[PATCH v10 00/11] Free some vmemmap pages of HugeTLB page

[PATCH v10 00/11] Free some vmemmap pages of HugeTLB page

[Muchun Song]

Hi all,

This patch series will free some vmemmap pages(struct page structures)
associated with each hugetlbpage when preallocated to save memory.

In order to reduce the difficulty of the first version of code review.
From this version, we disable PMD/huge page mapping of vmemmap if this
feature was enabled. This accutualy eliminate a bunch of the complex code
doing page table manipulation. When this patch series is solid, we cam add
the code of vmemmap page table manipulation in the future.

The struct page structures (page structs) are used to describe a physical
page frame. By default, there is a one-to-one mapping from a page frame to
it's corresponding page struct.

The HugeTLB pages consist of multiple base page size pages and is supported
by many architectures. See hugetlbpage.rst in the Documentation directory
for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
are currently supported. Since the base page size on x86 is 4KB, a 2MB
HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
4096 base pages. For each base page, there is a corresponding page struct.

Within the HugeTLB subsystem, only the first 4 page structs are used to
contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
provides this upper limit. The only 'useful' information in the remaining
page structs is the compound_head field, and this field is the same for all
tail pages.

By removing redundant page structs for HugeTLB pages, memory can returned to
the buddy allocator for other uses.

When the system boot up, every 2M HugeTLB has 512 struct page structs which
size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).

    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
 +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
 |           |                     |     0     | -------------> |     0     |
 |           |                     +-----------+                +-----------+
 |           |                     |     1     | -------------> |     1     |
 |           |                     +-----------+                +-----------+
 |           |                     |     2     | -------------> |     2     |
 |           |                     +-----------+                +-----------+
 |           |                     |     3     | -------------> |     3     |
 |           |                     +-----------+                +-----------+
 |           |                     |     4     | -------------> |     4     |
 |    2MB    |                     +-----------+                +-----------+
 |           |                     |     5     | -------------> |     5     |
 |           |                     +-----------+                +-----------+
 |           |                     |     6     | -------------> |     6     |
 |           |                     +-----------+                +-----------+
 |           |                     |     7     | -------------> |     7     |
 |           |                     +-----------+                +-----------+
 |           |
 |           |
 |           |
 +-----------+

The value of page->compound_head is the same for all tail pages. The first
page of page structs (page 0) associated with the HugeTLB page contains the 4
page structs necessary to describe the HugeTLB. The only use of the remaining
pages of page structs (page 1 to page 7) is to point to page->compound_head.
Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
will be used for each HugeTLB page. This will allow us to free the remaining
6 pages to the buddy allocator.

Here is how things look after remapping.

    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
 +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
 |           |                     |     0     | -------------> |     0     |
 |           |                     +-----------+                +-----------+
 |           |                     |     1     | -------------> |     1     |
 |           |                     +-----------+                +-----------+
 |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
 |           |                     +-----------+                   | | | | |
 |           |                     |     3     | ------------------+ | | | |
 |           |                     +-----------+                     | | | |
 |           |                     |     4     | --------------------+ | | |
 |    2MB    |                     +-----------+                       | | |
 |           |                     |     5     | ----------------------+ | |
 |           |                     +-----------+                         | |
 |           |                     |     6     | ------------------------+ |
 |           |                     +-----------+                           |
 |           |                     |     7     | --------------------------+
 |           |                     +-----------+
 |           |
 |           |
 |           |
 +-----------+

When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
vmemmap pages and restore the previous mapping relationship.

Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
pages.

In this case, for the 1GB HugeTLB page, we can save 4088 pages(There are
4096 pages for struct page structs, we reserve 2 pages for vmemmap and 8
pages for page tables. So we can save 4088 pages). This is a very substantial
gain. On our server, run some SPDK/QEMU applications which will use 1024GB
hugetlbpage. With this feature enabled, we can save ~16GB(1G hugepage)/~11GB
(2MB hugepage, the worst case is 10GB while the best is 12GB) memory.

Because there are vmemmap page tables reconstruction on the freeing/allocating
path, it increases some overhead. Here are some overhead analysis.

1) Allocating 10240 2MB hugetlb pages.

   a) With this patch series applied:
   # time echo 10240 > /proc/sys/vm/nr_hugepages

   real     0m0.166s
   user     0m0.000s
   sys      0m0.166s

   # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
   Attaching 2 probes...

   @latency:
   [8K, 16K)           8360 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
   [16K, 32K)          1868 |@@@@@@@@@@@                                         |
   [32K, 64K)            10 |                                                    |
   [64K, 128K)            2 |                                                    |

   b) Without this patch series:
   # time echo 10240 > /proc/sys/vm/nr_hugepages

   real     0m0.066s
   user     0m0.000s
   sys      0m0.066s

   # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
   Attaching 2 probes...

