[PATCH V8 0/2] arm64/mm: Enable memory hot remove

[PATCH V8 0/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

This series enables memory hot remove on arm64 after fixing a possible arm64
platform specific kernel page table race condition. This series is based on
linux-next (next-20190920).

Concurrent vmalloc() and hot-remove conflict:

As pointed out earlier on the v5 thread [2] there can be potential conflict
between concurrent vmalloc() and memory hot-remove operation. The problem here
is caused by inadequate locking in vmalloc() which protects installation of a
page table page but not the walk or the leaf entry modification.

Now free_empty_tables() and it's children functions take into account a maximum
possible range on which it operates as a floor-ceiling boundary. This along with
changed free_pxx_table() makes sure that no page table page is freed unless its
fully within the maximum possible range as decided by the caller.

Testing:

Memory hot remove has been tested on arm64 for 4K, 16K, 64K page config
options with all possible CONFIG_ARM64_VA_BITS and CONFIG_PGTABLE_LEVELS
combinations.

Changes in V8:

- Dropped the first patch (memblock_[free|remove] reorder) from the series which
  is no longer needed for arm64 hot-remove enablement and was posted separately
  as (https://patchwork.kernel.org/patch/11146361/)
- Dropped vmalloc-vmemmap detection and subsequent skipping of free_empty_tables()
- Changed free_empty_[pxx]_tables() functions which now accepts a possible maximum
  floor-ceiling address range on which it operates. Also changed free_pxx_table()
  functions to check against required alignment as well as maximum floor-ceiling
  range as another prerequisite before freeing the page table page.
- Dropped remove_pagetable(), instead call it's constituent functions directly

Changes in V7: (https://lkml.org/lkml/2019/9/3/326)

- vmalloc_vmemmap_overlap gets evaluated early during boot for a given config
- free_empty_tables() gets conditionally called based on vmalloc_vmemmap_overlap

Changes in V6: (https://lkml.org/lkml/2019/7/15/36)

- Implemented most of the suggestions from Mark Rutland
- Added <linux/memory_hotplug.h> in ptdump
- remove_pagetable() now has two distinct passes over the kernel page table
- First pass unmap_hotplug_range() removes leaf level entries at all level
- Second pass free_empty_tables() removes empty page table pages
- Kernel page table lock has been dropped completely
- vmemmap_free() does not call freee_empty_tables() to avoid conflict with vmalloc()
- All address range scanning are converted to do {} while() loop
- Added 'unsigned long end' in __remove_pgd_mapping()
- Callers need not provide starting pointer argument to free_[pte|pmd|pud]_table() 
- Drop the starting pointer argument from free_[pte|pmd|pud]_table() functions
- Fetching pxxp[i] in free_[pte|pmd|pud]_table() is wrapped around in READ_ONCE()
- free_[pte|pmd|pud]_table() now computes starting pointer inside the function
- Fixed TLB handling while freeing huge page section mappings at PMD or PUD level
- Added WARN_ON(!page) in free_hotplug_page_range()
- Added WARN_ON(![pm|pud]_table(pud|pmd)) when there is no section mapping

- [PATCH 1/3] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()
- Request earlier for separate merger (https://patchwork.kernel.org/patch/10986599/)
- s/__remove_memory/try_remove_memory in the subject line
- s/arch_remove_memory/memblock_[free|remove] in the subject line
- A small change in the commit message as re-order happens now for memblock remove
  functions not for arch_remove_memory()

Changes in V5: (https://lkml.org/lkml/2019/5/29/218)

- Have some agreement [1] over using memory_hotplug_lock for arm64 ptdump
- Change 7ba36eccb3f8 ("arm64/mm: Inhibit huge-vmap with ptdump") already merged
- Dropped the above patch from this series
- Fixed indentation problem in arch_[add|remove]_memory() as per David
- Collected all new Acked-by tags
 
Changes in V4: (https://lkml.org/lkml/2019/5/20/19)

- Implemented most of the suggestions from Mark Rutland
- Interchanged patch [PATCH 2/4] <---> [PATCH 3/4] and updated commit message
- Moved CONFIG_PGTABLE_LEVELS inside free_[pud|pmd]_table()
- Used READ_ONCE() in missing instances while accessing page table entries
- s/p???_present()/p???_none() for checking valid kernel page table entries
- WARN_ON() when an entry is !p???_none() and !p???_present() at the same time
- Updated memory hot-remove commit message with additional details as suggested
- Rebased the series on 5.2-rc1 with hotplug changes from David and Michal Hocko
- Collected all new Acked-by tags

Changes in V3: (https://lkml.org/lkml/2019/5/14/197)
 
- Implemented most of the suggestions from Mark Rutland for remove_pagetable()
- Fixed applicable PGTABLE_LEVEL wrappers around pgtable page freeing functions
- Replaced 'direct' with 'sparse_vmap' in remove_pagetable() with inverted polarity
- Changed pointer names ('p' at end) and removed tmp from iterations
- Perform intermediate TLB invalidation while clearing pgtable entries
- Dropped flush_tlb_kernel_range() in remove_pagetable()
- Added flush_tlb_kernel_range() in remove_pte_table() instead
- Renamed page freeing functions for pgtable page and mapped pages
- Used page range size instead of order while freeing mapped or pgtable pages
- Removed all PageReserved() handling while freeing mapped or pgtable pages
- Replaced XXX_index() with XXX_offset() while walking the kernel page table
- Used READ_ONCE() while fetching individual pgtable entries
- Taken overall init_mm.page_table_lock instead of just while changing an entry
- Dropped previously added [pmd|pud]_index() which are not required anymore
- Added a new patch to protect kernel page table race condition for ptdump
- Added a new patch from Mark Rutland to prevent huge-vmap with ptdump

Changes in V2: (https://lkml.org/lkml/2019/4/14/5)

- Added all received review and ack tags
- Split the series from ZONE_DEVICE enablement for better review
- Moved memblock re-order patch to the front as per Robin Murphy
- Updated commit message on memblock re-order patch per Michal Hocko
- Dropped [pmd|pud]_large() definitions
- Used existing [pmd|pud]_sect() instead of earlier [pmd|pud]_large()
- Removed __meminit and __ref tags as per Oscar Salvador
- Dropped unnecessary 'ret' init in arch_add_memory() per Robin Murphy
- Skipped calling into pgtable_page_dtor() for linear mapping page table
  pages and updated all relevant functions

Changes in V1: (https://lkml.org/lkml/2019/4/3/28)

References:

[1] https://lkml.org/lkml/2019/5/28/584
[2] https://lkml.org/lkml/2019/6/11/709

Anshuman Khandual (2):
  arm64/mm: Hold memory hotplug lock while walking for kernel page table dump
  arm64/mm: Enable memory hot remove

 arch/arm64/Kconfig              |   3 +
 arch/arm64/include/asm/memory.h |   1 +
 arch/arm64/mm/mmu.c             | 305 ++++++++++++++++++++++++++++++++++++++--
 arch/arm64/mm/ptdump_debugfs.c  |   4 +
 4 files changed, 304 insertions(+), 9 deletions(-)

-- 
2.7.4

[PATCH V8 0/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

This series enables memory hot remove on arm64 after fixing a possible arm64
platform specific kernel page table race condition. This series is based on
linux-next (next-20190920).

Concurrent vmalloc() and hot-remove conflict:

As pointed out earlier on the v5 thread [2] there can be potential conflict
between concurrent vmalloc() and memory hot-remove operation. The problem here
is caused by inadequate locking in vmalloc() which protects installation of a
page table page but not the walk or the leaf entry modification.

Now free_empty_tables() and it's children functions take into account a maximum
possible range on which it operates as a floor-ceiling boundary. This along with
changed free_pxx_table() makes sure that no page table page is freed unless its
fully within the maximum possible range as decided by the caller.

Testing:

Memory hot remove has been tested on arm64 for 4K, 16K, 64K page config
options with all possible CONFIG_ARM64_VA_BITS and CONFIG_PGTABLE_LEVELS
combinations.

Changes in V8:

- Dropped the first patch (memblock_[free|remove] reorder) from the series which
  is no longer needed for arm64 hot-remove enablement and was posted separately
  as (https://patchwork.kernel.org/patch/11146361/)
- Dropped vmalloc-vmemmap detection and subsequent skipping of free_empty_tables()
- Changed free_empty_[pxx]_tables() functions which now accepts a possible maximum
  floor-ceiling address range on which it operates. Also changed free_pxx_table()
  functions to check against required alignment as well as maximum floor-ceiling
  range as another prerequisite before freeing the page table page.
- Dropped remove_pagetable(), instead call it's constituent functions directly

Changes in V7: (https://lkml.org/lkml/2019/9/3/326)

- vmalloc_vmemmap_overlap gets evaluated early during boot for a given config
- free_empty_tables() gets conditionally called based on vmalloc_vmemmap_overlap

Changes in V6: (https://lkml.org/lkml/2019/7/15/36)

- Implemented most of the suggestions from Mark Rutland
- Added <linux/memory_hotplug.h> in ptdump
- remove_pagetable() now has two distinct passes over the kernel page table
- First pass unmap_hotplug_range() removes leaf level entries at all level
- Second pass free_empty_tables() removes empty page table pages
- Kernel page table lock has been dropped completely
- vmemmap_free() does not call freee_empty_tables() to avoid conflict with vmalloc()
- All address range scanning are converted to do {} while() loop
- Added 'unsigned long end' in __remove_pgd_mapping()
- Callers need not provide starting pointer argument to free_[pte|pmd|pud]_table() 
- Drop the starting pointer argument from free_[pte|pmd|pud]_table() functions
- Fetching pxxp[i] in free_[pte|pmd|pud]_table() is wrapped around in READ_ONCE()
- free_[pte|pmd|pud]_table() now computes starting pointer inside the function
- Fixed TLB handling while freeing huge page section mappings at PMD or PUD level
- Added WARN_ON(!page) in free_hotplug_page_range()
- Added WARN_ON(![pm|pud]_table(pud|pmd)) when there is no section mapping

- [PATCH 1/3] mm/hotplug: Reorder memblock_[free|remove]() calls in try_remove_memory()
- Request earlier for separate merger (https://patchwork.kernel.org/patch/10986599/)
- s/__remove_memory/try_remove_memory in the subject line
- s/arch_remove_memory/memblock_[free|remove] in the subject line
- A small change in the commit message as re-order happens now for memblock remove
  functions not for arch_remove_memory()

Changes in V5: (https://lkml.org/lkml/2019/5/29/218)

- Have some agreement [1] over using memory_hotplug_lock for arm64 ptdump
- Change 7ba36eccb3f8 ("arm64/mm: Inhibit huge-vmap with ptdump") already merged
- Dropped the above patch from this series
- Fixed indentation problem in arch_[add|remove]_memory() as per David
- Collected all new Acked-by tags
 
Changes in V4: (https://lkml.org/lkml/2019/5/20/19)

- Implemented most of the suggestions from Mark Rutland
- Interchanged patch [PATCH 2/4] <---> [PATCH 3/4] and updated commit message
- Moved CONFIG_PGTABLE_LEVELS inside free_[pud|pmd]_table()
- Used READ_ONCE() in missing instances while accessing page table entries
- s/p???_present()/p???_none() for checking valid kernel page table entries
- WARN_ON() when an entry is !p???_none() and !p???_present() at the same time
- Updated memory hot-remove commit message with additional details as suggested
- Rebased the series on 5.2-rc1 with hotplug changes from David and Michal Hocko
- Collected all new Acked-by tags

Changes in V3: (https://lkml.org/lkml/2019/5/14/197)
 
- Implemented most of the suggestions from Mark Rutland for remove_pagetable()
- Fixed applicable PGTABLE_LEVEL wrappers around pgtable page freeing functions
- Replaced 'direct' with 'sparse_vmap' in remove_pagetable() with inverted polarity
- Changed pointer names ('p' at end) and removed tmp from iterations
- Perform intermediate TLB invalidation while clearing pgtable entries
- Dropped flush_tlb_kernel_range() in remove_pagetable()
- Added flush_tlb_kernel_range() in remove_pte_table() instead
- Renamed page freeing functions for pgtable page and mapped pages
- Used page range size instead of order while freeing mapped or pgtable pages
- Removed all PageReserved() handling while freeing mapped or pgtable pages
- Replaced XXX_index() with XXX_offset() while walking the kernel page table
- Used READ_ONCE() while fetching individual pgtable entries
- Taken overall init_mm.page_table_lock instead of just while changing an entry
- Dropped previously added [pmd|pud]_index() which are not required anymore
- Added a new patch to protect kernel page table race condition for ptdump
- Added a new patch from Mark Rutland to prevent huge-vmap with ptdump

Changes in V2: (https://lkml.org/lkml/2019/4/14/5)

- Added all received review and ack tags
- Split the series from ZONE_DEVICE enablement for better review
- Moved memblock re-order patch to the front as per Robin Murphy
- Updated commit message on memblock re-order patch per Michal Hocko
- Dropped [pmd|pud]_large() definitions
- Used existing [pmd|pud]_sect() instead of earlier [pmd|pud]_large()
- Removed __meminit and __ref tags as per Oscar Salvador
- Dropped unnecessary 'ret' init in arch_add_memory() per Robin Murphy
- Skipped calling into pgtable_page_dtor() for linear mapping page table
  pages and updated all relevant functions

Changes in V1: (https://lkml.org/lkml/2019/4/3/28)

References:

[1] https://lkml.org/lkml/2019/5/28/584
[2] https://lkml.org/lkml/2019/6/11/709

Anshuman Khandual (2):
  arm64/mm: Hold memory hotplug lock while walking for kernel page table dump
  arm64/mm: Enable memory hot remove

 arch/arm64/Kconfig              |   3 +
 arch/arm64/include/asm/memory.h |   1 +
 arch/arm64/mm/mmu.c             | 305 ++++++++++++++++++++++++++++++++++++++--
 arch/arm64/mm/ptdump_debugfs.c  |   4 +
 4 files changed, 304 insertions(+), 9 deletions(-)

-- 
2.7.4


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

[PATCH V8 1/2] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

[Anshuman Khandual]

The arm64 page table dump code can race with concurrent modification of the
kernel page tables. When a leaf entries are modified concurrently, the dump
code may log stale or inconsistent information for a VA range, but this is
otherwise not harmful.

