_free_t page_free;
> >         struct vmem_altmap *altmap;
> >         const struct resource *res;
> >         struct percpu_ref *ref;
> >         struct device *dev;
> > +       void *data;
> > +       enum memory_type type;
> >  };
> 
> I think the only attribute that belongs here is @type, the rest are
> specific to the device_private case.
> 
> struct dev_private_pagemap {
>     dev_page_fault_t page_fault;
>     dev_page_free_t page_free;
>     void *data;
>     struct dev_pagemap pgmap;
> };
> 
> As Logan pointed out we can kill the internal struct page_map in
> kernel/memremap.c and let devm_memremap_pages() take a "struct
> dev_pagemap *" pointer. That way when future dev_pagemap users come
> along they can wrap the core structure with whatever specific data
> they need for their use case.

I don't want to make that change as part of HMM, this can be done
as a patchset on top. Also it is easier to add callback to dev_pagemap
especially as we want p2p and HMM side by side not as something
exclusive.

[...]

> > +#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
> > +static inline swp_entry_t make_device_entry(struct page *page, bool write)
> > +{
> > +       return swp_entry(write ? SWP_DEVICE_WRITE : SWP_DEVICE_READ,
> > +                        page_to_pfn(page));
> > +}
> > +
> > +static inline bool is_device_entry(swp_entry_t entry)
> > +{
> > +       int type = swp_type(entry);
> > +       return type == SWP_DEVICE_READ || type == SWP_DEVICE_WRITE;
> > +}
> > +
> > +static inline void make_device_entry_read(swp_entry_t *entry)
> > +{
> > +       *entry = swp_entry(SWP_DEVICE_READ, swp_offset(*entry));
> > +}
> > +
> > +static inline bool is_write_device_entry(swp_entry_t entry)
> > +{
> > +       return unlikely(swp_type(entry) == SWP_DEVICE_WRITE);
> > +}
> > +
> > +static inline struct page *device_entry_to_page(swp_entry_t entry)
> > +{
> > +       return pfn_to_page(swp_offset(entry));
> > +}
> > +
> > +int device_entry_fault(struct vm_area_struct *vma,
> > +                      unsigned long addr,
> > +                      swp_entry_t entry,
> > +                      unsigned int flags,
> > +                      pmd_t *pmdp);
> 
> 
> The use of the "device" term is ambiguous. Since these changes are
> specific to the MEMORY_DEVICE_PRIVATE can we reflect "private" in the
> name? Change "device" to "devpriv" or "device_private"?

Yes we can.

[...]

> > @@ -194,12 +196,47 @@ void put_zone_device_page(struct page *page)
> >          * ZONE_DEVICE page refcount should never reach 0 and never be freed
> >          * to kernel memory allocator.
> >          */
> > -       page_ref_dec(page);
> > +       int count = page_ref_dec_return(page);
> > +
> > +       /*
> > +        * If refcount is 1 then page is freed and refcount is stable as nobody
> > +        * holds a reference on the page.
> > +        */
> > +       if (page->pgmap->page_free && count == 1)
> > +               page->pgmap->page_free(page, page->pgmap->data);
> 
> 
> Reading this it isn't clear whether we are dealing with the public or
> private case. I think this should be:
> 
> if (count == 1)
>         zone_device_page_free(page);
> 
> ...where zone_device_page_free() is a helper that is explicit about
> the expectations of which ZONE_DEVICE types need to take action on a
> 'free' event.
> 
> static void zone_device_page_free(struct page *page)
> {
>         struct dev_private_pagemap *priv_pgmap;
> 
>         if (page->pgmap->type == MEMORY_DEVICE_PUBLIC)
>                 return;
> 
>         priv_pgmap = container_of(page->pgmap, typeof(*priv_pgmap), pgmap);
>         priv_pgmap->page_free(page);
> }

Ok.

> > @@ -332,6 +369,10 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
> >         }
> >         pgmap->ref = ref;
> >         pgmap->res = &page_map->res;
> > +       pgmap->type = MEMORY_DEVICE_PERSISTENT;
> > +       pgmap->page_fault = NULL;
> > +       pgmap->page_free = NULL;
> > +       pgmap->data = NULL;
> 
> 
> This goes away if we convert to passing in the dev_pagemap, and I
> think that is a useful cleanup.

Yes but i rather do it as a patchset on top as i am sure p2p folks
will want to give input and i don't want to delay until we can agree
on exact fields we want/need.

[...]

> > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > index 8edd0d5..dadb020 100644
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -126,6 +126,20 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> >
> >                                 pages++;
> >                         }
> > +
> > +                       if (is_write_device_entry(entry)) {
> > +                               pte_t newpte;
> > +
> > +                               /*
> > +                                * We do not preserve soft-dirtiness. See
> > +                                * copy_one_pte() for explanation.
> > +                                */
> > +                               make_device_entry_read(&entry);
> > +                               newpte = swp_entry_to_pte(entry);
> > +                               set_pte_at(mm, addr, pte, newpte);
> > +
> > +                               pages++;
> > +                       }
> >                 }
> >         } while (pte++, addr += PAGE_SIZE, addr != end);
> >         arch_leave_lazy_mmu_mode();
> 
> I think this is a good example of how the usage of the generic
> "device" term makes the new "private" code paths harder to spot /
> discern from the current "public" requirements.

I will change to devpriv

Thank for reviewing.

Cheers,
Jérôme

Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable memory

[Dan Williams]

On Sat, Apr 22, 2017 at 11:11 AM, Jerome Glisse <jglisse@redhat.com> wrote:
> On Fri, Apr 21, 2017 at 10:30:01PM -0700, Dan Williams wrote:
>> On Fri, Apr 21, 2017 at 8:30 PM, Jérôme Glisse <jglisse@redhat.com> wrote:
>
> [...]
>
>> > +/*
>> > + * Specialize ZONE_DEVICE memory into multiple types each having differents
>> > + * usage.
>> > + *
>> > + * MEMORY_DEVICE_PERSISTENT:
>> > + * Persistent device memory (pmem): struct page might be allocated in different
>> > + * memory and architecture might want to perform special actions. It is similar
>> > + * to regular memory, in that the CPU can access it transparently. However,
>> > + * it is likely to have different bandwidth and latency than regular memory.
>> > + * See Documentation/nvdimm/nvdimm.txt for more information.
>> > + *
>> > + * MEMORY_DEVICE_UNADDRESSABLE:
>> > + * Device memory that is not directly addressable by the CPU: CPU can neither
>> > + * read nor write _UNADDRESSABLE memory. In this case, we do still have struct
>> > + * pages backing the device memory. Doing so simplifies the implementation, but
>> > + * it is important to remember that there are certain points at which the struct
>> > + * page must be treated as an opaque object, rather than a "normal" struct page.
>> > + * A more complete discussion of unaddressable memory may be found in
>> > + * include/linux/hmm.h and Documentation/vm/hmm.txt.
>> > + */
>> > +enum memory_type {
>> > +       MEMORY_DEVICE_PERSISTENT = 0,
>> > +       MEMORY_DEVICE_UNADDRESSABLE,
>> > +};
>>
>> Ok, this is a bikeshed, but I think it is important. I think these
>> should be called MEMORY_DEVICE_PUBLIC and MEMORY_DEVICE_PRIVATE. The
>> reason is that persistence has nothing to do with the code paths that
>> deal with the pmem use case of ZONE_DEVICE. The only property the mm
>> cares about is that the address range behaves the same as host memory
>> for dma and cpu accesses. The "unaddressable" designation always
>> confuses me because a memory range isn't memory if it's
>> "unaddressable". It is addressable, it's just "private" to the device.
>
> I can change the name but the memory is truely unaddressable, the CPU
> can not access it whatsoever (well it can access a small window but
> even that is not guaranteed).
>

Understood, but that's still "addressable only by certain agents or
through a proxy" which seems closer to "private" to me.