   @latency:
   [4K, 8K)           10176 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
   [8K, 16K)             62 |                                                    |
   [16K, 32K)             2 |                                                    |

   Summarize: this feature is about ~2x slower than before.

2) Freeing 10240 2MB hugetlb pages.

   a) With this patch series applied:
   # time echo 0 > /proc/sys/vm/nr_hugepages

   real     0m0.004s
   user     0m0.000s
   sys      0m0.002s

   # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
   Attaching 2 probes...

   @latency:
   [16K, 32K)         10240 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|

   b) Without this patch series:
   # time echo 0 > /proc/sys/vm/nr_hugepages

   real     0m0.077s
   user     0m0.001s
   sys      0m0.075s

   # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
   Attaching 2 probes...

   @latency:
   [4K, 8K)            9950 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
   [8K, 16K)            287 |@                                                   |
   [16K, 32K)             3 |                                                    |

   Summarize: The overhead of __free_hugepage is about ~2-4x slower than before.
              But according to the allocation test above, I think that here is
	      also ~2x slower than before.

              But why the 'real' time of patched is smaller than before? Because
	      In this patch series, the freeing hugetlb is asynchronous(through
	      kwoker).

Although the overhead has increased, the overhead is not significant. Like Mike
said, "However, remember that the majority of use cases create hugetlb pages at
or shortly after boot time and add them to the pool. So, additional overhead is
at pool creation time. There is no change to 'normal run time' operations of
getting a page from or returning a page to the pool (think page fault/unmap)".

Todo:
  - Free all of the tail vmemmap pages
    Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. we really can
    free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page
    structures has beed set PG_head flag. If we can adjust compound_head()
    slightly and make compound_head() return the real head struct page when
    the parameter is the tail struct page but with PG_head flag set.

    In order to make the code evolution route clearer. This feature can can be
    a separate patch after this patchset is solid.

  - Support for other architectures (e.g. aarch64).
  - Enable PMD/huge page mapping of vmemmap even if this feature was enabled.

Changelog in v9 -> v10:
  - Fix a bug in patch #11. Thanks to Oscar for pointing that out.
  - Rework some commit log or comments. Thanks Mike and Oscar for the suggestions.
  - Drop VMEMMAP_TAIL_PAGE_REUSE in the patch #3.

  Thank you very much Mike and Oscar for reviewing the code.

Changelog in v8 -> v9:
  - Rework some code. Very thanks to Oscar.
  - Put all the non-hugetlb vmemmap functions under sparsemem-vmemmap.c.

Changelog in v7 -> v8:
  - Adjust the order of patches.

  Very thanks to David and Oscar. Your suggestions are very valuable.

Changelog in v6 -> v7:
  - Rebase to linux-next 20201130
  - Do not use basepage mapping for vmemmap when this feature is disabled.
  - Rework some patchs.
    [PATCH v6 08/16] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page
    [PATCH v6 10/16] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page

  Thanks to Oscar and Barry.

Changelog in v5 -> v6:
  - Disable PMD/huge page mapping of vmemmap if this feature was enabled.
  - Simplify the first version code.

Changelog in v4 -> v5:
  - Rework somme comments and code in the [PATCH v4 04/21] and [PATCH v4 05/21].

  Thanks to Mike and Oscar's suggestions.

Changelog in v3 -> v4:
  - Move all the vmemmap functions to hugetlb_vmemmap.c.
  - Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we want to
    disable this feature, we should disable it by a boot/kernel command line.
  - Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions.
  - Initialize page table lock for vmemmap through core_initcall mechanism.

  Thanks for Mike and Oscar's suggestions.

Changelog in v2 -> v3:
  - Rename some helps function name. Thanks Mike.
  - Rework some code. Thanks Mike and Oscar.
  - Remap the tail vmemmap page with PAGE_KERNEL_RO instead of PAGE_KERNEL.
    Thanks Matthew.
  - Add some overhead analysis in the cover letter.
  - Use vmemap pmd table lock instead of a hugetlb specific global lock.

Changelog in v1 -> v2:
  - Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap().
  - Fix some typo and code style problems.
  - Remove unused handle_vmemmap_fault().
  - Merge some commits to one commit suggested by Mike.

Muchun Song (11):
  mm/memory_hotplug: Factor out bootmem core functions to bootmem_info.c
  mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP
  mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page
  mm/hugetlb: Defer freeing of HugeTLB pages
  mm/hugetlb: Allocate the vmemmap pages associated with each HugeTLB
    page
  mm/hugetlb: Set the PageHWPoison to the raw error page
  mm/hugetlb: Flush work when dissolving hugetlb page
  mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap
  mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate
  mm/hugetlb: Gather discrete indexes of tail page
  mm/hugetlb: Optimize the code with the help of the compiler