When intermediate levels of table are freed, the dump code will continue to
use memory which has been freed and potentially reallocated for another
purpose. In such cases, the dump code may dereference bogus addresses,
leading to a number of potential problems.

Intermediate levels of table may by freed during memory hot-remove,
which will be enabled by a subsequent patch. To avoid racing with
this, take the memory hotplug lock when walking the kernel page table.

Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/mm/ptdump_debugfs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/mm/ptdump_debugfs.c b/arch/arm64/mm/ptdump_debugfs.c
index 064163f..b5eebc8 100644
--- a/arch/arm64/mm/ptdump_debugfs.c
+++ b/arch/arm64/mm/ptdump_debugfs.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/debugfs.h>
+#include <linux/memory_hotplug.h>
 #include <linux/seq_file.h>
 
 #include <asm/ptdump.h>
@@ -7,7 +8,10 @@
 static int ptdump_show(struct seq_file *m, void *v)
 {
 	struct ptdump_info *info = m->private;
+
+	get_online_mems();
 	ptdump_walk_pgd(m, info);
+	put_online_mems();
 	return 0;
 }
 DEFINE_SHOW_ATTRIBUTE(ptdump);
-- 
2.7.4

[PATCH V8 1/2] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump

[Anshuman Khandual]

The arm64 page table dump code can race with concurrent modification of the
kernel page tables. When a leaf entries are modified concurrently, the dump
code may log stale or inconsistent information for a VA range, but this is
otherwise not harmful.

When intermediate levels of table are freed, the dump code will continue to
use memory which has been freed and potentially reallocated for another
purpose. In such cases, the dump code may dereference bogus addresses,
leading to a number of potential problems.

Intermediate levels of table may by freed during memory hot-remove,
which will be enabled by a subsequent patch. To avoid racing with
this, take the memory hotplug lock when walking the kernel page table.

Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/mm/ptdump_debugfs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/mm/ptdump_debugfs.c b/arch/arm64/mm/ptdump_debugfs.c
index 064163f..b5eebc8 100644
--- a/arch/arm64/mm/ptdump_debugfs.c
+++ b/arch/arm64/mm/ptdump_debugfs.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/debugfs.h>
+#include <linux/memory_hotplug.h>
 #include <linux/seq_file.h>
 
 #include <asm/ptdump.h>
@@ -7,7 +8,10 @@
 static int ptdump_show(struct seq_file *m, void *v)
 {
 	struct ptdump_info *info = m->private;
+
+	get_online_mems();
 	ptdump_walk_pgd(m, info);
+	put_online_mems();
 	return 0;
 }
 DEFINE_SHOW_ATTRIBUTE(ptdump);
-- 
2.7.4


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

[PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

The arch code for hot-remove must tear down portions of the linear map and
vmemmap corresponding to memory being removed. In both cases the page
tables mapping these regions must be freed, and when sparse vmemmap is in
use the memory backing the vmemmap must also be freed.

This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
can be used to tear down either region and calls it from vmemmap_free() and
___remove_pgd_mapping(). The sparse_vmap argument determines whether the
backing memory will be freed.

It makes two distinct passes over the kernel page table. In the first pass
with unmap_hotplug_range() it unmaps, invalidates applicable TLB cache and
frees backing memory if required (vmemmap) for each mapped leaf entry. In
the second pass with free_empty_tables() it looks for empty page table
sections whose page table page can be unmapped, TLB invalidated and freed.

While freeing intermediate level page table pages bail out if any of its
entries are still valid. This can happen for partially filled kernel page
table either from a previously attempted failed memory hot add or while
removing an address range which does not span the entire page table page
range.

The vmemmap region may share levels of table with the vmalloc region.
There can be conflicts between hot remove freeing page table pages with
a concurrent vmalloc() walking the kernel page table. This conflict can
not just be solved by taking the init_mm ptl because of existing locking
scheme in vmalloc(). So free_empty_tables() implements a floor and ceiling
method which is borrowed from user page table tear with free_pgd_range()
which skips freeing page table pages if intermediate address range is not
aligned or maximum floor-ceiling might not own the entire page table page.

While here update arch_add_memory() to handle __add_pages() failures by
just unmapping recently added kernel linear mapping. Now enable memory hot
remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.

This implementation is overall inspired from kernel page table tear down
procedure on X86 architecture and user page table tear down method.

Acked-by: Steve Capper <steve.capper@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/Kconfig              |   3 +
 arch/arm64/include/asm/memory.h |   1 +
 arch/arm64/mm/mmu.c             | 305 ++++++++++++++++++++++++++++++++++++++--
 3 files changed, 300 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 41a9b42..511249f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -274,6 +274,9 @@ config ZONE_DMA32
 config ARCH_ENABLE_MEMORY_HOTPLUG
 	def_bool y
 
+config ARCH_ENABLE_MEMORY_HOTREMOVE
+	def_bool y
+
 config SMP
 	def_bool y
 
diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index b61b50b..615dcd0 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -54,6 +54,7 @@
 #define MODULES_VADDR		(BPF_JIT_REGION_END)
 #define MODULES_VSIZE		(SZ_128M)
 #define VMEMMAP_START		(-VMEMMAP_SIZE - SZ_2M)
+#define VMEMMAP_END		(VMEMMAP_START + VMEMMAP_SIZE)
 #define PCI_IO_END		(VMEMMAP_START - SZ_2M)
 #define PCI_IO_START		(PCI_IO_END - PCI_IO_SIZE)
 #define FIXADDR_TOP		(PCI_IO_START - SZ_2M)
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 60c929f..6178837 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -725,6 +725,277 @@ int kern_addr_valid(unsigned long addr)
 
 	return pfn_valid(pte_pfn(pte));
 }
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+static void free_hotplug_page_range(struct page *page, size_t size)
+{
+	WARN_ON(!page || PageReserved(page));
+	free_pages((unsigned long)page_address(page), get_order(size));
+}
+
+static void free_hotplug_pgtable_page(struct page *page)
+{
+	free_hotplug_page_range(page, PAGE_SIZE);
+}
+
+static bool pgtable_range_aligned(unsigned long start, unsigned long end,
+				  unsigned long floor, unsigned long ceiling,
+				  unsigned long mask)
+{
+	start &= mask;
+	if (start < floor)
+		return false;
+
+	if (ceiling) {
+		ceiling &= mask;
+		if (!ceiling)
+			return false;
+	}
+
+	if (end - 1 > ceiling - 1)
+		return false;
+	return true;
+}
+
+static void free_pte_table(pmd_t *pmdp, unsigned long addr, unsigned long end,
+			   unsigned long floor, unsigned long ceiling)
+{
+	struct page *page;
+	pte_t *ptep;
+	int i;
+
+	if (!pgtable_range_aligned(addr, end, floor, ceiling, PMD_MASK))
+		return;
+
+	ptep = pte_offset_kernel(pmdp, 0UL);
+	for (i = 0; i < PTRS_PER_PTE; i++) {
+		if (!pte_none(READ_ONCE(ptep[i])))
+			return;
+	}
+
+	page = pmd_page(READ_ONCE(*pmdp));
+	pmd_clear(pmdp);
+	__flush_tlb_kernel_pgtable(addr);
+	free_hotplug_pgtable_page(page);
+}
+
+static void free_pmd_table(pud_t *pudp, unsigned long addr, unsigned long end,
+			   unsigned long floor, unsigned long ceiling)
+{
+	struct page *page;
+	pmd_t *pmdp;
+	int i;
+
+	if (CONFIG_PGTABLE_LEVELS <= 2)
+		return;
+
+	if (!pgtable_range_aligned(addr, end, floor, ceiling, PUD_MASK))
+		return;
+
+	pmdp = pmd_offset(pudp, 0UL);
+	for (i = 0; i < PTRS_PER_PMD; i++) {
+		if (!pmd_none(READ_ONCE(pmdp[i])))
+			return;
+	}
+
+	page = pud_page(READ_ONCE(*pudp));
+	pud_clear(pudp);
+	__flush_tlb_kernel_pgtable(addr);
+	free_hotplug_pgtable_page(page);
+}
+
+static void free_pud_table(pgd_t *pgdp, unsigned long addr, unsigned  long end,
+			   unsigned long floor, unsigned long ceiling)
+{
+	struct page *page;
+	pud_t *pudp;
+	int i;
+
+	if (CONFIG_PGTABLE_LEVELS <= 3)
+		return;
+
+	if (!pgtable_range_aligned(addr, end, floor, ceiling, PGDIR_MASK))
+		return;
+
+	pudp = pud_offset(pgdp, 0UL);
+	for (i = 0; i < PTRS_PER_PUD; i++) {
+		if (!pud_none(READ_ONCE(pudp[i])))
+			return;
+	}
+
+	page = pgd_page(READ_ONCE(*pgdp));
+	pgd_clear(pgdp);
+	__flush_tlb_kernel_pgtable(addr);
+	free_hotplug_pgtable_page(page);
+}
+
+static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
+				    unsigned long end, bool sparse_vmap)
+{
+	struct page *page;
+	pte_t *ptep, pte;
+
+	do {
+		ptep = pte_offset_kernel(pmdp, addr);
+		pte = READ_ONCE(*ptep);
+		if (pte_none(pte))
+			continue;
+
+		WARN_ON(!pte_present(pte));
+		page = sparse_vmap ? pte_page(pte) : NULL;
+		pte_clear(&init_mm, addr, ptep);
+		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+		if (sparse_vmap)
+			free_hotplug_page_range(page, PAGE_SIZE);
+	} while (addr += PAGE_SIZE, addr < end);
+}
+
+static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
+				    unsigned long end, bool sparse_vmap)
+{
+	unsigned long next;
+	struct page *page;
+	pmd_t *pmdp, pmd;
+
+	do {
+		next = pmd_addr_end(addr, end);
+		pmdp = pmd_offset(pudp, addr);
+		pmd = READ_ONCE(*pmdp);
+		if (pmd_none(pmd))
+			continue;
+
+		WARN_ON(!pmd_present(pmd));
+		if (pmd_sect(pmd)) {
+			page = sparse_vmap ? pmd_page(pmd) : NULL;
+			pmd_clear(pmdp);
+			flush_tlb_kernel_range(addr, next);
+			if (sparse_vmap)
+				free_hotplug_page_range(page, PMD_SIZE);
+			continue;
+		}
+		WARN_ON(!pmd_table(pmd));
+		unmap_hotplug_pte_range(pmdp, addr, next, sparse_vmap);
+	} while (addr = next, addr < end);
+}
+
+static void unmap_hotplug_pud_range(pgd_t *pgdp, unsigned long addr,
+				    unsigned long end, bool sparse_vmap)
+{
+	unsigned long next;
+	struct page *page;
+	pud_t *pudp, pud;
+
+	do {
+		next = pud_addr_end(addr, end);
+		pudp = pud_offset(pgdp, addr);
+		pud = READ_ONCE(*pudp);
+		if (pud_none(pud))
+			continue;
+
+		WARN_ON(!pud_present(pud));
+		if (pud_sect(pud)) {
+			page = sparse_vmap ? pud_page(pud) : NULL;
+			pud_clear(pudp);
+			flush_tlb_kernel_range(addr, next);
+			if (sparse_vmap)
+				free_hotplug_page_range(page, PUD_SIZE);
+			continue;
+		}
+		WARN_ON(!pud_table(pud));
+		unmap_hotplug_pmd_range(pudp, addr, next, sparse_vmap);
+	} while (addr = next, addr < end);
+}
+
+static void unmap_hotplug_range(unsigned long addr, unsigned long end,
+				bool sparse_vmap)
+{
+	unsigned long next;
+	pgd_t *pgdp, pgd;
+
+	do {
+		next = pgd_addr_end(addr, end);
+		pgdp = pgd_offset_k(addr);
+		pgd = READ_ONCE(*pgdp);
+		if (pgd_none(pgd))
+			continue;
+
+		WARN_ON(!pgd_present(pgd));
+		unmap_hotplug_pud_range(pgdp, addr, next, sparse_vmap);
+	} while (addr = next, addr < end);
+}
+
+static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
+				 unsigned long end)
+{
+	pte_t *ptep, pte;
+
+	do {
+		ptep = pte_offset_kernel(pmdp, addr);
+		pte = READ_ONCE(*ptep);
+		WARN_ON(!pte_none(pte));
+	} while (addr += PAGE_SIZE, addr < end);
+}
+
+static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
+				 unsigned long end, unsigned long floor,
+				 unsigned long ceiling)
+{
+	unsigned long next;
+	pmd_t *pmdp, pmd;
+
+	do {
+		next = pmd_addr_end(addr, end);
+		pmdp = pmd_offset(pudp, addr);
+		pmd = READ_ONCE(*pmdp);
+		if (pmd_none(pmd))
+			continue;
+
+		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
+		free_empty_pte_table(pmdp, addr, next);
+		free_pte_table(pmdp, addr, next, floor, ceiling);
+	} while (addr = next, addr < end);
+}
+
+static void free_empty_pud_table(pgd_t *pgdp, unsigned long addr,
+				 unsigned long end, unsigned long floor,
+				 unsigned long ceiling)
+{
+	unsigned long next;
+	pud_t *pudp, pud;
+
+	do {
+		next = pud_addr_end(addr, end);
+		pudp = pud_offset(pgdp, addr);
+		pud = READ_ONCE(*pudp);
+		if (pud_none(pud))
+			continue;
+
+		WARN_ON(!pud_present(pud) || !pud_table(pud) || pud_sect(pud));
+		free_empty_pmd_table(pudp, addr, next, floor, ceiling);
+		free_pmd_table(pudp, addr, next, floor, ceiling);
+	} while (addr = next, addr < end);
+}
+
+static void free_empty_tables(unsigned long addr, unsigned long end,
+				unsigned long floor, unsigned long ceiling)
+{
+	unsigned long next;
+	pgd_t *pgdp, pgd;
+
+	do {
+		next = pgd_addr_end(addr, end);
+		pgdp = pgd_offset_k(addr);
+		pgd = READ_ONCE(*pgdp);
+		if (pgd_none(pgd))
+			continue;
+
+		WARN_ON(!pgd_present(pgd));
+		free_empty_pud_table(pgdp, addr, next, floor, ceiling);
+		free_pud_table(pgdp, addr, next, floor, ceiling);
+	} while (addr = next, addr < end);
+}
+#endif
+
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 #if !ARM64_SWAPPER_USES_SECTION_MAPS
 int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
@@ -772,6 +1043,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 void vmemmap_free(unsigned long start, unsigned long end,
 		struct vmem_altmap *altmap)
 {
+#ifdef CONFIG_MEMORY_HOTPLUG
+	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
+
+	unmap_hotplug_range(start, end, true);
+	free_empty_tables(start, end, VMEMMAP_START, VMEMMAP_END);
+#endif
 }
 #endif	/* CONFIG_SPARSEMEM_VMEMMAP */
 