>
>> > +/*
>> > + * For MEMORY_DEVICE_UNADDRESSABLE we use ZONE_DEVICE and extend it with two
>> > + * callbacks:
>> > + *   page_fault()
>> > + *   page_free()
>> > + *
>> > + * Additional notes about MEMORY_DEVICE_UNADDRESSABLE may be found in
>> > + * include/linux/hmm.h and Documentation/vm/hmm.txt. There is also a brief
>> > + * explanation in include/linux/memory_hotplug.h.
>> > + *
>> > + * The page_fault() callback must migrate page back, from device memory to
>> > + * system memory, so that the CPU can access it. This might fail for various
>> > + * reasons (device issues,  device have been unplugged, ...). When such error
>> > + * conditions happen, the page_fault() callback must return VM_FAULT_SIGBUS and
>> > + * set the CPU page table entry to "poisoned".
>> > + *
>> > + * Note that because memory cgroup charges are transferred to the device memory,
>> > + * this should never fail due to memory restrictions. However, allocation
>> > + * of a regular system page might still fail because we are out of memory. If
>> > + * that happens, the page_fault() callback must return VM_FAULT_OOM.
>> > + *
>> > + * The page_fault() callback can also try to migrate back multiple pages in one
>> > + * chunk, as an optimization. It must, however, prioritize the faulting address
>> > + * over all the others.
>> > + *
>> > + *
>> > + * The page_free() callback is called once the page refcount reaches 1
>> > + * (ZONE_DEVICE pages never reach 0 refcount unless there is a refcount bug.
>> > + * This allows the device driver to implement its own memory management.)
>> > + */
>> > +typedef int (*dev_page_fault_t)(struct vm_area_struct *vma,
>> > +                               unsigned long addr,
>> > +                               struct page *page,
>> > +                               unsigned int flags,
>> > +                               pmd_t *pmdp);
>> > +typedef void (*dev_page_free_t)(struct page *page, void *data);
>> > +
>> >  /**
>> >   * struct dev_pagemap - metadata for ZONE_DEVICE mappings
>> > + * @page_fault: callback when CPU fault on an unaddressable device page
>> > + * @page_free: free page callback when page refcount reaches 1
>> >   * @altmap: pre-allocated/reserved memory for vmemmap allocations
>> >   * @res: physical address range covered by @ref
>> >   * @ref: reference count that pins the devm_memremap_pages() mapping
>> >   * @dev: host device of the mapping for debug
>> > + * @data: private data pointer for page_free()
>> > + * @type: memory type: see MEMORY_* in memory_hotplug.h
>> >   */
>> >  struct dev_pagemap {
>> > +       dev_page_fault_t page_fault;
>> > +       dev_page_free_t page_free;
>> >         struct vmem_altmap *altmap;
>> >         const struct resource *res;
>> >         struct percpu_ref *ref;
>> >         struct device *dev;
>> > +       void *data;
>> > +       enum memory_type type;
>> >  };
>>
>> I think the only attribute that belongs here is @type, the rest are
>> specific to the device_private case.
>>
>> struct dev_private_pagemap {
>>     dev_page_fault_t page_fault;
>>     dev_page_free_t page_free;
>>     void *data;
>>     struct dev_pagemap pgmap;
>> };
>>
>> As Logan pointed out we can kill the internal struct page_map in
>> kernel/memremap.c and let devm_memremap_pages() take a "struct
>> dev_pagemap *" pointer. That way when future dev_pagemap users come
>> along they can wrap the core structure with whatever specific data
>> they need for their use case.
>
> I don't want to make that change as part of HMM, this can be done
> as a patchset on top. Also it is easier to add callback to dev_pagemap
> especially as we want p2p and HMM side by side not as something
> exclusive.

Fair enough, I'm ok to do the conversion as a post HMM merge cleanup.
Does the p2p case actually need ->fault() and ->free()? I'd rather we
start with dev_private_pagemap and if p2p needs a subset of the same
fields we can think about a rename or shuffling things around.