 Documentation/admin-guide/kernel-parameters.txt |  14 ++
 Documentation/admin-guide/mm/hugetlbpage.rst    |   3 +
 arch/x86/mm/init_64.c                           |  13 +-
 fs/Kconfig                                      |  18 ++
 include/linux/bootmem_info.h                    |  65 ++++++
 include/linux/hugetlb.h                         |  36 ++++
 include/linux/hugetlb_cgroup.h                  |  15 +-
 include/linux/memory_hotplug.h                  |  27 ---
 include/linux/mm.h                              |   3 +
 mm/Makefile                                     |   2 +
 mm/bootmem_info.c                               | 124 +++++++++++
 mm/hugetlb.c                                    | 164 ++++++++++++--
 mm/hugetlb_vmemmap.c                            | 270 ++++++++++++++++++++++++
 mm/hugetlb_vmemmap.h                            |  45 ++++
 mm/memory_hotplug.c                             | 116 ----------
 mm/sparse-vmemmap.c                             | 248 ++++++++++++++++++++++
 mm/sparse.c                                     |   1 +
 17 files changed, 990 insertions(+), 174 deletions(-)
 create mode 100644 include/linux/bootmem_info.h
 create mode 100644 mm/bootmem_info.c
 create mode 100644 mm/hugetlb_vmemmap.c
 create mode 100644 mm/hugetlb_vmemmap.h

-- 
2.11.0

[PATCH v10 01/11] mm/memory_hotplug: Factor out bootmem core functions to bootmem_info.c

[Muchun Song]

Move bootmem info registration common API to individual bootmem_info.c.
And we will use {get,put}_page_bootmem() to initialize the page for the
vmemmap pages or free the vmemmap pages to buddy in the later patch.
So move them out of CONFIG_MEMORY_HOTPLUG_SPARSE. This is just code
movement without any functional change.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 arch/x86/mm/init_64.c          |   3 +-
 include/linux/bootmem_info.h   |  40 +++++++++++++
 include/linux/memory_hotplug.h |  27 ---------
 mm/Makefile                    |   1 +
 mm/bootmem_info.c              | 124 +++++++++++++++++++++++++++++++++++++++++
 mm/memory_hotplug.c            | 116 --------------------------------------
 mm/sparse.c                    |   1 +
 7 files changed, 168 insertions(+), 144 deletions(-)
 create mode 100644 include/linux/bootmem_info.h
 create mode 100644 mm/bootmem_info.c

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index b5a3fa4033d3..0a45f062826e 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -33,6 +33,7 @@
 #include <linux/nmi.h>
 #include <linux/gfp.h>
 #include <linux/kcore.h>
+#include <linux/bootmem_info.h>
 
 #include <asm/processor.h>
 #include <asm/bios_ebda.h>
@@ -1571,7 +1572,7 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 	return err;
 }
 
-#if defined(CONFIG_MEMORY_HOTPLUG_SPARSE) && defined(CONFIG_HAVE_BOOTMEM_INFO_NODE)
+#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
 void register_page_bootmem_memmap(unsigned long section_nr,
 				  struct page *start_page, unsigned long nr_pages)
 {
diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
new file mode 100644
index 000000000000..4ed6dee1adc9
--- /dev/null
+++ b/include/linux/bootmem_info.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_BOOTMEM_INFO_H
+#define __LINUX_BOOTMEM_INFO_H
+
+#include <linux/mmzone.h>
+
+/*
+ * Types for free bootmem stored in page->lru.next. These have to be in
+ * some random range in unsigned long space for debugging purposes.
+ */
+enum {
+	MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE = 12,
+	SECTION_INFO = MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE,
+	MIX_SECTION_INFO,
+	NODE_INFO,
+	MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE = NODE_INFO,
+};
+
+#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
+void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
+
+void get_page_bootmem(unsigned long info, struct page *page,
+		      unsigned long type);
+void put_page_bootmem(struct page *page);
+#else
+static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
+{
+}
+
+static inline void put_page_bootmem(struct page *page)
+{
+}
+
+static inline void get_page_bootmem(unsigned long info, struct page *page,
+				    unsigned long type)
+{
+}
+#endif
+
+#endif /* __LINUX_BOOTMEM_INFO_H */
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 15acce5ab106..84590964ad35 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -33,18 +33,6 @@ struct vmem_altmap;
 	___page;						   \
 })
 
-/*
- * Types for free bootmem stored in page->lru.next. These have to be in
- * some random range in unsigned long space for debugging purposes.
- */
-enum {
-	MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE = 12,
-	SECTION_INFO = MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE,
-	MIX_SECTION_INFO,
-	NODE_INFO,
-	MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE = NODE_INFO,
-};
-
 /* Types for control the zone type of onlined and offlined memory */
 enum {
 	/* Offline the memory. */
@@ -222,17 +210,6 @@ static inline void arch_refresh_nodedata(int nid, pg_data_t *pgdat)
 #endif /* CONFIG_NUMA */
 #endif /* CONFIG_HAVE_ARCH_NODEDATA_EXTENSION */
 