@@ -1050,10 +1327,21 @@ int p4d_free_pud_page(p4d_t *p4d, unsigned long addr)
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG
+static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
+{
+	unsigned long end = start + size;
+
+	WARN_ON(pgdir != init_mm.pgd);
+	WARN_ON((start < PAGE_OFFSET) || (end > PAGE_END));
+
+	unmap_hotplug_range(start, end, false);
+	free_empty_tables(start, end, PAGE_OFFSET, PAGE_END);
+}
+
 int arch_add_memory(int nid, u64 start, u64 size,
 			struct mhp_restrictions *restrictions)
 {
-	int flags = 0;
+	int ret, flags = 0;
 
 	if (rodata_full || debug_pagealloc_enabled())
 		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
@@ -1061,9 +1349,14 @@ int arch_add_memory(int nid, u64 start, u64 size,
 	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
 			     size, PAGE_KERNEL, __pgd_pgtable_alloc, flags);
 
-	return __add_pages(nid, start >> PAGE_SHIFT, size >> PAGE_SHIFT,
+	ret = __add_pages(nid, start >> PAGE_SHIFT, size >> PAGE_SHIFT,
 			   restrictions);
+	if (ret)
+		__remove_pgd_mapping(swapper_pg_dir,
+				     __phys_to_virt(start), size);
+	return ret;
 }
+
 void arch_remove_memory(int nid, u64 start, u64 size,
 			struct vmem_altmap *altmap)
 {
@@ -1071,14 +1364,8 @@ void arch_remove_memory(int nid, u64 start, u64 size,
 	unsigned long nr_pages = size >> PAGE_SHIFT;
 	struct zone *zone;
 
-	/*
-	 * FIXME: Cleanup page tables (also in arch_add_memory() in case
-	 * adding fails). Until then, this function should only be used
-	 * during memory hotplug (adding memory), not for memory
-	 * unplug. ARCH_ENABLE_MEMORY_HOTREMOVE must not be
-	 * unlocked yet.
-	 */
 	zone = page_zone(pfn_to_page(start_pfn));
 	__remove_pages(zone, start_pfn, nr_pages, altmap);
+	__remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), size);
 }
 #endif
-- 
2.7.4

[PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

The arch code for hot-remove must tear down portions of the linear map and
vmemmap corresponding to memory being removed. In both cases the page
tables mapping these regions must be freed, and when sparse vmemmap is in
use the memory backing the vmemmap must also be freed.

This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
can be used to tear down either region and calls it from vmemmap_free() and
___remove_pgd_mapping(). The sparse_vmap argument determines whether the
backing memory will be freed.

It makes two distinct passes over the kernel page table. In the first pass
with unmap_hotplug_range() it unmaps, invalidates applicable TLB cache and
frees backing memory if required (vmemmap) for each mapped leaf entry. In
the second pass with free_empty_tables() it looks for empty page table
sections whose page table page can be unmapped, TLB invalidated and freed.

While freeing intermediate level page table pages bail out if any of its
entries are still valid. This can happen for partially filled kernel page
table either from a previously attempted failed memory hot add or while
removing an address range which does not span the entire page table page
range.

The vmemmap region may share levels of table with the vmalloc region.
There can be conflicts between hot remove freeing page table pages with
a concurrent vmalloc() walking the kernel page table. This conflict can
not just be solved by taking the init_mm ptl because of existing locking
scheme in vmalloc(). So free_empty_tables() implements a floor and ceiling
method which is borrowed from user page table tear with free_pgd_range()
which skips freeing page table pages if intermediate address range is not
aligned or maximum floor-ceiling might not own the entire page table page.

While here update arch_add_memory() to handle __add_pages() failures by
just unmapping recently added kernel linear mapping. Now enable memory hot
remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.

This implementation is overall inspired from kernel page table tear down
procedure on X86 architecture and user page table tear down method.

Acked-by: Steve Capper <steve.capper@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
 arch/arm64/Kconfig              |   3 +
 arch/arm64/include/asm/memory.h |   1 +
 arch/arm64/mm/mmu.c             | 305 ++++++++++++++++++++++++++++++++++++++--
 3 files changed, 300 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 41a9b42..511249f 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -274,6 +274,9 @@ config ZONE_DMA32
 config ARCH_ENABLE_MEMORY_HOTPLUG
 	def_bool y
 
+config ARCH_ENABLE_MEMORY_HOTREMOVE
+	def_bool y
+
 config SMP
 	def_bool y
 
diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index b61b50b..615dcd0 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -54,6 +54,7 @@
 #define MODULES_VADDR		(BPF_JIT_REGION_END)
 #define MODULES_VSIZE		(SZ_128M)
 #define VMEMMAP_START		(-VMEMMAP_SIZE - SZ_2M)
+#define VMEMMAP_END		(VMEMMAP_START + VMEMMAP_SIZE)
 #define PCI_IO_END		(VMEMMAP_START - SZ_2M)
 #define PCI_IO_START		(PCI_IO_END - PCI_IO_SIZE)
 #define FIXADDR_TOP		(PCI_IO_START - SZ_2M)
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 60c929f..6178837 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -725,6 +725,277 @@ int kern_addr_valid(unsigned long addr)
 
 	return pfn_valid(pte_pfn(pte));
 }
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+static void free_hotplug_page_range(struct page *page, size_t size)
+{
+	WARN_ON(!page || PageReserved(page));
+	free_pages((unsigned long)page_address(page), get_order(size));
+}
+
+static void free_hotplug_pgtable_page(struct page *page)
+{
+	free_hotplug_page_range(page, PAGE_SIZE);
+}
+
+static bool pgtable_range_aligned(unsigned long start, unsigned long end,
+				  unsigned long floor, unsigned long ceiling,
+				  unsigned long mask)
+{
+	start &= mask;
+	if (start < floor)
+		return false;
+
+	if (ceiling) {
+		ceiling &= mask;
+		if (!ceiling)
+			return false;
+	}
+
+	if (end - 1 > ceiling - 1)
+		return false;
+	return true;
+}
+
+static void free_pte_table(pmd_t *pmdp, unsigned long addr, unsigned long end,
+			   unsigned long floor, unsigned long ceiling)
+{
+	struct page *page;
+	pte_t *ptep;
+	int i;
+
+	if (!pgtable_range_aligned(addr, end, floor, ceiling, PMD_MASK))
+		return;
+
+	ptep = pte_offset_kernel(pmdp, 0UL);
+	for (i = 0; i < PTRS_PER_PTE; i++) {
+		if (!pte_none(READ_ONCE(ptep[i])))
+			return;
+	}
+
+	page = pmd_page(READ_ONCE(*pmdp));
+	pmd_clear(pmdp);
+	__flush_tlb_kernel_pgtable(addr);
+	free_hotplug_pgtable_page(page);
+}
+
+static void free_pmd_table(pud_t *pudp, unsigned long addr, unsigned long end,
+			   unsigned long floor, unsigned long ceiling)
+{
+	struct page *page;
+	pmd_t *pmdp;
+	int i;
+
+	if (CONFIG_PGTABLE_LEVELS <= 2)
+		return;
+
+	if (!pgtable_range_aligned(addr, end, floor, ceiling, PUD_MASK))
+		return;
+
+	pmdp = pmd_offset(pudp, 0UL);
+	for (i = 0; i < PTRS_PER_PMD; i++) {
+		if (!pmd_none(READ_ONCE(pmdp[i])))
+			return;
+	}
+
+	page = pud_page(READ_ONCE(*pudp));
+	pud_clear(pudp);
+	__flush_tlb_kernel_pgtable(addr);
+	free_hotplug_pgtable_page(page);
+}
+
+static void free_pud_table(pgd_t *pgdp, unsigned long addr, unsigned  long end,
+			   unsigned long floor, unsigned long ceiling)
+{
+	struct page *page;
+	pud_t *pudp;
+	int i;
+
+	if (CONFIG_PGTABLE_LEVELS <= 3)
+		return;
+
+	if (!pgtable_range_aligned(addr, end, floor, ceiling, PGDIR_MASK))
+		return;
+
+	pudp = pud_offset(pgdp, 0UL);
+	for (i = 0; i < PTRS_PER_PUD; i++) {
+		if (!pud_none(READ_ONCE(pudp[i])))
+			return;
+	}
+
+	page = pgd_page(READ_ONCE(*pgdp));
+	pgd_clear(pgdp);
+	__flush_tlb_kernel_pgtable(addr);
+	free_hotplug_pgtable_page(page);
+}
+
+static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
+				    unsigned long end, bool sparse_vmap)
+{
+	struct page *page;
+	pte_t *ptep, pte;
+
+	do {
+		ptep = pte_offset_kernel(pmdp, addr);
+		pte = READ_ONCE(*ptep);
+		if (pte_none(pte))
+			continue;
+
+		WARN_ON(!pte_present(pte));
+		page = sparse_vmap ? pte_page(pte) : NULL;
+		pte_clear(&init_mm, addr, ptep);
+		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+		if (sparse_vmap)
+			free_hotplug_page_range(page, PAGE_SIZE);
+	} while (addr += PAGE_SIZE, addr < end);
+}
+
+static void unmap_hotplug_pmd_range(pud_t *pudp, unsigned long addr,
+				    unsigned long end, bool sparse_vmap)
+{
+	unsigned long next;
+	struct page *page;
+	pmd_t *pmdp, pmd;
+
+	do {
+		next = pmd_addr_end(addr, end);
+		pmdp = pmd_offset(pudp, addr);
+		pmd = READ_ONCE(*pmdp);
+		if (pmd_none(pmd))
+			continue;
+
+		WARN_ON(!pmd_present(pmd));
+		if (pmd_sect(pmd)) {
+			page = sparse_vmap ? pmd_page(pmd) : NULL;
+			pmd_clear(pmdp);
+			flush_tlb_kernel_range(addr, next);
+			if (sparse_vmap)
+				free_hotplug_page_range(page, PMD_SIZE);
+			continue;
+		}
+		WARN_ON(!pmd_table(pmd));
+		unmap_hotplug_pte_range(pmdp, addr, next, sparse_vmap);
+	} while (addr = next, addr < end);
+}
+
+static void unmap_hotplug_pud_range(pgd_t *pgdp, unsigned long addr,
+				    unsigned long end, bool sparse_vmap)
+{
+	unsigned long next;
+	struct page *page;
+	pud_t *pudp, pud;
+
+	do {
+		next = pud_addr_end(addr, end);
+		pudp = pud_offset(pgdp, addr);
+		pud = READ_ONCE(*pudp);
+		if (pud_none(pud))
+			continue;
+
+		WARN_ON(!pud_present(pud));
+		if (pud_sect(pud)) {
+			page = sparse_vmap ? pud_page(pud) : NULL;
+			pud_clear(pudp);
+			flush_tlb_kernel_range(addr, next);
+			if (sparse_vmap)
+				free_hotplug_page_range(page, PUD_SIZE);
+			continue;
+		}
+		WARN_ON(!pud_table(pud));
+		unmap_hotplug_pmd_range(pudp, addr, next, sparse_vmap);
+	} while (addr = next, addr < end);
+}
+
+static void unmap_hotplug_range(unsigned long addr, unsigned long end,
+				bool sparse_vmap)
+{
+	unsigned long next;
+	pgd_t *pgdp, pgd;
+
+	do {
+		next = pgd_addr_end(addr, end);
+		pgdp = pgd_offset_k(addr);
+		pgd = READ_ONCE(*pgdp);
+		if (pgd_none(pgd))
+			continue;
+
+		WARN_ON(!pgd_present(pgd));
+		unmap_hotplug_pud_range(pgdp, addr, next, sparse_vmap);
+	} while (addr = next, addr < end);
+}
+
+static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
+				 unsigned long end)
+{
+	pte_t *ptep, pte;
+
+	do {
+		ptep = pte_offset_kernel(pmdp, addr);
+		pte = READ_ONCE(*ptep);
+		WARN_ON(!pte_none(pte));
+	} while (addr += PAGE_SIZE, addr < end);
+}
+
+static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
+				 unsigned long end, unsigned long floor,
+				 unsigned long ceiling)
+{
+	unsigned long next;
+	pmd_t *pmdp, pmd;
+
+	do {
+		next = pmd_addr_end(addr, end);
+		pmdp = pmd_offset(pudp, addr);
+		pmd = READ_ONCE(*pmdp);
+		if (pmd_none(pmd))
+			continue;
+
+		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
+		free_empty_pte_table(pmdp, addr, next);
+		free_pte_table(pmdp, addr, next, floor, ceiling);
+	} while (addr = next, addr < end);
+}
+
+static void free_empty_pud_table(pgd_t *pgdp, unsigned long addr,
+				 unsigned long end, unsigned long floor,
+				 unsigned long ceiling)
+{
+	unsigned long next;
+	pud_t *pudp, pud;
+
+	do {
+		next = pud_addr_end(addr, end);
+		pudp = pud_offset(pgdp, addr);
+		pud = READ_ONCE(*pudp);
+		if (pud_none(pud))
+			continue;
+
+		WARN_ON(!pud_present(pud) || !pud_table(pud) || pud_sect(pud));
+		free_empty_pmd_table(pudp, addr, next, floor, ceiling);
+		free_pmd_table(pudp, addr, next, floor, ceiling);
+	} while (addr = next, addr < end);
+}
+
+static void free_empty_tables(unsigned long addr, unsigned long end,
+				unsigned long floor, unsigned long ceiling)
+{
+	unsigned long next;
+	pgd_t *pgdp, pgd;
+
+	do {
+		next = pgd_addr_end(addr, end);
+		pgdp = pgd_offset_k(addr);
+		pgd = READ_ONCE(*pgdp);
+		if (pgd_none(pgd))
+			continue;
+
+		WARN_ON(!pgd_present(pgd));
+		free_empty_pud_table(pgdp, addr, next, floor, ceiling);
+		free_pud_table(pgdp, addr, next, floor, ceiling);
+	} while (addr = next, addr < end);
+}
+#endif
+
 #ifdef CONFIG_SPARSEMEM_VMEMMAP
 #if !ARM64_SWAPPER_USES_SECTION_MAPS
 int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
@@ -772,6 +1043,12 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 void vmemmap_free(unsigned long start, unsigned long end,
 		struct vmem_altmap *altmap)
 {
+#ifdef CONFIG_MEMORY_HOTPLUG
+	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
+
+	unmap_hotplug_range(start, end, true);
+	free_empty_tables(start, end, VMEMMAP_START, VMEMMAP_END);
+#endif
 }
 #endif	/* CONFIG_SPARSEMEM_VMEMMAP */
 