>
> [...]
>
>> > +#if IS_ENABLED(CONFIG_DEVICE_UNADDRESSABLE)
>> > +static inline swp_entry_t make_device_entry(struct page *page, bool write)
>> > +{
>> > +       return swp_entry(write ? SWP_DEVICE_WRITE : SWP_DEVICE_READ,
>> > +                        page_to_pfn(page));
>> > +}
>> > +
>> > +static inline bool is_device_entry(swp_entry_t entry)
>> > +{
>> > +       int type = swp_type(entry);
>> > +       return type == SWP_DEVICE_READ || type == SWP_DEVICE_WRITE;
>> > +}
>> > +
>> > +static inline void make_device_entry_read(swp_entry_t *entry)
>> > +{
>> > +       *entry = swp_entry(SWP_DEVICE_READ, swp_offset(*entry));
>> > +}
>> > +
>> > +static inline bool is_write_device_entry(swp_entry_t entry)
>> > +{
>> > +       return unlikely(swp_type(entry) == SWP_DEVICE_WRITE);
>> > +}
>> > +
>> > +static inline struct page *device_entry_to_page(swp_entry_t entry)
>> > +{
>> > +       return pfn_to_page(swp_offset(entry));
>> > +}
>> > +
>> > +int device_entry_fault(struct vm_area_struct *vma,
>> > +                      unsigned long addr,
>> > +                      swp_entry_t entry,
>> > +                      unsigned int flags,
>> > +                      pmd_t *pmdp);
>>
>>
>> The use of the "device" term is ambiguous. Since these changes are
>> specific to the MEMORY_DEVICE_PRIVATE can we reflect "private" in the
>> name? Change "device" to "devpriv" or "device_private"?
>
> Yes we can.
>
> [...]
>
>> > @@ -194,12 +196,47 @@ void put_zone_device_page(struct page *page)
>> >          * ZONE_DEVICE page refcount should never reach 0 and never be freed
>> >          * to kernel memory allocator.
>> >          */
>> > -       page_ref_dec(page);
>> > +       int count = page_ref_dec_return(page);
>> > +
>> > +       /*
>> > +        * If refcount is 1 then page is freed and refcount is stable as nobody
>> > +        * holds a reference on the page.
>> > +        */
>> > +       if (page->pgmap->page_free && count == 1)
>> > +               page->pgmap->page_free(page, page->pgmap->data);
>>
>>
>> Reading this it isn't clear whether we are dealing with the public or
>> private case. I think this should be:
>>
>> if (count == 1)
>>         zone_device_page_free(page);
>>
>> ...where zone_device_page_free() is a helper that is explicit about
>> the expectations of which ZONE_DEVICE types need to take action on a
>> 'free' event.
>>
>> static void zone_device_page_free(struct page *page)
>> {
>>         struct dev_private_pagemap *priv_pgmap;
>>
>>         if (page->pgmap->type == MEMORY_DEVICE_PUBLIC)
>>                 return;
>>
>>         priv_pgmap = container_of(page->pgmap, typeof(*priv_pgmap), pgmap);
>>         priv_pgmap->page_free(page);
>> }
>
> Ok.
>
>> > @@ -332,6 +369,10 @@ void *devm_memremap_pages(struct device *dev, struct resource *res,
>> >         }
>> >         pgmap->ref = ref;
>> >         pgmap->res = &page_map->res;
>> > +       pgmap->type = MEMORY_DEVICE_PERSISTENT;
>> > +       pgmap->page_fault = NULL;
>> > +       pgmap->page_free = NULL;
>> > +       pgmap->data = NULL;
>>
>>
>> This goes away if we convert to passing in the dev_pagemap, and I
>> think that is a useful cleanup.
>
> Yes but i rather do it as a patchset on top as i am sure p2p folks
> will want to give input and i don't want to delay until we can agree
> on exact fields we want/need.

Ok.

>
> [...]
>
>> > diff --git a/mm/mprotect.c b/mm/mprotect.c
>> > index 8edd0d5..dadb020 100644
>> > --- a/mm/mprotect.c
>> > +++ b/mm/mprotect.c
>> > @@ -126,6 +126,20 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>> >
>> >                                 pages++;
>> >                         }
>> > +
>> > +                       if (is_write_device_entry(entry)) {
>> > +                               pte_t newpte;
>> > +
>> > +                               /*
>> > +                                * We do not preserve soft-dirtiness. See
>> > +                                * copy_one_pte() for explanation.
>> > +                                */
>> > +                               make_device_entry_read(&entry);
>> > +                               newpte = swp_entry_to_pte(entry);
>> > +                               set_pte_at(mm, addr, pte, newpte);
>> > +
>> > +                               pages++;
>> > +                       }
>> >                 }
>> >         } while (pte++, addr += PAGE_SIZE, addr != end);
>> >         arch_leave_lazy_mmu_mode();
>>
>> I think this is a good example of how the usage of the generic
>> "device" term makes the new "private" code paths harder to spot /
>> discern from the current "public" requirements.
>
> I will change to devpriv
>
> Thank for reviewing.

Thanks for working through this late breaking feedback.

Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable memory

[John Hubbard]