-#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
-extern void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
-#else
-static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
-{
-}
-#endif
-extern void put_page_bootmem(struct page *page);
-extern void get_page_bootmem(unsigned long ingo, struct page *page,
-			     unsigned long type);
-
 void get_online_mems(void);
 void put_online_mems(void);
 
@@ -260,10 +237,6 @@ static inline void zone_span_writelock(struct zone *zone) {}
 static inline void zone_span_writeunlock(struct zone *zone) {}
 static inline void zone_seqlock_init(struct zone *zone) {}
 
-static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
-{
-}
-
 static inline int try_online_node(int nid)
 {
 	return 0;
diff --git a/mm/Makefile b/mm/Makefile
index a1af02ba8f3f..ed4b88fa0f5e 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -83,6 +83,7 @@ obj-$(CONFIG_SLUB) += slub.o
 obj-$(CONFIG_KASAN)	+= kasan/
 obj-$(CONFIG_KFENCE) += kfence/
 obj-$(CONFIG_FAILSLAB) += failslab.o
+obj-$(CONFIG_HAVE_BOOTMEM_INFO_NODE) += bootmem_info.o
 obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
 obj-$(CONFIG_MEMTEST)		+= memtest.o
 obj-$(CONFIG_MIGRATION) += migrate.o
diff --git a/mm/bootmem_info.c b/mm/bootmem_info.c
new file mode 100644
index 000000000000..fcab5a3f8cc0
--- /dev/null
+++ b/mm/bootmem_info.c
@@ -0,0 +1,124 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  linux/mm/bootmem_info.c
+ *
+ *  Copyright (C)
+ */
+#include <linux/mm.h>
+#include <linux/compiler.h>
+#include <linux/memblock.h>
+#include <linux/bootmem_info.h>
+#include <linux/memory_hotplug.h>
+
+void get_page_bootmem(unsigned long info, struct page *page, unsigned long type)
+{
+	page->freelist = (void *)type;
+	SetPagePrivate(page);
+	set_page_private(page, info);
+	page_ref_inc(page);
+}
+
+void put_page_bootmem(struct page *page)
+{
+	unsigned long type;
+
+	type = (unsigned long) page->freelist;
+	BUG_ON(type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||
+	       type > MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE);
+
+	if (page_ref_dec_return(page) == 1) {
+		page->freelist = NULL;
+		ClearPagePrivate(page);
+		set_page_private(page, 0);
+		INIT_LIST_HEAD(&page->lru);
+		free_reserved_page(page);
+	}
+}
+
+#ifndef CONFIG_SPARSEMEM_VMEMMAP
+static void register_page_bootmem_info_section(unsigned long start_pfn)
+{
+	unsigned long mapsize, section_nr, i;
+	struct mem_section *ms;
+	struct page *page, *memmap;
+	struct mem_section_usage *usage;
+
+	section_nr = pfn_to_section_nr(start_pfn);
+	ms = __nr_to_section(section_nr);
+
+	/* Get section's memmap address */
+	memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
+
+	/*
+	 * Get page for the memmap's phys address
+	 * XXX: need more consideration for sparse_vmemmap...
+	 */
+	page = virt_to_page(memmap);
+	mapsize = sizeof(struct page) * PAGES_PER_SECTION;
+	mapsize = PAGE_ALIGN(mapsize) >> PAGE_SHIFT;
+
+	/* remember memmap's page */
+	for (i = 0; i < mapsize; i++, page++)
+		get_page_bootmem(section_nr, page, SECTION_INFO);
+
+	usage = ms->usage;
+	page = virt_to_page(usage);
+
+	mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
+
+	for (i = 0; i < mapsize; i++, page++)
+		get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
+
+}
+#else /* CONFIG_SPARSEMEM_VMEMMAP */
+static void register_page_bootmem_info_section(unsigned long start_pfn)
+{
+	unsigned long mapsize, section_nr, i;
+	struct mem_section *ms;
+	struct page *page, *memmap;
+	struct mem_section_usage *usage;
+
+	section_nr = pfn_to_section_nr(start_pfn);
+	ms = __nr_to_section(section_nr);
+
+	memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
+
+	register_page_bootmem_memmap(section_nr, memmap, PAGES_PER_SECTION);
+
+	usage = ms->usage;
+	page = virt_to_page(usage);
+
+	mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
+
+	for (i = 0; i < mapsize; i++, page++)
+		get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
+}
+#endif /* !CONFIG_SPARSEMEM_VMEMMAP */
+
+void __init register_page_bootmem_info_node(struct pglist_data *pgdat)
+{
+	unsigned long i, pfn, end_pfn, nr_pages;
+	int node = pgdat->node_id;
+	struct page *page;
+
+	nr_pages = PAGE_ALIGN(sizeof(struct pglist_data)) >> PAGE_SHIFT;
+	page = virt_to_page(pgdat);
+
+	for (i = 0; i < nr_pages; i++, page++)
+		get_page_bootmem(node, page, NODE_INFO);
+
+	pfn = pgdat->node_start_pfn;
+	end_pfn = pgdat_end_pfn(pgdat);
+
+	/* register section info */
+	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
+		/*
+		 * Some platforms can assign the same pfn to multiple nodes - on
+		 * node0 as well as nodeN.  To avoid registering a pfn against
+		 * multiple nodes we check that this pfn does not already
+		 * reside in some other nodes.
+		 */
+		if (pfn_valid(pfn) && (early_pfn_to_nid(pfn) == node))
+			register_page_bootmem_info_section(pfn);
+	}
+}
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index a8cef4955907..4c4ca99745b7 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -141,122 +141,6 @@ static void release_memory_resource(struct resource *res)
 }
 