@@ -1050,10 +1327,21 @@ int p4d_free_pud_page(p4d_t *p4d, unsigned long addr)
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG
+static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
+{
+	unsigned long end = start + size;
+
+	WARN_ON(pgdir != init_mm.pgd);
+	WARN_ON((start < PAGE_OFFSET) || (end > PAGE_END));
+
+	unmap_hotplug_range(start, end, false);
+	free_empty_tables(start, end, PAGE_OFFSET, PAGE_END);
+}
+
 int arch_add_memory(int nid, u64 start, u64 size,
 			struct mhp_restrictions *restrictions)
 {
-	int flags = 0;
+	int ret, flags = 0;
 
 	if (rodata_full || debug_pagealloc_enabled())
 		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
@@ -1061,9 +1349,14 @@ int arch_add_memory(int nid, u64 start, u64 size,
 	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
 			     size, PAGE_KERNEL, __pgd_pgtable_alloc, flags);
 
-	return __add_pages(nid, start >> PAGE_SHIFT, size >> PAGE_SHIFT,
+	ret = __add_pages(nid, start >> PAGE_SHIFT, size >> PAGE_SHIFT,
 			   restrictions);
+	if (ret)
+		__remove_pgd_mapping(swapper_pg_dir,
+				     __phys_to_virt(start), size);
+	return ret;
 }
+
 void arch_remove_memory(int nid, u64 start, u64 size,
 			struct vmem_altmap *altmap)
 {
@@ -1071,14 +1364,8 @@ void arch_remove_memory(int nid, u64 start, u64 size,
 	unsigned long nr_pages = size >> PAGE_SHIFT;
 	struct zone *zone;
 
-	/*
-	 * FIXME: Cleanup page tables (also in arch_add_memory() in case
-	 * adding fails). Until then, this function should only be used
-	 * during memory hotplug (adding memory), not for memory
-	 * unplug. ARCH_ENABLE_MEMORY_HOTREMOVE must not be
-	 * unlocked yet.
-	 */
 	zone = page_zone(pfn_to_page(start_pfn));
 	__remove_pages(zone, start_pfn, nr_pages, altmap);
+	__remove_pgd_mapping(swapper_pg_dir, __phys_to_virt(start), size);
 }
 #endif
-- 
2.7.4


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Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Matthew Wilcox]

On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +static void free_hotplug_page_range(struct page *page, size_t size)
> +{
> +	WARN_ON(!page || PageReserved(page));

WARN_ON(!page) isn't terribly useful.  You're going to crash on the very
next line when you call page_address() anyway.  If this line were

	if (WARN_ON(!page || PageReserved(page)))
		return;

it would make sense, or if it were just

	WARN_ON(PageReserved(page))

it would also make sense.

> +	free_pages((unsigned long)page_address(page), get_order(size));
> +}

Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Matthew Wilcox]

On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +static void free_hotplug_page_range(struct page *page, size_t size)
> +{
> +	WARN_ON(!page || PageReserved(page));

WARN_ON(!page) isn't terribly useful.  You're going to crash on the very
next line when you call page_address() anyway.  If this line were

	if (WARN_ON(!page || PageReserved(page)))
		return;

it would make sense, or if it were just

	WARN_ON(PageReserved(page))

it would also make sense.

> +	free_pages((unsigned long)page_address(page), get_order(size));
> +}

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Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 4255 bytes --]

Hi Anshuman,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on linus/master]
[cannot apply to v5.3 next-20190920]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Anshuman-Khandual/arm64-mm-Hold-memory-hotplug-lock-while-walking-for-kernel-page-table-dump/20190923-134733
config: arm64-allmodconfig (attached as .config)
compiler: aarch64-linux-gcc (GCC) 7.4.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.4.0 make.cross ARCH=arm64 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

>> mm/memremap.c:16:0: warning: "SECTION_MASK" redefined
    #define SECTION_MASK ~((1UL << PA_SECTION_SHIFT) - 1)
    
   In file included from arch/arm64/include/asm/processor.h:34:0,
                    from include/linux/mutex.h:19,
                    from include/linux/kernfs.h:12,
                    from include/linux/sysfs.h:16,
                    from include/linux/kobject.h:20,
                    from include/linux/device.h:16,
                    from mm/memremap.c:3:
   arch/arm64/include/asm/pgtable-hwdef.h:79:0: note: this is the location of the previous definition
    #define SECTION_MASK  (~(SECTION_SIZE-1))
    
>> mm/memremap.c:17:0: warning: "SECTION_SIZE" redefined
    #define SECTION_SIZE (1UL << PA_SECTION_SHIFT)
    
   In file included from arch/arm64/include/asm/processor.h:34:0,
                    from include/linux/mutex.h:19,
                    from include/linux/kernfs.h:12,
                    from include/linux/sysfs.h:16,
                    from include/linux/kobject.h:20,
                    from include/linux/device.h:16,
                    from mm/memremap.c:3:
   arch/arm64/include/asm/pgtable-hwdef.h:78:0: note: this is the location of the previous definition
    #define SECTION_SIZE  (_AC(1, UL) << SECTION_SHIFT)
    

vim +/SECTION_MASK +16 mm/memremap.c

7d3dcf26a6559f kernel/memremap.c Christoph Hellwig 2015-08-10  @3  #include <linux/device.h>
92281dee825f6d kernel/memremap.c Dan Williams      2015-08-10   4  #include <linux/io.h>
0207df4fa1a869 kernel/memremap.c Andrey Ryabinin   2018-08-17   5  #include <linux/kasan.h>
41e94a851304f7 kernel/memremap.c Christoph Hellwig 2015-08-17   6  #include <linux/memory_hotplug.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15   7  #include <linux/mm.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15   8  #include <linux/pfn_t.h>
5042db43cc26f5 kernel/memremap.c Jérôme Glisse     2017-09-08   9  #include <linux/swap.h>
5042db43cc26f5 kernel/memremap.c Jérôme Glisse     2017-09-08  10  #include <linux/swapops.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15  11  #include <linux/types.h>
e7638488434415 kernel/memremap.c Dan Williams      2018-05-16  12  #include <linux/wait_bit.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15  13  #include <linux/xarray.h>
92281dee825f6d kernel/memremap.c Dan Williams      2015-08-10  14  
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15  15  static DEFINE_XARRAY(pgmap_array);
9476df7d80dfc4 kernel/memremap.c Dan Williams      2016-01-15 @16  #define SECTION_MASK ~((1UL << PA_SECTION_SHIFT) - 1)
9476df7d80dfc4 kernel/memremap.c Dan Williams      2016-01-15 @17  #define SECTION_SIZE (1UL << PA_SECTION_SHIFT)
9476df7d80dfc4 kernel/memremap.c Dan Williams      2016-01-15  18  

:::::: The code at line 16 was first introduced by commit
:::::: 9476df7d80dfc425b37bfecf1d89edf8ec81fcb6 mm: introduce find_dev_pagemap()

:::::: TO: Dan Williams <dan.j.williams@intel.com>
:::::: CC: Linus Torvalds <torvalds@linux-foundation.org>

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 67082 bytes --]

Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[kbuild test robot]

[-- Attachment #1: Type: text/plain, Size: 4255 bytes --]

Hi Anshuman,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on linus/master]
[cannot apply to v5.3 next-20190920]
[if your patch is applied to the wrong git tree, please drop us a note to help
improve the system. BTW, we also suggest to use '--base' option to specify the
base tree in git format-patch, please see https://stackoverflow.com/a/37406982]

url:    https://github.com/0day-ci/linux/commits/Anshuman-Khandual/arm64-mm-Hold-memory-hotplug-lock-while-walking-for-kernel-page-table-dump/20190923-134733
config: arm64-allmodconfig (attached as .config)
compiler: aarch64-linux-gcc (GCC) 7.4.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        GCC_VERSION=7.4.0 make.cross ARCH=arm64 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

>> mm/memremap.c:16:0: warning: "SECTION_MASK" redefined
    #define SECTION_MASK ~((1UL << PA_SECTION_SHIFT) - 1)
    
   In file included from arch/arm64/include/asm/processor.h:34:0,
                    from include/linux/mutex.h:19,
                    from include/linux/kernfs.h:12,
                    from include/linux/sysfs.h:16,
                    from include/linux/kobject.h:20,
                    from include/linux/device.h:16,
                    from mm/memremap.c:3:
   arch/arm64/include/asm/pgtable-hwdef.h:79:0: note: this is the location of the previous definition
    #define SECTION_MASK  (~(SECTION_SIZE-1))
    
>> mm/memremap.c:17:0: warning: "SECTION_SIZE" redefined
    #define SECTION_SIZE (1UL << PA_SECTION_SHIFT)
    
   In file included from arch/arm64/include/asm/processor.h:34:0,
                    from include/linux/mutex.h:19,
                    from include/linux/kernfs.h:12,
                    from include/linux/sysfs.h:16,
                    from include/linux/kobject.h:20,
                    from include/linux/device.h:16,
                    from mm/memremap.c:3:
   arch/arm64/include/asm/pgtable-hwdef.h:78:0: note: this is the location of the previous definition
    #define SECTION_SIZE  (_AC(1, UL) << SECTION_SHIFT)
    

vim +/SECTION_MASK +16 mm/memremap.c

7d3dcf26a6559f kernel/memremap.c Christoph Hellwig 2015-08-10  @3  #include <linux/device.h>
92281dee825f6d kernel/memremap.c Dan Williams      2015-08-10   4  #include <linux/io.h>
0207df4fa1a869 kernel/memremap.c Andrey Ryabinin   2018-08-17   5  #include <linux/kasan.h>
41e94a851304f7 kernel/memremap.c Christoph Hellwig 2015-08-17   6  #include <linux/memory_hotplug.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15   7  #include <linux/mm.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15   8  #include <linux/pfn_t.h>
5042db43cc26f5 kernel/memremap.c Jérôme Glisse     2017-09-08   9  #include <linux/swap.h>
5042db43cc26f5 kernel/memremap.c Jérôme Glisse     2017-09-08  10  #include <linux/swapops.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15  11  #include <linux/types.h>
e7638488434415 kernel/memremap.c Dan Williams      2018-05-16  12  #include <linux/wait_bit.h>
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15  13  #include <linux/xarray.h>
92281dee825f6d kernel/memremap.c Dan Williams      2015-08-10  14  
bcfa4b72111c9a kernel/memremap.c Matthew Wilcox    2018-08-15  15  static DEFINE_XARRAY(pgmap_array);
9476df7d80dfc4 kernel/memremap.c Dan Williams      2016-01-15 @16  #define SECTION_MASK ~((1UL << PA_SECTION_SHIFT) - 1)
9476df7d80dfc4 kernel/memremap.c Dan Williams      2016-01-15 @17  #define SECTION_SIZE (1UL << PA_SECTION_SHIFT)
9476df7d80dfc4 kernel/memremap.c Dan Williams      2016-01-15  18  

:::::: The code at line 16 was first introduced by commit
:::::: 9476df7d80dfc425b37bfecf1d89edf8ec81fcb6 mm: introduce find_dev_pagemap()

:::::: TO: Dan Williams <dan.j.williams@intel.com>
:::::: CC: Linus Torvalds <torvalds@linux-foundation.org>

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 67082 bytes --]

[-- Attachment #3: Type: text/plain, Size: 176 bytes --]

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Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

On 09/23/2019 04:47 PM, Matthew Wilcox wrote:
> On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +static void free_hotplug_page_range(struct page *page, size_t size)
>> +{
>> +	WARN_ON(!page || PageReserved(page));
> 
> WARN_ON(!page) isn't terribly useful.  You're going to crash on the very
> next line when you call page_address() anyway.  If this line were
> 
> 	if (WARN_ON(!page || PageReserved(page)))
> 		return;
> 
> it would make sense, or if it were just
> 
> 	WARN_ON(PageReserved(page))
> 
> it would also make sense.