On 4/23/17 6:13 AM, Dan Williams wrote:
> On Sat, Apr 22, 2017 at 11:11 AM, Jerome Glisse <jglisse@redhat.com> wrote:
>> On Fri, Apr 21, 2017 at 10:30:01PM -0700, Dan Williams wrote:
>>> On Fri, Apr 21, 2017 at 8:30 PM, Jérôme Glisse <jglisse@redhat.com> wrote:
>>
>> [...]
>>
>>>> +/*
>>>> + * Specialize ZONE_DEVICE memory into multiple types each having differents
>>>> + * usage.
>>>> + *
>>>> + * MEMORY_DEVICE_PERSISTENT:
>>>> + * Persistent device memory (pmem): struct page might be allocated in different
>>>> + * memory and architecture might want to perform special actions. It is similar
>>>> + * to regular memory, in that the CPU can access it transparently. However,
>>>> + * it is likely to have different bandwidth and latency than regular memory.
>>>> + * See Documentation/nvdimm/nvdimm.txt for more information.
>>>> + *
>>>> + * MEMORY_DEVICE_UNADDRESSABLE:
>>>> + * Device memory that is not directly addressable by the CPU: CPU can neither
>>>> + * read nor write _UNADDRESSABLE memory. In this case, we do still have struct
>>>> + * pages backing the device memory. Doing so simplifies the implementation, but
>>>> + * it is important to remember that there are certain points at which the struct
>>>> + * page must be treated as an opaque object, rather than a "normal" struct page.
>>>> + * A more complete discussion of unaddressable memory may be found in
>>>> + * include/linux/hmm.h and Documentation/vm/hmm.txt.
>>>> + */
>>>> +enum memory_type {
>>>> +       MEMORY_DEVICE_PERSISTENT = 0,
>>>> +       MEMORY_DEVICE_UNADDRESSABLE,
>>>> +};
>>>
>>> Ok, this is a bikeshed, but I think it is important. I think these
>>> should be called MEMORY_DEVICE_PUBLIC and MEMORY_DEVICE_PRIVATE. The
>>> reason is that persistence has nothing to do with the code paths that
>>> deal with the pmem use case of ZONE_DEVICE. The only property the mm
>>> cares about is that the address range behaves the same as host memory
>>> for dma and cpu accesses. The "unaddressable" designation always
>>> confuses me because a memory range isn't memory if it's
>>> "unaddressable". It is addressable, it's just "private" to the device.
>>
>> I can change the name but the memory is truely unaddressable, the CPU
>> can not access it whatsoever (well it can access a small window but
>> even that is not guaranteed).
>>
>
> Understood, but that's still "addressable only by certain agents or
> through a proxy" which seems closer to "private" to me.
>

Actually, MEMORY_DEVICE_PRIVATE / _PUBLIC seems like a good choice to 
me, because the memory may not remain CPU-unaddressable in the future. 
By that, I mean that I know of at least one company (ours) that is 
working on products that will support hardware-based memory coherence 
(and access counters to go along with that). If someone were to enable 
HMM on such a system, then the device memory would be, in fact, directly 
addressable by a CPU--thus exactly contradicting the "unaddressable" name.

Yes, it is true that we would have to modify HMM anyway, in order to 
work in that situation, partly because HMM today relies on CPU and 
device page faults in order to work. And it's also true that we might 
want to take a different approach than HMM, to support that kind of 
device: for example, making it a NUMA node has been debated here, recently.

But even so, I think the potential for the "unaddressable" memory 
actually becoming "addressable" someday, is a good argument for using a 
different name.

thanks,

--
John Hubbard
NVIDIA

Re: [HMM 03/15] mm/unaddressable-memory: new type of ZONE_DEVICE for unaddressable memory

[Jon Masters]

On 04/23/2017 08:39 PM, John Hubbard wrote:

> Actually, MEMORY_DEVICE_PRIVATE / _PUBLIC seems like a good choice to 
> me, because the memory may not remain CPU-unaddressable in the future. 
> By that, I mean that I know of at least one company (ours) that is 
> working on products that will support hardware-based memory coherence 
> (and access counters to go along with that). If someone were to enable 
> HMM on such a system, then the device memory would be, in fact, directly 
> addressable by a CPU--thus exactly contradicting the "unaddressable" name.

I'm expecting similar with CCIX-like coherently attached accelerators
running within FPGAs and as discrete devices as well. Everyone and their
dog is working on hardware based coherence as a programming convenience
and so the notion of ZONE_DEVICE as it stood is going to rapidly evolve
over the next 18 months, maybe less. Short term anyway.

Jon.

[HMM 15/15] hmm: heterogeneous memory management documentation

[Jérôme Glisse]

This add documentation for HMM (Heterogeneous Memory Management). It
presents the motivation behind it, the features necessary for it to
be useful and and gives an overview of how this is implemented.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
---
 Documentation/vm/hmm.txt | 362 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 362 insertions(+)
 create mode 100644 Documentation/vm/hmm.txt