 #ifdef CONFIG_MEMORY_HOTPLUG_SPARSE
-void get_page_bootmem(unsigned long info,  struct page *page,
-		      unsigned long type)
-{
-	page->freelist = (void *)type;
-	SetPagePrivate(page);
-	set_page_private(page, info);
-	page_ref_inc(page);
-}
-
-void put_page_bootmem(struct page *page)
-{
-	unsigned long type;
-
-	type = (unsigned long) page->freelist;
-	BUG_ON(type < MEMORY_HOTPLUG_MIN_BOOTMEM_TYPE ||
-	       type > MEMORY_HOTPLUG_MAX_BOOTMEM_TYPE);
-
-	if (page_ref_dec_return(page) == 1) {
-		page->freelist = NULL;
-		ClearPagePrivate(page);
-		set_page_private(page, 0);
-		INIT_LIST_HEAD(&page->lru);
-		free_reserved_page(page);
-	}
-}
-
-#ifdef CONFIG_HAVE_BOOTMEM_INFO_NODE
-#ifndef CONFIG_SPARSEMEM_VMEMMAP
-static void register_page_bootmem_info_section(unsigned long start_pfn)
-{
-	unsigned long mapsize, section_nr, i;
-	struct mem_section *ms;
-	struct page *page, *memmap;
-	struct mem_section_usage *usage;
-
-	section_nr = pfn_to_section_nr(start_pfn);
-	ms = __nr_to_section(section_nr);
-
-	/* Get section's memmap address */
-	memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
-
-	/*
-	 * Get page for the memmap's phys address
-	 * XXX: need more consideration for sparse_vmemmap...
-	 */
-	page = virt_to_page(memmap);
-	mapsize = sizeof(struct page) * PAGES_PER_SECTION;
-	mapsize = PAGE_ALIGN(mapsize) >> PAGE_SHIFT;
-
-	/* remember memmap's page */
-	for (i = 0; i < mapsize; i++, page++)
-		get_page_bootmem(section_nr, page, SECTION_INFO);
-
-	usage = ms->usage;
-	page = virt_to_page(usage);
-
-	mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
-
-	for (i = 0; i < mapsize; i++, page++)
-		get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
-
-}
-#else /* CONFIG_SPARSEMEM_VMEMMAP */
-static void register_page_bootmem_info_section(unsigned long start_pfn)
-{
-	unsigned long mapsize, section_nr, i;
-	struct mem_section *ms;
-	struct page *page, *memmap;
-	struct mem_section_usage *usage;
-
-	section_nr = pfn_to_section_nr(start_pfn);
-	ms = __nr_to_section(section_nr);
-
-	memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr);
-
-	register_page_bootmem_memmap(section_nr, memmap, PAGES_PER_SECTION);
-
-	usage = ms->usage;
-	page = virt_to_page(usage);
-
-	mapsize = PAGE_ALIGN(mem_section_usage_size()) >> PAGE_SHIFT;
-
-	for (i = 0; i < mapsize; i++, page++)
-		get_page_bootmem(section_nr, page, MIX_SECTION_INFO);
-}
-#endif /* !CONFIG_SPARSEMEM_VMEMMAP */
-
-void __init register_page_bootmem_info_node(struct pglist_data *pgdat)
-{
-	unsigned long i, pfn, end_pfn, nr_pages;
-	int node = pgdat->node_id;
-	struct page *page;
-
-	nr_pages = PAGE_ALIGN(sizeof(struct pglist_data)) >> PAGE_SHIFT;
-	page = virt_to_page(pgdat);
-
-	for (i = 0; i < nr_pages; i++, page++)
-		get_page_bootmem(node, page, NODE_INFO);
-
-	pfn = pgdat->node_start_pfn;
-	end_pfn = pgdat_end_pfn(pgdat);
-
-	/* register section info */
-	for (; pfn < end_pfn; pfn += PAGES_PER_SECTION) {
-		/*
-		 * Some platforms can assign the same pfn to multiple nodes - on
-		 * node0 as well as nodeN.  To avoid registering a pfn against
-		 * multiple nodes we check that this pfn does not already
-		 * reside in some other nodes.
-		 */
-		if (pfn_valid(pfn) && (early_pfn_to_nid(pfn) == node))
-			register_page_bootmem_info_section(pfn);
-	}
-}
-#endif /* CONFIG_HAVE_BOOTMEM_INFO_NODE */
-
 static int check_pfn_span(unsigned long pfn, unsigned long nr_pages,
 		const char *reason)
 {
diff --git a/mm/sparse.c b/mm/sparse.c
index 7bd23f9d6cef..87676bf3af40 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -13,6 +13,7 @@
 #include <linux/vmalloc.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
+#include <linux/bootmem_info.h>
 