I guess WARN_ON(PageReserved(page)) should be good enough to make sure
that page being freed here was originally allocated at runtime for a
previous memory hot add operation and did not some how come from the
memblock reserved area. That was the original objective for this check.
Will change it.

> 
>> +	free_pages((unsigned long)page_address(page), get_order(size));
>> +}
> 

Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

On 09/23/2019 04:47 PM, Matthew Wilcox wrote:
> On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
>> +#ifdef CONFIG_MEMORY_HOTPLUG
>> +static void free_hotplug_page_range(struct page *page, size_t size)
>> +{
>> +	WARN_ON(!page || PageReserved(page));
> 
> WARN_ON(!page) isn't terribly useful.  You're going to crash on the very
> next line when you call page_address() anyway.  If this line were
> 
> 	if (WARN_ON(!page || PageReserved(page)))
> 		return;
> 
> it would make sense, or if it were just
> 
> 	WARN_ON(PageReserved(page))
> 
> it would also make sense.

I guess WARN_ON(PageReserved(page)) should be good enough to make sure
that page being freed here was originally allocated at runtime for a
previous memory hot add operation and did not some how come from the
memblock reserved area. That was the original objective for this check.
Will change it.

> 
>> +	free_pages((unsigned long)page_address(page), get_order(size));
>> +}
> 

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Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Catalin Marinas]

On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
> The arch code for hot-remove must tear down portions of the linear map and
> vmemmap corresponding to memory being removed. In both cases the page
> tables mapping these regions must be freed, and when sparse vmemmap is in
> use the memory backing the vmemmap must also be freed.
> 
> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
> can be used to tear down either region and calls it from vmemmap_free() and
> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
> backing memory will be freed.

Can you change the 'sparse_vmap' name to something more meaningful which
would suggest freeing of the backing memory?

> It makes two distinct passes over the kernel page table. In the first pass
> with unmap_hotplug_range() it unmaps, invalidates applicable TLB cache and
> frees backing memory if required (vmemmap) for each mapped leaf entry. In
> the second pass with free_empty_tables() it looks for empty page table
> sections whose page table page can be unmapped, TLB invalidated and freed.
> 
> While freeing intermediate level page table pages bail out if any of its
> entries are still valid. This can happen for partially filled kernel page
> table either from a previously attempted failed memory hot add or while
> removing an address range which does not span the entire page table page
> range.
> 
> The vmemmap region may share levels of table with the vmalloc region.
> There can be conflicts between hot remove freeing page table pages with
> a concurrent vmalloc() walking the kernel page table. This conflict can
> not just be solved by taking the init_mm ptl because of existing locking
> scheme in vmalloc(). So free_empty_tables() implements a floor and ceiling
> method which is borrowed from user page table tear with free_pgd_range()
> which skips freeing page table pages if intermediate address range is not
> aligned or maximum floor-ceiling might not own the entire page table page.
> 
> While here update arch_add_memory() to handle __add_pages() failures by
> just unmapping recently added kernel linear mapping. Now enable memory hot
> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.
> 
> This implementation is overall inspired from kernel page table tear down
> procedure on X86 architecture and user page table tear down method.
> 
> Acked-by: Steve Capper <steve.capper@arm.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Given the amount of changes since version 7, do the acks still stand?

[...]
> +static void free_pte_table(pmd_t *pmdp, unsigned long addr, unsigned long end,
> +			   unsigned long floor, unsigned long ceiling)
> +{
> +	struct page *page;
> +	pte_t *ptep;
> +	int i;
> +
> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PMD_MASK))
> +		return;
> +
> +	ptep = pte_offset_kernel(pmdp, 0UL);
> +	for (i = 0; i < PTRS_PER_PTE; i++) {
> +		if (!pte_none(READ_ONCE(ptep[i])))
> +			return;
> +	}
> +
> +	page = pmd_page(READ_ONCE(*pmdp));

Arguably, that's not the pmd page we are freeing here. Even if you get
the same result, pmd_page() is normally used for huge pages pointed at
by the pmd entry. Since you have the ptep already, why not use
virt_to_page(ptep)?

> +	pmd_clear(pmdp);
> +	__flush_tlb_kernel_pgtable(addr);
> +	free_hotplug_pgtable_page(page);
> +}
> +
> +static void free_pmd_table(pud_t *pudp, unsigned long addr, unsigned long end,
> +			   unsigned long floor, unsigned long ceiling)
> +{
> +	struct page *page;
> +	pmd_t *pmdp;
> +	int i;
> +
> +	if (CONFIG_PGTABLE_LEVELS <= 2)
> +		return;
> +
> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PUD_MASK))
> +		return;
> +
> +	pmdp = pmd_offset(pudp, 0UL);
> +	for (i = 0; i < PTRS_PER_PMD; i++) {
> +		if (!pmd_none(READ_ONCE(pmdp[i])))
> +			return;
> +	}
> +
> +	page = pud_page(READ_ONCE(*pudp));

Same here, virt_to_page(pmdp).

> +	pud_clear(pudp);
> +	__flush_tlb_kernel_pgtable(addr);
> +	free_hotplug_pgtable_page(page);
> +}
> +
> +static void free_pud_table(pgd_t *pgdp, unsigned long addr, unsigned  long end,
> +			   unsigned long floor, unsigned long ceiling)
> +{
> +	struct page *page;
> +	pud_t *pudp;
> +	int i;
> +
> +	if (CONFIG_PGTABLE_LEVELS <= 3)
> +		return;
> +
> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PGDIR_MASK))
> +		return;
> +
> +	pudp = pud_offset(pgdp, 0UL);
> +	for (i = 0; i < PTRS_PER_PUD; i++) {
> +		if (!pud_none(READ_ONCE(pudp[i])))
> +			return;
> +	}
> +
> +	page = pgd_page(READ_ONCE(*pgdp));

As above.

> +	pgd_clear(pgdp);
> +	__flush_tlb_kernel_pgtable(addr);
> +	free_hotplug_pgtable_page(page);
> +}
> +
> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
> +				    unsigned long end, bool sparse_vmap)
> +{
> +	struct page *page;
> +	pte_t *ptep, pte;
> +
> +	do {
> +		ptep = pte_offset_kernel(pmdp, addr);
> +		pte = READ_ONCE(*ptep);
> +		if (pte_none(pte))
> +			continue;
> +
> +		WARN_ON(!pte_present(pte));
> +		page = sparse_vmap ? pte_page(pte) : NULL;
> +		pte_clear(&init_mm, addr, ptep);
> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> +		if (sparse_vmap)
> +			free_hotplug_page_range(page, PAGE_SIZE);

You could only set 'page' if sparse_vmap (or even drop 'page' entirely).
The compiler is probably smart enough to optimise it but using a
pointless ternary operator just makes the code harder to follow.

> +	} while (addr += PAGE_SIZE, addr < end);
> +}
[...]
> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
> +				 unsigned long end)
> +{
> +	pte_t *ptep, pte;
> +
> +	do {
> +		ptep = pte_offset_kernel(pmdp, addr);
> +		pte = READ_ONCE(*ptep);
> +		WARN_ON(!pte_none(pte));
> +	} while (addr += PAGE_SIZE, addr < end);
> +}
> +
> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
> +				 unsigned long end, unsigned long floor,
> +				 unsigned long ceiling)
> +{
> +	unsigned long next;
> +	pmd_t *pmdp, pmd;
> +
> +	do {
> +		next = pmd_addr_end(addr, end);
> +		pmdp = pmd_offset(pudp, addr);
> +		pmd = READ_ONCE(*pmdp);
> +		if (pmd_none(pmd))
> +			continue;
> +
> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
> +		free_empty_pte_table(pmdp, addr, next);
> +		free_pte_table(pmdp, addr, next, floor, ceiling);

Do we need two closely named functions here? Can you not collapse
free_empty_pud_table() and free_pte_table() into a single one? The same
comment for the pmd/pud variants. I just find this confusing.

> +	} while (addr = next, addr < end);

You could make these function in two steps: first, as above, invoke the
next level recursively; second, after the do..while loop, check whether
it's empty and free the pmd page as in free_pmd_table().

> +}
[...]

-- 
Catalin

Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Catalin Marinas]

On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
> The arch code for hot-remove must tear down portions of the linear map and
> vmemmap corresponding to memory being removed. In both cases the page
> tables mapping these regions must be freed, and when sparse vmemmap is in
> use the memory backing the vmemmap must also be freed.
> 
> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
> can be used to tear down either region and calls it from vmemmap_free() and
> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
> backing memory will be freed.

Can you change the 'sparse_vmap' name to something more meaningful which
would suggest freeing of the backing memory?

> It makes two distinct passes over the kernel page table. In the first pass
> with unmap_hotplug_range() it unmaps, invalidates applicable TLB cache and
> frees backing memory if required (vmemmap) for each mapped leaf entry. In
> the second pass with free_empty_tables() it looks for empty page table
> sections whose page table page can be unmapped, TLB invalidated and freed.
> 
> While freeing intermediate level page table pages bail out if any of its
> entries are still valid. This can happen for partially filled kernel page
> table either from a previously attempted failed memory hot add or while
> removing an address range which does not span the entire page table page
> range.
> 
> The vmemmap region may share levels of table with the vmalloc region.
> There can be conflicts between hot remove freeing page table pages with
> a concurrent vmalloc() walking the kernel page table. This conflict can
> not just be solved by taking the init_mm ptl because of existing locking
> scheme in vmalloc(). So free_empty_tables() implements a floor and ceiling
> method which is borrowed from user page table tear with free_pgd_range()
> which skips freeing page table pages if intermediate address range is not
> aligned or maximum floor-ceiling might not own the entire page table page.
> 
> While here update arch_add_memory() to handle __add_pages() failures by
> just unmapping recently added kernel linear mapping. Now enable memory hot
> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.
> 
> This implementation is overall inspired from kernel page table tear down
> procedure on X86 architecture and user page table tear down method.
> 
> Acked-by: Steve Capper <steve.capper@arm.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>

Given the amount of changes since version 7, do the acks still stand?

[...]
> +static void free_pte_table(pmd_t *pmdp, unsigned long addr, unsigned long end,
> +			   unsigned long floor, unsigned long ceiling)
> +{
> +	struct page *page;
> +	pte_t *ptep;
> +	int i;
> +
> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PMD_MASK))
> +		return;
> +
> +	ptep = pte_offset_kernel(pmdp, 0UL);
> +	for (i = 0; i < PTRS_PER_PTE; i++) {
> +		if (!pte_none(READ_ONCE(ptep[i])))
> +			return;
> +	}
> +
> +	page = pmd_page(READ_ONCE(*pmdp));

Arguably, that's not the pmd page we are freeing here. Even if you get
the same result, pmd_page() is normally used for huge pages pointed at
by the pmd entry. Since you have the ptep already, why not use
virt_to_page(ptep)?

> +	pmd_clear(pmdp);
> +	__flush_tlb_kernel_pgtable(addr);
> +	free_hotplug_pgtable_page(page);
> +}
> +
> +static void free_pmd_table(pud_t *pudp, unsigned long addr, unsigned long end,
> +			   unsigned long floor, unsigned long ceiling)
> +{
> +	struct page *page;
> +	pmd_t *pmdp;
> +	int i;
> +
> +	if (CONFIG_PGTABLE_LEVELS <= 2)
> +		return;
> +
> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PUD_MASK))
> +		return;
> +
> +	pmdp = pmd_offset(pudp, 0UL);
> +	for (i = 0; i < PTRS_PER_PMD; i++) {
> +		if (!pmd_none(READ_ONCE(pmdp[i])))
> +			return;
> +	}
> +
> +	page = pud_page(READ_ONCE(*pudp));

Same here, virt_to_page(pmdp).

> +	pud_clear(pudp);
> +	__flush_tlb_kernel_pgtable(addr);
> +	free_hotplug_pgtable_page(page);
> +}
> +
> +static void free_pud_table(pgd_t *pgdp, unsigned long addr, unsigned  long end,
> +			   unsigned long floor, unsigned long ceiling)
> +{
> +	struct page *page;
> +	pud_t *pudp;
> +	int i;
> +
> +	if (CONFIG_PGTABLE_LEVELS <= 3)
> +		return;
> +
> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PGDIR_MASK))
> +		return;
> +
> +	pudp = pud_offset(pgdp, 0UL);
> +	for (i = 0; i < PTRS_PER_PUD; i++) {
> +		if (!pud_none(READ_ONCE(pudp[i])))
> +			return;
> +	}
> +
> +	page = pgd_page(READ_ONCE(*pgdp));

As above.