diff --git a/Documentation/vm/hmm.txt b/Documentation/vm/hmm.txt
new file mode 100644
index 0000000..a18ffc0
--- /dev/null
+++ b/Documentation/vm/hmm.txt
@@ -0,0 +1,362 @@
+Heterogeneous Memory Management (HMM)
+
+Transparently allow any component of a program to use any memory region of said
+program with a device without using device specific memory allocator. This is
+becoming a requirement to simplify the use of advance heterogeneous computing
+where GPU, DSP or FPGA are use to perform various computations.
+
+This document is divided as follow, in the first section i expose the problems
+related to the use of a device specific allocator. The second section i expose
+the hardware limitations that are inherent to many platforms. The third section
+gives an overview of HMM designs. The fourth section explains how CPU page-
+table mirroring works and what is HMM purpose in this context. Fifth section
+deals with how device memory is represented inside the kernel. Finaly the last
+section present the new migration helper that allow to leverage the device DMA
+engine.
+
+
+-------------------------------------------------------------------------------
+
+1) Problems of using device specific memory allocator:
+
+Device with large amount of on board memory (several giga bytes) like GPU have
+historically manage their memory through dedicated driver specific API. This
+creates a disconnect between memory allocated and managed by device driver and
+regular application memory (private anonymous, share memory or regular file
+back memory). From here on i will refer to this aspect as split address space.
+I use share address space to refer to the opposite situation ie one in which
+any memory region can be use by device transparently.
+
+Split address space because device can only access memory allocated through the
+device specific API. This imply that all memory object in a program are not
+equal from device point of view which complicate large program that rely on a
+wide set of libraries.
+
+Concretly this means that code that wants to leverage device like GPU need to
+copy object between genericly allocated memory (malloc, mmap private/share/)
+and memory allocated through the device driver API (this still end up with an
+mmap but of the device file).
+
+For flat dataset (array, grid, image, ...) this isn't too hard to achieve but
+complex data-set (list, tree, ...) are hard to get right. Duplicating a complex
+data-set need to re-map all the pointer relations between each of its elements.
+This is error prone and program gets harder to debug because of the duplicate
+data-set.
+
+Split address space also means that library can not transparently use data they
+are getting from core program or other library and thus each library might have
+to duplicate its input data-set using specific memory allocator. Large project
+suffer from this and waste resources because of the various memory copy.
+
+Duplicating each library API to accept as input or output memory allocted by
+each device specific allocator is not a viable option. It would lead to a
+combinatorial explosions in the library entry points.
+
+Finaly with the advance of high level language constructs (in C++ but in other
+language too) it is now possible for compiler to leverage GPU or other devices
+without even the programmer knowledge. Some of compiler identified patterns are
+only do-able with a share address. It is as well more reasonable to use a share
+address space for all the other patterns.
+
+
+-------------------------------------------------------------------------------
+
+2) System bus, device memory characteristics
+
+System bus cripple share address due to few limitations. Most system bus only
+allow basic memory access from device to main memory, even cache coherency is
+often optional. Access to device memory from CPU is even more limited, most
+often than not it is not cache coherent.
+
+If we only consider the PCIE bus than device can access main memory (often
+through an IOMMU) and be cache coherent with the CPUs. However it only allows
+a limited set of atomic operation from device on main memory. This is worse
+in the other direction the CPUs can only access a limited range of the device
+memory and can not perform atomic operations on it. Thus device memory can not
+be consider like regular memory from kernel point of view.
+
+Another crippling factor is the limited bandwidth (~32GBytes/s with PCIE 4.0
+and 16 lanes). This is 33 times less that fastest GPU memory (1 TBytes/s).
+The final limitation is latency, access to main memory from the device has an
+order of magnitude higher latency than when the device access its own memory.
+
+Some platform are developing new system bus or additions/modifications to PCIE
+to address some of those limitations (OpenCAPI, CCIX). They mainly allow two
+way cache coherency between CPU and device and allow all atomic operations the
+architecture supports. Saddly not all platform are following this trends and
+some major architecture are left without hardware solutions to those problems.
+
+So for share address space to make sense not only we must allow device to
+access any memory memory but we must also permit any memory to be migrated to
+device memory while device is using it (blocking CPU access while it happens).
+
+
+-------------------------------------------------------------------------------
+
+3) Share address space and migration
+
+HMM intends to provide two main features. First one is to share the address
+space by duplication the CPU page table into the device page table so same
+address point to same memory and this for any valid main memory address in
+the process address space.
+
+To achieve this, HMM offer a set of helpers to populate the device page table
+while keeping track of CPU page table updates. Device page table updates are
+not as easy as CPU page table updates. To update the device page table you must
+allow a buffer (or use a pool of pre-allocated buffer) and write GPU specifics