 #include "internal.h"
 #include <asm/dma.h>
-- 
2.11.0

[PATCH v10 02/11] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song]

The HUGETLB_PAGE_FREE_VMEMMAP option is used to enable the freeing
of unnecessary vmemmap associated with HugeTLB pages. The config
option is introduced early so that supporting code can be written
to depend on the option. The initial version of the code only
provides support for x86-64.

Like other code which frees vmemmap, this config option depends on
HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
used to register bootmem info. Therefore, make sure
register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
is defined.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 arch/x86/mm/init_64.c |  2 +-
 fs/Kconfig            | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0a45f062826e..0435bee2e172 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1225,7 +1225,7 @@ static struct kcore_list kcore_vsyscall;
 
 static void __init register_page_bootmem_info(void)
 {
-#ifdef CONFIG_NUMA
+#if defined(CONFIG_NUMA) || defined(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)
 	int i;
 
 	for_each_online_node(i)
diff --git a/fs/Kconfig b/fs/Kconfig
index 976e8b9033c4..e7c4c2a79311 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -245,6 +245,24 @@ config HUGETLBFS
 config HUGETLB_PAGE
 	def_bool HUGETLBFS
 
+config HUGETLB_PAGE_FREE_VMEMMAP
+	def_bool HUGETLB_PAGE
+	depends on X86_64
+	depends on SPARSEMEM_VMEMMAP
+	depends on HAVE_BOOTMEM_INFO_NODE
+	help
+	  The option HUGETLB_PAGE_FREE_VMEMMAP allows for the freeing of
+	  some vmemmap pages associated with pre-allocated HugeTLB pages.
+	  For example, on X86_64 6 vmemmap pages of size 4KB each can be
+	  saved for each 2MB HugeTLB page.  4094 vmemmap pages of size 4KB
+	  each can be saved for each 1GB HugeTLB page.
+
+	  When a HugeTLB page is allocated or freed, the vmemmap array
+	  representing the range associated with the page will need to be
+	  remapped.  When a page is allocated, vmemmap pages are freed
+	  after remapping.  When a page is freed, previously discarded
+	  vmemmap pages must be allocated before remapping.
+
 config MEMFD_CREATE
 	def_bool TMPFS || HUGETLBFS
 
-- 
2.11.0

[PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

Every HugeTLB has more than one struct page structure. We __know__ that
we only use the first 4(HUGETLB_CGROUP_MIN_ORDER) struct page structures
to store metadata associated with each HugeTLB.

There are a lot of struct page structures associated with each HugeTLB
page. For tail pages, the value of compound_head is the same. So we can
reuse first page of tail page structures. We map the virtual addresses
of the remaining pages of tail page structures to the first tail page
struct, and then free these page frames. Therefore, we need to reserve
two pages as vmemmap areas.

When we allocate a HugeTLB page from the buddy, we can free some vmemmap
pages associated with each HugeTLB page. It is more appropriate to do it
in the prep_new_huge_page().

The free_vmemmap_pages_per_hpage(), which indicates how many vmemmap
pages associated with a HugeTLB page can be freed, returns zero for
now, which means the feature is disabled. We will enable it once all
the infrastructure is there.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/bootmem_info.h |  27 +++++-
 include/linux/mm.h           |   2 +
 mm/Makefile                  |   1 +
 mm/hugetlb.c                 |   3 +
 mm/hugetlb_vmemmap.c         | 207 +++++++++++++++++++++++++++++++++++++++++++
 mm/hugetlb_vmemmap.h         |  20 +++++
 mm/sparse-vmemmap.c          | 177 ++++++++++++++++++++++++++++++++++++
 7 files changed, 436 insertions(+), 1 deletion(-)
 create mode 100644 mm/hugetlb_vmemmap.c
 create mode 100644 mm/hugetlb_vmemmap.h

diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
index 4ed6dee1adc9..4c80b7be1771 100644
--- a/include/linux/bootmem_info.h
+++ b/include/linux/bootmem_info.h
@@ -2,7 +2,7 @@
 #ifndef __LINUX_BOOTMEM_INFO_H
 #define __LINUX_BOOTMEM_INFO_H
 