> +	pgd_clear(pgdp);
> +	__flush_tlb_kernel_pgtable(addr);
> +	free_hotplug_pgtable_page(page);
> +}
> +
> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
> +				    unsigned long end, bool sparse_vmap)
> +{
> +	struct page *page;
> +	pte_t *ptep, pte;
> +
> +	do {
> +		ptep = pte_offset_kernel(pmdp, addr);
> +		pte = READ_ONCE(*ptep);
> +		if (pte_none(pte))
> +			continue;
> +
> +		WARN_ON(!pte_present(pte));
> +		page = sparse_vmap ? pte_page(pte) : NULL;
> +		pte_clear(&init_mm, addr, ptep);
> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> +		if (sparse_vmap)
> +			free_hotplug_page_range(page, PAGE_SIZE);

You could only set 'page' if sparse_vmap (or even drop 'page' entirely).
The compiler is probably smart enough to optimise it but using a
pointless ternary operator just makes the code harder to follow.

> +	} while (addr += PAGE_SIZE, addr < end);
> +}
[...]
> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
> +				 unsigned long end)
> +{
> +	pte_t *ptep, pte;
> +
> +	do {
> +		ptep = pte_offset_kernel(pmdp, addr);
> +		pte = READ_ONCE(*ptep);
> +		WARN_ON(!pte_none(pte));
> +	} while (addr += PAGE_SIZE, addr < end);
> +}
> +
> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
> +				 unsigned long end, unsigned long floor,
> +				 unsigned long ceiling)
> +{
> +	unsigned long next;
> +	pmd_t *pmdp, pmd;
> +
> +	do {
> +		next = pmd_addr_end(addr, end);
> +		pmdp = pmd_offset(pudp, addr);
> +		pmd = READ_ONCE(*pmdp);
> +		if (pmd_none(pmd))
> +			continue;
> +
> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
> +		free_empty_pte_table(pmdp, addr, next);
> +		free_pte_table(pmdp, addr, next, floor, ceiling);

Do we need two closely named functions here? Can you not collapse
free_empty_pud_table() and free_pte_table() into a single one? The same
comment for the pmd/pud variants. I just find this confusing.

> +	} while (addr = next, addr < end);

You could make these function in two steps: first, as above, invoke the
next level recursively; second, after the do..while loop, check whether
it's empty and free the pmd page as in free_pmd_table().

> +}
[...]

-- 
Catalin

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Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

On 10/07/2019 07:47 PM, Catalin Marinas wrote:
> On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
>> The arch code for hot-remove must tear down portions of the linear map and
>> vmemmap corresponding to memory being removed. In both cases the page
>> tables mapping these regions must be freed, and when sparse vmemmap is in
>> use the memory backing the vmemmap must also be freed.
>>
>> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
>> can be used to tear down either region and calls it from vmemmap_free() and
>> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
>> backing memory will be freed.
> 
> Can you change the 'sparse_vmap' name to something more meaningful which
> would suggest freeing of the backing memory?

free_mapped_mem or free_backed_mem ? Even shorter forms like free_mapped or
free_backed might do as well. Do you have a particular preference here ? But
yes, sparse_vmap has been very much specific to vmemmap for these functions
which are now very generic in nature.

> 
>> It makes two distinct passes over the kernel page table. In the first pass
>> with unmap_hotplug_range() it unmaps, invalidates applicable TLB cache and
>> frees backing memory if required (vmemmap) for each mapped leaf entry. In
>> the second pass with free_empty_tables() it looks for empty page table
>> sections whose page table page can be unmapped, TLB invalidated and freed.
>>
>> While freeing intermediate level page table pages bail out if any of its
>> entries are still valid. This can happen for partially filled kernel page
>> table either from a previously attempted failed memory hot add or while
>> removing an address range which does not span the entire page table page
>> range.
>>
>> The vmemmap region may share levels of table with the vmalloc region.
>> There can be conflicts between hot remove freeing page table pages with
>> a concurrent vmalloc() walking the kernel page table. This conflict can
>> not just be solved by taking the init_mm ptl because of existing locking
>> scheme in vmalloc(). So free_empty_tables() implements a floor and ceiling
>> method which is borrowed from user page table tear with free_pgd_range()
>> which skips freeing page table pages if intermediate address range is not
>> aligned or maximum floor-ceiling might not own the entire page table page.
>>
>> While here update arch_add_memory() to handle __add_pages() failures by
>> just unmapping recently added kernel linear mapping. Now enable memory hot
>> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.
>>
>> This implementation is overall inspired from kernel page table tear down
>> procedure on X86 architecture and user page table tear down method.
>>
>> Acked-by: Steve Capper <steve.capper@arm.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> 
> Given the amount of changes since version 7, do the acks still stand?

I had taken the liberty to carry them till V7 where the implementation has
been sort of structurally similar but as you mention, there have been some
basic changes in the approach since V7. Will drop these tags in next version
and request their fresh ACKs once again.

> 
> [...]
>> +static void free_pte_table(pmd_t *pmdp, unsigned long addr, unsigned long end,
>> +			   unsigned long floor, unsigned long ceiling)
>> +{
>> +	struct page *page;
>> +	pte_t *ptep;
>> +	int i;
>> +
>> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PMD_MASK))
>> +		return;
>> +
>> +	ptep = pte_offset_kernel(pmdp, 0UL);
>> +	for (i = 0; i < PTRS_PER_PTE; i++) {
>> +		if (!pte_none(READ_ONCE(ptep[i])))
>> +			return;
>> +	}
>> +
>> +	page = pmd_page(READ_ONCE(*pmdp));
> 
> Arguably, that's not the pmd page we are freeing here. Even if you get
> the same result, pmd_page() is normally used for huge pages pointed at
> by the pmd entry. Since you have the ptep already, why not use
> virt_to_page(ptep)?

Makes sense, will do.

> 
>> +	pmd_clear(pmdp);
>> +	__flush_tlb_kernel_pgtable(addr);
>> +	free_hotplug_pgtable_page(page);
>> +}
>> +
>> +static void free_pmd_table(pud_t *pudp, unsigned long addr, unsigned long end,
>> +			   unsigned long floor, unsigned long ceiling)
>> +{
>> +	struct page *page;
>> +	pmd_t *pmdp;
>> +	int i;
>> +
>> +	if (CONFIG_PGTABLE_LEVELS <= 2)
>> +		return;
>> +
>> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PUD_MASK))
>> +		return;
>> +
>> +	pmdp = pmd_offset(pudp, 0UL);
>> +	for (i = 0; i < PTRS_PER_PMD; i++) {
>> +		if (!pmd_none(READ_ONCE(pmdp[i])))
>> +			return;
>> +	}
>> +
>> +	page = pud_page(READ_ONCE(*pudp));
> 
> Same here, virt_to_page(pmdp).

Will do.

> 
>> +	pud_clear(pudp);
>> +	__flush_tlb_kernel_pgtable(addr);
>> +	free_hotplug_pgtable_page(page);
>> +}
>> +
>> +static void free_pud_table(pgd_t *pgdp, unsigned long addr, unsigned  long end,
>> +			   unsigned long floor, unsigned long ceiling)
>> +{
>> +	struct page *page;
>> +	pud_t *pudp;
>> +	int i;
>> +
>> +	if (CONFIG_PGTABLE_LEVELS <= 3)
>> +		return;
>> +
>> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PGDIR_MASK))
>> +		return;
>> +
>> +	pudp = pud_offset(pgdp, 0UL);
>> +	for (i = 0; i < PTRS_PER_PUD; i++) {
>> +		if (!pud_none(READ_ONCE(pudp[i])))
>> +			return;
>> +	}
>> +
>> +	page = pgd_page(READ_ONCE(*pgdp));
> 
> As above.

Will do.

> 
>> +	pgd_clear(pgdp);
>> +	__flush_tlb_kernel_pgtable(addr);
>> +	free_hotplug_pgtable_page(page);
>> +}
>> +
>> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
>> +				    unsigned long end, bool sparse_vmap)
>> +{
>> +	struct page *page;
>> +	pte_t *ptep, pte;
>> +
>> +	do {
>> +		ptep = pte_offset_kernel(pmdp, addr);
>> +		pte = READ_ONCE(*ptep);
>> +		if (pte_none(pte))
>> +			continue;
>> +
>> +		WARN_ON(!pte_present(pte));
>> +		page = sparse_vmap ? pte_page(pte) : NULL;
>> +		pte_clear(&init_mm, addr, ptep);
>> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
>> +		if (sparse_vmap)
>> +			free_hotplug_page_range(page, PAGE_SIZE);
> 
> You could only set 'page' if sparse_vmap (or even drop 'page' entirely).

I am afraid 'page' is being used to hold pte_page(pte) extraction which
needs to be freed (sparse_vmap) as we are going to clear the ptep entry
in the next statement and lose access to it for good. We will need some
where to hold onto pte_page(pte) across pte_clear() as we cannot free it
before clearing it's entry and flushing the TLB. Hence wondering how the
'page' can be completely dropped.

> The compiler is probably smart enough to optimise it but using a
> pointless ternary operator just makes the code harder to follow.

Not sure I got this but are you suggesting for an 'if' statement here

if (sparse_vmap)
	page = pte_page(pte);

instead of the current assignment ?

page = sparse_vmap ? pte_page(pte) : NULL;

> 
>> +	} while (addr += PAGE_SIZE, addr < end);
>> +}
> [...]
>> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
>> +				 unsigned long end)
>> +{
>> +	pte_t *ptep, pte;
>> +
>> +	do {
>> +		ptep = pte_offset_kernel(pmdp, addr);
>> +		pte = READ_ONCE(*ptep);
>> +		WARN_ON(!pte_none(pte));
>> +	} while (addr += PAGE_SIZE, addr < end);
>> +}
>> +
>> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
>> +				 unsigned long end, unsigned long floor,
>> +				 unsigned long ceiling)
>> +{
>> +	unsigned long next;
>> +	pmd_t *pmdp, pmd;
>> +
>> +	do {
>> +		next = pmd_addr_end(addr, end);
>> +		pmdp = pmd_offset(pudp, addr);
>> +		pmd = READ_ONCE(*pmdp);
>> +		if (pmd_none(pmd))
>> +			continue;
>> +
>> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
>> +		free_empty_pte_table(pmdp, addr, next);
>> +		free_pte_table(pmdp, addr, next, floor, ceiling);
> 
> Do we need two closely named functions here? Can you not collapse
> free_empty_pud_table() and free_pte_table() into a single one? The same
> comment for the pmd/pud variants. I just find this confusing.

The two functions could be collapsed into a single one. But just wanted to
keep free_pxx_table() part which checks floor/ceiling alignment, non-zero
entries clear off the actual page table walking.

> 
>> +	} while (addr = next, addr < end);
> 
> You could make these function in two steps: first, as above, invoke the
> next level recursively; second, after the do..while loop, check whether
> it's empty and free the pmd page as in free_pmd_table().

free_pte_table() freeing attempt actually belongs to free_empty_pte_table().
Yes, free_pte_table() part can be moved inside free_empty_pte_table() after
it's do..while(). Also s/free_pte_table/free_pte_page to make it sound more
distinct with respect to free_empty_pte_table(). Just that the pgtable page
freeing part is still wrapped in a helper function to hide it's details.

But if you prefer not to have these helpers free_pxx_page() and directly
encode the second step in free_empty_pxx_table(), then will that instead.

> 
>> +}
> [...]
> 

Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

On 10/07/2019 07:47 PM, Catalin Marinas wrote:
> On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
>> The arch code for hot-remove must tear down portions of the linear map and
>> vmemmap corresponding to memory being removed. In both cases the page
>> tables mapping these regions must be freed, and when sparse vmemmap is in
>> use the memory backing the vmemmap must also be freed.
>>
>> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
>> can be used to tear down either region and calls it from vmemmap_free() and
>> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
>> backing memory will be freed.
> 
> Can you change the 'sparse_vmap' name to something more meaningful which
> would suggest freeing of the backing memory?

free_mapped_mem or free_backed_mem ? Even shorter forms like free_mapped or
free_backed might do as well. Do you have a particular preference here ? But
yes, sparse_vmap has been very much specific to vmemmap for these functions
which are now very generic in nature.

> 
>> It makes two distinct passes over the kernel page table. In the first pass
>> with unmap_hotplug_range() it unmaps, invalidates applicable TLB cache and
>> frees backing memory if required (vmemmap) for each mapped leaf entry. In
>> the second pass with free_empty_tables() it looks for empty page table
>> sections whose page table page can be unmapped, TLB invalidated and freed.
>>
>> While freeing intermediate level page table pages bail out if any of its
>> entries are still valid. This can happen for partially filled kernel page
>> table either from a previously attempted failed memory hot add or while
>> removing an address range which does not span the entire page table page
>> range.
>>
>> The vmemmap region may share levels of table with the vmalloc region.
>> There can be conflicts between hot remove freeing page table pages with
>> a concurrent vmalloc() walking the kernel page table. This conflict can
>> not just be solved by taking the init_mm ptl because of existing locking
>> scheme in vmalloc(). So free_empty_tables() implements a floor and ceiling
>> method which is borrowed from user page table tear with free_pgd_range()
>> which skips freeing page table pages if intermediate address range is not
>> aligned or maximum floor-ceiling might not own the entire page table page.
>>
>> While here update arch_add_memory() to handle __add_pages() failures by
>> just unmapping recently added kernel linear mapping. Now enable memory hot
>> remove on arm64 platforms by default with ARCH_ENABLE_MEMORY_HOTREMOVE.
>>
>> This implementation is overall inspired from kernel page table tear down
>> procedure on X86 architecture and user page table tear down method.
>>
>> Acked-by: Steve Capper <steve.capper@arm.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> 
> Given the amount of changes since version 7, do the acks still stand?