+commands in it to perform the update (unmap, cache invalidations and flush,
+...). This can not be done through common code for all device. Hence why HMM
+provides helpers to factor out everything that can be while leaving the gory
+details to the device driver.
+
+The second mechanism HMM provide is a new kind of ZONE_DEVICE memory that does
+allow to allocate a struct page for each page of the device memory. Those page
+are special because the CPU can not map them. They however allow to migrate
+main memory to device memory using exhisting migration mechanism and everything
+looks like if page was swap out to disk from CPU point of view. Using a struct
+page gives the easiest and cleanest integration with existing mm mechanisms.
+Again here HMM only provide helpers, first to hotplug new ZONE_DEVICE memory
+for the device memory and second to perform migration. Policy decision of what
+and when to migrate things is left to the device driver.
+
+Note that any CPU access to a device page trigger a page fault and a migration
+back to main memory ie when a page backing an given address A is migrated from
+a main memory page to a device page then any CPU access to address A trigger a
+page fault and initiate a migration back to main memory.
+
+
+With this two features, HMM not only allow a device to mirror a process address
+space and keeps both CPU and device page table synchronize, but also allow to
+leverage device memory by migrating part of data-set that is actively use by a
+device.
+
+
+-------------------------------------------------------------------------------
+
+4) Address space mirroring implementation and API
+
+Address space mirroring main objective is to allow to duplicate range of CPU
+page table into a device page table and HMM helps keeping both synchronize. A
+device driver that want to mirror a process address space must start with the
+registration of an hmm_mirror struct:
+
+ int hmm_mirror_register(struct hmm_mirror *mirror,
+                         struct mm_struct *mm);
+ int hmm_mirror_register_locked(struct hmm_mirror *mirror,
+                                struct mm_struct *mm);
+
+The locked variant is to be use when the driver is already holding the mmap_sem
+of the mm in write mode. The mirror struct has a set of callback that are use
+to propagate CPU page table:
+
+ struct hmm_mirror_ops {
+     /* sync_cpu_device_pagetables() - synchronize page tables
+      *
+      * @mirror: pointer to struct hmm_mirror
+      * @update_type: type of update that occurred to the CPU page table
+      * @start: virtual start address of the range to update
+      * @end: virtual end address of the range to update
+      *
+      * This callback ultimately originates from mmu_notifiers when the CPU
+      * page table is updated. The device driver must update its page table
+      * in response to this callback. The update argument tells what action
+      * to perform.
+      *
+      * The device driver must not return from this callback until the device
+      * page tables are completely updated (TLBs flushed, etc); this is a
+      * synchronous call.
+      */
+      void (*update)(struct hmm_mirror *mirror,
+                     enum hmm_update action,
+                     unsigned long start,
+                     unsigned long end);
+ };
+
+Device driver must perform update to the range following action (turn range
+read only, or fully unmap, ...). Once driver callback returns the device must
+be done with the update.
+
+
+When device driver wants to populate a range of virtual address it can use
+either:
+ int hmm_vma_get_pfns(struct vm_area_struct *vma,
+                      struct hmm_range *range,
+                      unsigned long start,
+                      unsigned long end,
+                      hmm_pfn_t *pfns);
+ int hmm_vma_fault(struct vm_area_struct *vma,
+                   struct hmm_range *range,
+                   unsigned long start,
+                   unsigned long end,
+                   hmm_pfn_t *pfns,
+                   bool write,
+                   bool block);
+
+First one (hmm_vma_get_pfns()) will only fetch present CPU page table entry and
+will not trigger a page fault on missing or non present entry. The second one
+do trigger page fault on missing or read only entry if write parameter is true.
+Page fault use the generic mm page fault code path just like a CPU page fault.
+
+Both function copy CPU page table into their pfns array argument. Each entry in
+that array correspond to an address in the virtual range. HMM provide a set of
+flags to help driver identify special CPU page table entries.
+
+Locking with the update() callback is the most important aspect the driver must
+respect in order to keep things properly synchronize. The usage pattern is :
+
+ int driver_populate_range(...)
+ {
+      struct hmm_range range;
+      ...
+ again:
+      ret = hmm_vma_get_pfns(vma, &range, start, end, pfns);
+      if (ret)
+          return ret;
+      take_lock(driver->update);
+      if (!hmm_vma_range_done(vma, &range)) {
+          release_lock(driver->update);
+          goto again;
+      }
+
+      // Use pfns array content to update device page table
+
+      release_lock(driver->update);
+      return 0;
+ }
+
+The driver->update lock is the same lock that driver takes inside its update()
+callback. That lock must be call before hmm_vma_range_done() to avoid any race
+with a concurrent CPU page table update.
+
+HMM implements all this on top of the mmu_notifier API because we wanted to a
+simpler API and also to be able to perform optimization latter own like doing
+concurrent device update in multi-devices scenario.
+
+HMM also serve as an impedence missmatch between how CPU page table update are
+done (by CPU write to the page table and TLB flushes) from how device update
+their own page table. Device update is a multi-step process, first appropriate
+commands are write to a buffer, then this buffer is schedule for execution on