-#include <linux/mmzone.h>
+#include <linux/mm.h>
 
 /*
  * Types for free bootmem stored in page->lru.next. These have to be in
@@ -22,6 +22,27 @@ void __init register_page_bootmem_info_node(struct pglist_data *pgdat);
 void get_page_bootmem(unsigned long info, struct page *page,
 		      unsigned long type);
 void put_page_bootmem(struct page *page);
+
+/*
+ * Any memory allocated via the memblock allocator and not via the
+ * buddy will be marked reserved already in the memmap. For those
+ * pages, we can call this function to free it to buddy allocator.
+ */
+static inline void free_bootmem_page(struct page *page)
+{
+	unsigned long magic = (unsigned long)page->freelist;
+
+	/*
+	 * The reserve_bootmem_region sets the reserved flag on bootmem
+	 * pages.
+	 */
+	VM_WARN_ON(page_ref_count(page) != 2);
+
+	if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
+		put_page_bootmem(page);
+	else
+		VM_WARN_ON(1);
+}
 #else
 static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
 {
@@ -35,6 +56,10 @@ static inline void get_page_bootmem(unsigned long info, struct page *page,
 				    unsigned long type)
 {
 }
+
+static inline void free_bootmem_page(struct page *page)
+{
+}
 #endif
 
 #endif /* __LINUX_BOOTMEM_INFO_H */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index eabe7d9f80d8..0ecad1a41190 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3005,6 +3005,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
 }
 #endif
 
+void vmemmap_remap_free(unsigned long start, unsigned long size);
+
 void *sparse_buffer_alloc(unsigned long size);
 struct page * __populate_section_memmap(unsigned long pfn,
 		unsigned long nr_pages, int nid, struct vmem_altmap *altmap);
diff --git a/mm/Makefile b/mm/Makefile
index ed4b88fa0f5e..056801d8daae 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -71,6 +71,7 @@ obj-$(CONFIG_FRONTSWAP)	+= frontswap.o
 obj-$(CONFIG_ZSWAP)	+= zswap.o
 obj-$(CONFIG_HAS_DMA)	+= dmapool.o
 obj-$(CONFIG_HUGETLBFS)	+= hugetlb.o
+obj-$(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)	+= hugetlb_vmemmap.o
 obj-$(CONFIG_NUMA) 	+= mempolicy.o
 obj-$(CONFIG_SPARSEMEM)	+= sparse.o
 obj-$(CONFIG_SPARSEMEM_VMEMMAP) += sparse-vmemmap.o
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 1f3bf1710b66..140135fc8113 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -42,6 +42,7 @@
 #include <linux/userfaultfd_k.h>
 #include <linux/page_owner.h>
 #include "internal.h"
+#include "hugetlb_vmemmap.h"
 
 int hugetlb_max_hstate __read_mostly;
 unsigned int default_hstate_idx;
@@ -1497,6 +1498,8 @@ void free_huge_page(struct page *page)
 