I had taken the liberty to carry them till V7 where the implementation has
been sort of structurally similar but as you mention, there have been some
basic changes in the approach since V7. Will drop these tags in next version
and request their fresh ACKs once again.

> 
> [...]
>> +static void free_pte_table(pmd_t *pmdp, unsigned long addr, unsigned long end,
>> +			   unsigned long floor, unsigned long ceiling)
>> +{
>> +	struct page *page;
>> +	pte_t *ptep;
>> +	int i;
>> +
>> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PMD_MASK))
>> +		return;
>> +
>> +	ptep = pte_offset_kernel(pmdp, 0UL);
>> +	for (i = 0; i < PTRS_PER_PTE; i++) {
>> +		if (!pte_none(READ_ONCE(ptep[i])))
>> +			return;
>> +	}
>> +
>> +	page = pmd_page(READ_ONCE(*pmdp));
> 
> Arguably, that's not the pmd page we are freeing here. Even if you get
> the same result, pmd_page() is normally used for huge pages pointed at
> by the pmd entry. Since you have the ptep already, why not use
> virt_to_page(ptep)?

Makes sense, will do.

> 
>> +	pmd_clear(pmdp);
>> +	__flush_tlb_kernel_pgtable(addr);
>> +	free_hotplug_pgtable_page(page);
>> +}
>> +
>> +static void free_pmd_table(pud_t *pudp, unsigned long addr, unsigned long end,
>> +			   unsigned long floor, unsigned long ceiling)
>> +{
>> +	struct page *page;
>> +	pmd_t *pmdp;
>> +	int i;
>> +
>> +	if (CONFIG_PGTABLE_LEVELS <= 2)
>> +		return;
>> +
>> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PUD_MASK))
>> +		return;
>> +
>> +	pmdp = pmd_offset(pudp, 0UL);
>> +	for (i = 0; i < PTRS_PER_PMD; i++) {
>> +		if (!pmd_none(READ_ONCE(pmdp[i])))
>> +			return;
>> +	}
>> +
>> +	page = pud_page(READ_ONCE(*pudp));
> 
> Same here, virt_to_page(pmdp).

Will do.

> 
>> +	pud_clear(pudp);
>> +	__flush_tlb_kernel_pgtable(addr);
>> +	free_hotplug_pgtable_page(page);
>> +}
>> +
>> +static void free_pud_table(pgd_t *pgdp, unsigned long addr, unsigned  long end,
>> +			   unsigned long floor, unsigned long ceiling)
>> +{
>> +	struct page *page;
>> +	pud_t *pudp;
>> +	int i;
>> +
>> +	if (CONFIG_PGTABLE_LEVELS <= 3)
>> +		return;
>> +
>> +	if (!pgtable_range_aligned(addr, end, floor, ceiling, PGDIR_MASK))
>> +		return;
>> +
>> +	pudp = pud_offset(pgdp, 0UL);
>> +	for (i = 0; i < PTRS_PER_PUD; i++) {
>> +		if (!pud_none(READ_ONCE(pudp[i])))
>> +			return;
>> +	}
>> +
>> +	page = pgd_page(READ_ONCE(*pgdp));
> 
> As above.

Will do.

> 
>> +	pgd_clear(pgdp);
>> +	__flush_tlb_kernel_pgtable(addr);
>> +	free_hotplug_pgtable_page(page);
>> +}
>> +
>> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
>> +				    unsigned long end, bool sparse_vmap)
>> +{
>> +	struct page *page;
>> +	pte_t *ptep, pte;
>> +
>> +	do {
>> +		ptep = pte_offset_kernel(pmdp, addr);
>> +		pte = READ_ONCE(*ptep);
>> +		if (pte_none(pte))
>> +			continue;
>> +
>> +		WARN_ON(!pte_present(pte));
>> +		page = sparse_vmap ? pte_page(pte) : NULL;
>> +		pte_clear(&init_mm, addr, ptep);
>> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
>> +		if (sparse_vmap)
>> +			free_hotplug_page_range(page, PAGE_SIZE);
> 
> You could only set 'page' if sparse_vmap (or even drop 'page' entirely).

I am afraid 'page' is being used to hold pte_page(pte) extraction which
needs to be freed (sparse_vmap) as we are going to clear the ptep entry
in the next statement and lose access to it for good. We will need some
where to hold onto pte_page(pte) across pte_clear() as we cannot free it
before clearing it's entry and flushing the TLB. Hence wondering how the
'page' can be completely dropped.

> The compiler is probably smart enough to optimise it but using a
> pointless ternary operator just makes the code harder to follow.

Not sure I got this but are you suggesting for an 'if' statement here

if (sparse_vmap)
	page = pte_page(pte);

instead of the current assignment ?

page = sparse_vmap ? pte_page(pte) : NULL;

> 
>> +	} while (addr += PAGE_SIZE, addr < end);
>> +}
> [...]
>> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
>> +				 unsigned long end)
>> +{
>> +	pte_t *ptep, pte;
>> +
>> +	do {
>> +		ptep = pte_offset_kernel(pmdp, addr);
>> +		pte = READ_ONCE(*ptep);
>> +		WARN_ON(!pte_none(pte));
>> +	} while (addr += PAGE_SIZE, addr < end);
>> +}
>> +
>> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
>> +				 unsigned long end, unsigned long floor,
>> +				 unsigned long ceiling)
>> +{
>> +	unsigned long next;
>> +	pmd_t *pmdp, pmd;
>> +
>> +	do {
>> +		next = pmd_addr_end(addr, end);
>> +		pmdp = pmd_offset(pudp, addr);
>> +		pmd = READ_ONCE(*pmdp);
>> +		if (pmd_none(pmd))
>> +			continue;
>> +
>> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
>> +		free_empty_pte_table(pmdp, addr, next);
>> +		free_pte_table(pmdp, addr, next, floor, ceiling);
> 
> Do we need two closely named functions here? Can you not collapse
> free_empty_pud_table() and free_pte_table() into a single one? The same
> comment for the pmd/pud variants. I just find this confusing.

The two functions could be collapsed into a single one. But just wanted to
keep free_pxx_table() part which checks floor/ceiling alignment, non-zero
entries clear off the actual page table walking.

> 
>> +	} while (addr = next, addr < end);
> 
> You could make these function in two steps: first, as above, invoke the
> next level recursively; second, after the do..while loop, check whether
> it's empty and free the pmd page as in free_pmd_table().

free_pte_table() freeing attempt actually belongs to free_empty_pte_table().
Yes, free_pte_table() part can be moved inside free_empty_pte_table() after
it's do..while(). Also s/free_pte_table/free_pte_page to make it sound more
distinct with respect to free_empty_pte_table(). Just that the pgtable page
freeing part is still wrapped in a helper function to hide it's details.

But if you prefer not to have these helpers free_pxx_page() and directly
encode the second step in free_empty_pxx_table(), then will that instead.

> 
>> +}
> [...]
> 

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Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Catalin Marinas]

On Tue, Oct 08, 2019 at 10:06:26AM +0530, Anshuman Khandual wrote:
> On 10/07/2019 07:47 PM, Catalin Marinas wrote:
> > On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
> >> The arch code for hot-remove must tear down portions of the linear map and
> >> vmemmap corresponding to memory being removed. In both cases the page
> >> tables mapping these regions must be freed, and when sparse vmemmap is in
> >> use the memory backing the vmemmap must also be freed.
> >>
> >> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
> >> can be used to tear down either region and calls it from vmemmap_free() and
> >> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
> >> backing memory will be freed.
> > 
> > Can you change the 'sparse_vmap' name to something more meaningful which
> > would suggest freeing of the backing memory?
> 
> free_mapped_mem or free_backed_mem ? Even shorter forms like free_mapped or
> free_backed might do as well. Do you have a particular preference here ? But
> yes, sparse_vmap has been very much specific to vmemmap for these functions
> which are now very generic in nature.

free_mapped would do.

> >> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
> >> +				    unsigned long end, bool sparse_vmap)
> >> +{
> >> +	struct page *page;
> >> +	pte_t *ptep, pte;
> >> +
> >> +	do {
> >> +		ptep = pte_offset_kernel(pmdp, addr);
> >> +		pte = READ_ONCE(*ptep);
> >> +		if (pte_none(pte))
> >> +			continue;
> >> +
> >> +		WARN_ON(!pte_present(pte));
> >> +		page = sparse_vmap ? pte_page(pte) : NULL;
> >> +		pte_clear(&init_mm, addr, ptep);
> >> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> >> +		if (sparse_vmap)
> >> +			free_hotplug_page_range(page, PAGE_SIZE);
> > 
> > You could only set 'page' if sparse_vmap (or even drop 'page' entirely).
> 
> I am afraid 'page' is being used to hold pte_page(pte) extraction which
> needs to be freed (sparse_vmap) as we are going to clear the ptep entry
> in the next statement and lose access to it for good.

You clear *ptep, not pte.

> We will need some
> where to hold onto pte_page(pte) across pte_clear() as we cannot free it
> before clearing it's entry and flushing the TLB. Hence wondering how the
> 'page' can be completely dropped.
> 
> > The compiler is probably smart enough to optimise it but using a
> > pointless ternary operator just makes the code harder to follow.
> 
> Not sure I got this but are you suggesting for an 'if' statement here
> 
> if (sparse_vmap)
> 	page = pte_page(pte);
> 
> instead of the current assignment ?
> 
> page = sparse_vmap ? pte_page(pte) : NULL;

I suggest:

	if (sparse_vmap)
		free_hotplug_pgtable_page(pte_page(pte), PAGE_SIZE);

> >> +	} while (addr += PAGE_SIZE, addr < end);
> >> +}
> > [...]
> >> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
> >> +				 unsigned long end)
> >> +{
> >> +	pte_t *ptep, pte;
> >> +
> >> +	do {
> >> +		ptep = pte_offset_kernel(pmdp, addr);
> >> +		pte = READ_ONCE(*ptep);
> >> +		WARN_ON(!pte_none(pte));
> >> +	} while (addr += PAGE_SIZE, addr < end);
> >> +}
> >> +
> >> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
> >> +				 unsigned long end, unsigned long floor,
> >> +				 unsigned long ceiling)
> >> +{
> >> +	unsigned long next;
> >> +	pmd_t *pmdp, pmd;
> >> +
> >> +	do {
> >> +		next = pmd_addr_end(addr, end);
> >> +		pmdp = pmd_offset(pudp, addr);
> >> +		pmd = READ_ONCE(*pmdp);
> >> +		if (pmd_none(pmd))
> >> +			continue;
> >> +
> >> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
> >> +		free_empty_pte_table(pmdp, addr, next);
> >> +		free_pte_table(pmdp, addr, next, floor, ceiling);
> > 
> > Do we need two closely named functions here? Can you not collapse
> > free_empty_pud_table() and free_pte_table() into a single one? The same
> > comment for the pmd/pud variants. I just find this confusing.
> 
> The two functions could be collapsed into a single one. But just wanted to
> keep free_pxx_table() part which checks floor/ceiling alignment, non-zero
> entries clear off the actual page table walking.

With the pmd variant, they both take the floor/ceiling argument while
the free_empty_pte_table() doesn't even free anything. So not entirely
consistent.

Can you not just copy the free_pgd_range() functions but instead of
p*d_free_tlb() just do the TLB invalidation followed by page freeing?
That seems to be an easier pattern to follow.

-- 
Catalin

Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Catalin Marinas]

On Tue, Oct 08, 2019 at 10:06:26AM +0530, Anshuman Khandual wrote:
> On 10/07/2019 07:47 PM, Catalin Marinas wrote:
> > On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
> >> The arch code for hot-remove must tear down portions of the linear map and
> >> vmemmap corresponding to memory being removed. In both cases the page
> >> tables mapping these regions must be freed, and when sparse vmemmap is in
> >> use the memory backing the vmemmap must also be freed.
> >>
> >> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
> >> can be used to tear down either region and calls it from vmemmap_free() and
> >> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
> >> backing memory will be freed.
> > 
> > Can you change the 'sparse_vmap' name to something more meaningful which
> > would suggest freeing of the backing memory?
> 
> free_mapped_mem or free_backed_mem ? Even shorter forms like free_mapped or
> free_backed might do as well. Do you have a particular preference here ? But
> yes, sparse_vmap has been very much specific to vmemmap for these functions
> which are now very generic in nature.

free_mapped would do.

> >> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
> >> +				    unsigned long end, bool sparse_vmap)
> >> +{
> >> +	struct page *page;
> >> +	pte_t *ptep, pte;
> >> +
> >> +	do {
> >> +		ptep = pte_offset_kernel(pmdp, addr);
> >> +		pte = READ_ONCE(*ptep);
> >> +		if (pte_none(pte))
> >> +			continue;
> >> +
> >> +		WARN_ON(!pte_present(pte));
> >> +		page = sparse_vmap ? pte_page(pte) : NULL;
> >> +		pte_clear(&init_mm, addr, ptep);
> >> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> >> +		if (sparse_vmap)
> >> +			free_hotplug_page_range(page, PAGE_SIZE);
> > 
> > You could only set 'page' if sparse_vmap (or even drop 'page' entirely).
> 
> I am afraid 'page' is being used to hold pte_page(pte) extraction which
> needs to be freed (sparse_vmap) as we are going to clear the ptep entry
> in the next statement and lose access to it for good.