+the device. It is only once the device has executed commands in the buffer that
+the update is done. Creating and scheduling update command buffer can happen
+concurrently for multiple devices. Waiting for each device to report commands
+as executed is serialize (there is no point in doing this concurrently).
+
+
+-------------------------------------------------------------------------------
+
+5) Represent and manage device memory from core kernel point of view
+
+Several differents design were try to support device memory. First one use
+device specific data structure to keep information about migrated memory and
+HMM hooked itself in various place of mm code to handle any access to address
+that were back by device memory. It turns out that this ended up replicating
+most of the fields of struct page and also needed many kernel code path to be
+updated to understand this new kind of memory.
+
+Thing is most kernel code path never try to access the memory behind a page
+but only care about struct page contents. Because of this HMM switchted to
+directly using struct page for device memory which left most kernel code path
+un-aware of the difference. We only need to make sure that no one ever try to
+map those page from the CPU side.
+
+HMM provide a set of helpers to register and hotplug device memory as a new
+region needing struct page. This is offer through a very simple API:
+
+ struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
+                                   struct device *device,
+                                   unsigned long size);
+ void hmm_devmem_remove(struct hmm_devmem *devmem);
+
+The hmm_devmem_ops is where most of the important things are:
+
+ struct hmm_devmem_ops {
+     void (*free)(struct hmm_devmem *devmem, struct page *page);
+     int (*fault)(struct hmm_devmem *devmem,
+                  struct vm_area_struct *vma,
+                  unsigned long addr,
+                  struct page *page,
+                  unsigned flags,
+                  pmd_t *pmdp);
+ };
+
+The first callback (free()) happens when the last reference on a device page is
+drop. This means the device page is now free and no longer use by anyone. The
+second callback happens whenever CPU try to access a device page which it can
+not do. This second callback must trigger a migration back to system memory,
+HMM provides an helper to do just that:
+
+ int hmm_devmem_fault_range(struct hmm_devmem *devmem,
+                            struct vm_area_struct *vma,
+                            const struct migrate_vma_ops *ops,
+                            unsigned long mentry,
+                            unsigned long *src,
+                            unsigned long *dst,
+                            unsigned long start,
+                            unsigned long addr,
+                            unsigned long end,
+                            void *private);
+
+It relies on new migrate_vma() helper which is a generic page migration helper
+that work on range of virtual address instead of working on individual pages,
+it also allow to leverage device DMA engine to perform the copy from device to
+main memory (or in the other direction). The next section goes over this new
+helper.
+
+
+-------------------------------------------------------------------------------
+
+6) Migrate to and from device memory
+
+Because CPU can not access device memory, migration must use device DMA engine
+to perform copy from and to device memory. For this we need a new migration
+helper:
+
+ int migrate_vma(const struct migrate_vma_ops *ops,
+                 struct vm_area_struct *vma,
+                 unsigned long mentries,
+                 unsigned long start,
+                 unsigned long end,
+                 unsigned long *src,
+                 unsigned long *dst,
+                 void *private);
+
+Unlike other migration function it works on a range of virtual address, there
+is two reasons for that. First device DMA copy has a high setup overhead cost
+and thus batching multiple pages is needed as otherwise the migration overhead
+make the whole excersie pointless. The second reason is because driver trigger
+such migration base on range of address the device is actively accessing.
+
+The migrate_vma_ops struct define two callbacks. First one (alloc_and_copy())
+control destination memory allocation and copy operation. Second one is there
+to allow device driver to perform cleanup operation after migration.
+
+ struct migrate_vma_ops {
+     void (*alloc_and_copy)(struct vm_area_struct *vma,
+                            const unsigned long *src,
+                            unsigned long *dst,
+                            unsigned long start,
+                            unsigned long end,
+                            void *private);
+     void (*finalize_and_map)(struct vm_area_struct *vma,
+                              const unsigned long *src,
+                              const unsigned long *dst,
+                              unsigned long start,
+                              unsigned long end,
+                              void *private);
+ };
+
+It is important to stress that this migration helpers allow for hole in the
+virtual address range. Some pages in the range might not be migrated for all
+the usual reasons (page is pin, page is lock, ...). This helper does not fail
+but just skip over those pages.
+
+The alloc_and_copy() might as well decide to not migrate all pages in the
+range (for reasons under the callback control). For those the callback just
+have to leave the corresponding dst entry empty.
+
+Finaly the migration of the struct page might fails (for file back page) for
+various reasons (failure to freeze reference, or update page cache, ...). If
+that happens then the finalize_and_map() can catch any pages that was not
+migrated. Note those page were still copied to new page and thus we wasted
+bandwidth but this is considered as a rare event and a price that we are
+willing to pay to keep all the code simpler.
-- 
2.9.3