You clear *ptep, not pte.

> We will need some
> where to hold onto pte_page(pte) across pte_clear() as we cannot free it
> before clearing it's entry and flushing the TLB. Hence wondering how the
> 'page' can be completely dropped.
> 
> > The compiler is probably smart enough to optimise it but using a
> > pointless ternary operator just makes the code harder to follow.
> 
> Not sure I got this but are you suggesting for an 'if' statement here
> 
> if (sparse_vmap)
> 	page = pte_page(pte);
> 
> instead of the current assignment ?
> 
> page = sparse_vmap ? pte_page(pte) : NULL;

I suggest:

	if (sparse_vmap)
		free_hotplug_pgtable_page(pte_page(pte), PAGE_SIZE);

> >> +	} while (addr += PAGE_SIZE, addr < end);
> >> +}
> > [...]
> >> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
> >> +				 unsigned long end)
> >> +{
> >> +	pte_t *ptep, pte;
> >> +
> >> +	do {
> >> +		ptep = pte_offset_kernel(pmdp, addr);
> >> +		pte = READ_ONCE(*ptep);
> >> +		WARN_ON(!pte_none(pte));
> >> +	} while (addr += PAGE_SIZE, addr < end);
> >> +}
> >> +
> >> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
> >> +				 unsigned long end, unsigned long floor,
> >> +				 unsigned long ceiling)
> >> +{
> >> +	unsigned long next;
> >> +	pmd_t *pmdp, pmd;
> >> +
> >> +	do {
> >> +		next = pmd_addr_end(addr, end);
> >> +		pmdp = pmd_offset(pudp, addr);
> >> +		pmd = READ_ONCE(*pmdp);
> >> +		if (pmd_none(pmd))
> >> +			continue;
> >> +
> >> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
> >> +		free_empty_pte_table(pmdp, addr, next);
> >> +		free_pte_table(pmdp, addr, next, floor, ceiling);
> > 
> > Do we need two closely named functions here? Can you not collapse
> > free_empty_pud_table() and free_pte_table() into a single one? The same
> > comment for the pmd/pud variants. I just find this confusing.
> 
> The two functions could be collapsed into a single one. But just wanted to
> keep free_pxx_table() part which checks floor/ceiling alignment, non-zero
> entries clear off the actual page table walking.

With the pmd variant, they both take the floor/ceiling argument while
the free_empty_pte_table() doesn't even free anything. So not entirely
consistent.

Can you not just copy the free_pgd_range() functions but instead of
p*d_free_tlb() just do the TLB invalidation followed by page freeing?
That seems to be an easier pattern to follow.

-- 
Catalin

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Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

On 10/08/2019 04:25 PM, Catalin Marinas wrote:
> On Tue, Oct 08, 2019 at 10:06:26AM +0530, Anshuman Khandual wrote:
>> On 10/07/2019 07:47 PM, Catalin Marinas wrote:
>>> On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
>>>> The arch code for hot-remove must tear down portions of the linear map and
>>>> vmemmap corresponding to memory being removed. In both cases the page
>>>> tables mapping these regions must be freed, and when sparse vmemmap is in
>>>> use the memory backing the vmemmap must also be freed.
>>>>
>>>> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
>>>> can be used to tear down either region and calls it from vmemmap_free() and
>>>> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
>>>> backing memory will be freed.
>>>
>>> Can you change the 'sparse_vmap' name to something more meaningful which
>>> would suggest freeing of the backing memory?
>>
>> free_mapped_mem or free_backed_mem ? Even shorter forms like free_mapped or
>> free_backed might do as well. Do you have a particular preference here ? But
>> yes, sparse_vmap has been very much specific to vmemmap for these functions
>> which are now very generic in nature.
> 
> free_mapped would do.

Sure.

> 
>>>> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
>>>> +				    unsigned long end, bool sparse_vmap)
>>>> +{
>>>> +	struct page *page;
>>>> +	pte_t *ptep, pte;
>>>> +
>>>> +	do {
>>>> +		ptep = pte_offset_kernel(pmdp, addr);
>>>> +		pte = READ_ONCE(*ptep);
>>>> +		if (pte_none(pte))
>>>> +			continue;
>>>> +
>>>> +		WARN_ON(!pte_present(pte));
>>>> +		page = sparse_vmap ? pte_page(pte) : NULL;
>>>> +		pte_clear(&init_mm, addr, ptep);
>>>> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
>>>> +		if (sparse_vmap)
>>>> +			free_hotplug_page_range(page, PAGE_SIZE);
>>>
>>> You could only set 'page' if sparse_vmap (or even drop 'page' entirely).
>>
>> I am afraid 'page' is being used to hold pte_page(pte) extraction which
>> needs to be freed (sparse_vmap) as we are going to clear the ptep entry
>> in the next statement and lose access to it for good.
> 
> You clear *ptep, not pte.

Ahh, missed that. We have already captured the contents with READ_ONCE().

> 
>> We will need some
>> where to hold onto pte_page(pte) across pte_clear() as we cannot free it
>> before clearing it's entry and flushing the TLB. Hence wondering how the
>> 'page' can be completely dropped.
>>
>>> The compiler is probably smart enough to optimise it but using a
>>> pointless ternary operator just makes the code harder to follow.
>>
>> Not sure I got this but are you suggesting for an 'if' statement here
>>
>> if (sparse_vmap)
>> 	page = pte_page(pte);
>>
>> instead of the current assignment ?
>>
>> page = sparse_vmap ? pte_page(pte) : NULL;
> 
> I suggest:
> 
> 	if (sparse_vmap)
> 		free_hotplug_pgtable_page(pte_page(pte), PAGE_SIZE);

Sure, will do.

> 
>>>> +	} while (addr += PAGE_SIZE, addr < end);
>>>> +}
>>> [...]
>>>> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
>>>> +				 unsigned long end)
>>>> +{
>>>> +	pte_t *ptep, pte;
>>>> +
>>>> +	do {
>>>> +		ptep = pte_offset_kernel(pmdp, addr);
>>>> +		pte = READ_ONCE(*ptep);
>>>> +		WARN_ON(!pte_none(pte));
>>>> +	} while (addr += PAGE_SIZE, addr < end);
>>>> +}
>>>> +
>>>> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
>>>> +				 unsigned long end, unsigned long floor,
>>>> +				 unsigned long ceiling)
>>>> +{
>>>> +	unsigned long next;
>>>> +	pmd_t *pmdp, pmd;
>>>> +
>>>> +	do {
>>>> +		next = pmd_addr_end(addr, end);
>>>> +		pmdp = pmd_offset(pudp, addr);
>>>> +		pmd = READ_ONCE(*pmdp);
>>>> +		if (pmd_none(pmd))
>>>> +			continue;
>>>> +
>>>> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
>>>> +		free_empty_pte_table(pmdp, addr, next);
>>>> +		free_pte_table(pmdp, addr, next, floor, ceiling);
>>>
>>> Do we need two closely named functions here? Can you not collapse
>>> free_empty_pud_table() and free_pte_table() into a single one? The same
>>> comment for the pmd/pud variants. I just find this confusing.
>>
>> The two functions could be collapsed into a single one. But just wanted to
>> keep free_pxx_table() part which checks floor/ceiling alignment, non-zero
>> entries clear off the actual page table walking.
> 
> With the pmd variant, they both take the floor/ceiling argument while
> the free_empty_pte_table() doesn't even free anything. So not entirely
> consistent.> 
> Can you not just copy the free_pgd_range() functions but instead of
> p*d_free_tlb() just do the TLB invalidation followed by page freeing?
> That seems to be an easier pattern to follow.
> 

Sure, will follow that pattern.

Re: [PATCH V8 2/2] arm64/mm: Enable memory hot remove

[Anshuman Khandual]

On 10/08/2019 04:25 PM, Catalin Marinas wrote:
> On Tue, Oct 08, 2019 at 10:06:26AM +0530, Anshuman Khandual wrote:
>> On 10/07/2019 07:47 PM, Catalin Marinas wrote:
>>> On Mon, Sep 23, 2019 at 11:13:45AM +0530, Anshuman Khandual wrote:
>>>> The arch code for hot-remove must tear down portions of the linear map and
>>>> vmemmap corresponding to memory being removed. In both cases the page
>>>> tables mapping these regions must be freed, and when sparse vmemmap is in
>>>> use the memory backing the vmemmap must also be freed.
>>>>
>>>> This patch adds unmap_hotplug_range() and free_empty_tables() helpers which
>>>> can be used to tear down either region and calls it from vmemmap_free() and
>>>> ___remove_pgd_mapping(). The sparse_vmap argument determines whether the
>>>> backing memory will be freed.
>>>
>>> Can you change the 'sparse_vmap' name to something more meaningful which
>>> would suggest freeing of the backing memory?
>>
>> free_mapped_mem or free_backed_mem ? Even shorter forms like free_mapped or
>> free_backed might do as well. Do you have a particular preference here ? But
>> yes, sparse_vmap has been very much specific to vmemmap for these functions
>> which are now very generic in nature.
> 
> free_mapped would do.

Sure.

> 
>>>> +static void unmap_hotplug_pte_range(pmd_t *pmdp, unsigned long addr,
>>>> +				    unsigned long end, bool sparse_vmap)
>>>> +{
>>>> +	struct page *page;
>>>> +	pte_t *ptep, pte;
>>>> +
>>>> +	do {
>>>> +		ptep = pte_offset_kernel(pmdp, addr);
>>>> +		pte = READ_ONCE(*ptep);
>>>> +		if (pte_none(pte))
>>>> +			continue;
>>>> +
>>>> +		WARN_ON(!pte_present(pte));
>>>> +		page = sparse_vmap ? pte_page(pte) : NULL;
>>>> +		pte_clear(&init_mm, addr, ptep);
>>>> +		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
>>>> +		if (sparse_vmap)
>>>> +			free_hotplug_page_range(page, PAGE_SIZE);
>>>
>>> You could only set 'page' if sparse_vmap (or even drop 'page' entirely).
>>
>> I am afraid 'page' is being used to hold pte_page(pte) extraction which
>> needs to be freed (sparse_vmap) as we are going to clear the ptep entry
>> in the next statement and lose access to it for good.
> 
> You clear *ptep, not pte.

Ahh, missed that. We have already captured the contents with READ_ONCE().

> 
>> We will need some
>> where to hold onto pte_page(pte) across pte_clear() as we cannot free it
>> before clearing it's entry and flushing the TLB. Hence wondering how the
>> 'page' can be completely dropped.
>>
>>> The compiler is probably smart enough to optimise it but using a
>>> pointless ternary operator just makes the code harder to follow.
>>
>> Not sure I got this but are you suggesting for an 'if' statement here
>>
>> if (sparse_vmap)
>> 	page = pte_page(pte);
>>
>> instead of the current assignment ?
>>
>> page = sparse_vmap ? pte_page(pte) : NULL;
> 
> I suggest:
> 
> 	if (sparse_vmap)
> 		free_hotplug_pgtable_page(pte_page(pte), PAGE_SIZE);

Sure, will do.

> 
>>>> +	} while (addr += PAGE_SIZE, addr < end);
>>>> +}
>>> [...]
>>>> +static void free_empty_pte_table(pmd_t *pmdp, unsigned long addr,
>>>> +				 unsigned long end)
>>>> +{
>>>> +	pte_t *ptep, pte;
>>>> +
>>>> +	do {
>>>> +		ptep = pte_offset_kernel(pmdp, addr);
>>>> +		pte = READ_ONCE(*ptep);
>>>> +		WARN_ON(!pte_none(pte));
>>>> +	} while (addr += PAGE_SIZE, addr < end);
>>>> +}
>>>> +
>>>> +static void free_empty_pmd_table(pud_t *pudp, unsigned long addr,
>>>> +				 unsigned long end, unsigned long floor,
>>>> +				 unsigned long ceiling)
>>>> +{
>>>> +	unsigned long next;
>>>> +	pmd_t *pmdp, pmd;
>>>> +
>>>> +	do {
>>>> +		next = pmd_addr_end(addr, end);
>>>> +		pmdp = pmd_offset(pudp, addr);
>>>> +		pmd = READ_ONCE(*pmdp);
>>>> +		if (pmd_none(pmd))
>>>> +			continue;
>>>> +
>>>> +		WARN_ON(!pmd_present(pmd) || !pmd_table(pmd) || pmd_sect(pmd));
>>>> +		free_empty_pte_table(pmdp, addr, next);
>>>> +		free_pte_table(pmdp, addr, next, floor, ceiling);
>>>
>>> Do we need two closely named functions here? Can you not collapse
>>> free_empty_pud_table() and free_pte_table() into a single one? The same
>>> comment for the pmd/pud variants. I just find this confusing.
>>
>> The two functions could be collapsed into a single one. But just wanted to
>> keep free_pxx_table() part which checks floor/ceiling alignment, non-zero
>> entries clear off the actual page table walking.
> 
> With the pmd variant, they both take the floor/ceiling argument while
> the free_empty_pte_table() doesn't even free anything. So not entirely
> consistent.> 
> Can you not just copy the free_pgd_range() functions but instead of
> p*d_free_tlb() just do the TLB invalidation followed by page freeing?
> That seems to be an easier pattern to follow.
> 

Sure, will follow that pattern.

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