 static void prep_new_huge_page(struct hstate *h, struct page *page, int nid)
 {
+	free_huge_page_vmemmap(h, page);
+
 	INIT_LIST_HEAD(&page->lru);
 	set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
 	set_hugetlb_cgroup(page, NULL);
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
new file mode 100644
index 000000000000..5cf7b6122c86
--- /dev/null
+++ b/mm/hugetlb_vmemmap.c
@@ -0,0 +1,207 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Free some vmemmap pages of HugeTLB
+ *
+ * Copyright (c) 2020, Bytedance. All rights reserved.
+ *
+ *     Author: Muchun Song <songmuchun@bytedance.com>
+ *
+ * The struct page structures (page structs) are used to describe a physical
+ * page frame. By default, there is a one-to-one mapping from a page frame to
+ * it's corresponding page struct.
+ *
+ * The HugeTLB pages consist of multiple base page size pages and is supported
+ * by many architectures. See hugetlbpage.rst in the Documentation directory
+ * for more details. On the x86-64 architecture, HugeTLB pages of size 2MB and
+ * 1GB are currently supported. Since the base page size on x86 is 4KB, a 2MB
+ * HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
+ * 4096 base pages. For each base page, there is a corresponding page struct.
+ *
+ * Within the HugeTLB subsystem, only the first 4 page structs are used to
+ * contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
+ * provides this upper limit. The only 'useful' information in the remaining
+ * page structs is the compound_head field, and this field is the same for all
+ * tail pages.
+ *
+ * By removing redundant page structs for HugeTLB pages, memory can returned to
+ * the buddy allocator for other uses.
+ *
+ * Different architectures support different HugeTLB pages. For example, the
+ * following table is the HugeTLB page size supported by x86 and arm64
+ * architectures. Becasue arm64 supports 4k, 16k, and 64k base pages and
+ * supports contiguous entries, so it supports many kinds of sizes of HugeTLB
+ * page.
+ *
+ * +--------------+-----------+-----------------------------------------------+
+ * | Architecture | Page Size |                HugeTLB Page Size              |
+ * +--------------+-----------+-----------+-----------+-----------+-----------+
+ * |    x86-64    |    4KB    |    2MB    |    1GB    |           |           |
+ * +--------------+-----------+-----------+-----------+-----------+-----------+
+ * |              |    4KB    |   64KB    |    2MB    |    32MB   |    1GB    |
+ * |              +-----------+-----------+-----------+-----------+-----------+
+ * |    arm64     |   16KB    |    2MB    |   32MB    |     1GB   |           |
+ * |              +-----------+-----------+-----------+-----------+-----------+
+ * |              |   64KB    |    2MB    |  512MB    |    16GB   |           |
+ * +--------------+-----------+-----------+-----------+-----------+-----------+
+ *
+ * When the system boot up, every HugeTLB page has more than one struct page
+ * structs whose size is (unit: pages):
+ *
+ *    struct_size = HugeTLB_Size / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
+ *
+ * Where HugeTLB_Size is the size of the HugeTLB page. We know that the size
+ * of the HugeTLB page is always n times PAGE_SIZE. So we can get the following
+ * relationship.
+ *
+ *    HugeTLB_Size = n * PAGE_SIZE
+ *
+ * Then,
+ *
+ *    struct_size = n * PAGE_SIZE / PAGE_SIZE * sizeof(struct page) / PAGE_SIZE
+ *                = n * sizeof(struct page) / PAGE_SIZE
+ *
+ * We can use huge mapping at the pud/pmd level for the HugeTLB page.
+ *
+ * For the HugeTLB page of the pmd level mapping, then
+ *
+ *    struct_size = n * sizeof(struct page) / PAGE_SIZE
+ *                = PAGE_SIZE / sizeof(pte_t) * sizeof(struct page) / PAGE_SIZE
+ *                = sizeof(struct page) / sizeof(pte_t)
+ *                = 64 / 8
+ *                = 8 (pages)
+ *
+ * Where n is how many pte entries which one page can contains. So the value of
+ * n is (PAGE_SIZE / sizeof(pte_t)).
+ *
+ * This optimization only supports 64-bit system, so the value of sizeof(pte_t)
+ * is 8. And this optimization also applicable only when the size of struct page
+ * is a power of two. In most cases, the size of struct page is 64 (e.g. x86-64
+ * and arm64). So if we use pmd level mapping for a HugeTLB page, the size of
+ * struct page structs of it is 8 pages whose size depends on the size of the
+ * base page.
+ *
+ * For the HugeTLB page of the pud level mapping, then
+ *
+ *    struct_size = PAGE_SIZE / sizeof(pmd_t) * struct_size(pmd)
+ *                = PAGE_SIZE / 8 * 8 (pages)
+ *                = PAGE_SIZE (pages)
+ *
+ * Where the struct_size(pmd) is the size of the struct page structs of a
+ * HugeTLB page of the pmd level mapping.
+ *
+ * Next, we take the pmd level mapping of the HugeTLB page as an example to
+ * show the internal implementation of this optimization. There are 8 pages
+ * struct page structs associated with a HugeTLB page which is pmd mapped.
+ *
+ * Here is how things look before optimization.
+ *
+ *    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
+ * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
+ * |           |                     |     0     | -------------> |     0     |
+ * |           |                     +-----------+                +-----------+
+ * |           |                     |     1     | -------------> |     1     |
+ * |           |                     +-----------+                +-----------+
+ * |           |                     |     2     | -------------> |     2     |
+ * |           |                     +-----------+                +-----------+
+ * |           |                     |     3     | -------------> |     3     |
+ * |           |                     +-----------+                +-----------+
+ * |           |                     |     4     | -------------> |     4     |
+ * |    PMD    |                     +-----------+                +-----------+
+ * |   level   |                     |     5     | -------------> |     5     |
+ * |  mapping  |                     +-----------+                +-----------+
+ * |           |                     |     6     | -------------> |     6     |
+ * |           |                     +-----------+                +-----------+
+ * |           |                     |     7     | -------------> |     7     |
+ * |           |                     +-----------+                +-----------+
+ * |           |
+ * |           |
+ * |           |
+ * +-----------+
+ *
+ * The value of page->compound_head is the same for all tail pages. The first
+ * page of page structs (page 0) associated with the HugeTLB page contains the 4
+ * page structs necessary to describe the HugeTLB. The only use of the remaining
+ * pages of page structs (page 1 to page 7) is to point to page->compound_head.
+ * Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
+ * will be used for each HugeTLB page. This will allow us to free the remaining
+ * 6 pages to the buddy allocator.
+ *
+ * Here is how things look after remapping.
+ *
+ *    HugeTLB                  struct pages(8 pages)         page frame(8 pages)
+ * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
+ * |           |                     |     0     | -------------> |     0     |
+ * |           |                     +-----------+                +-----------+
+ * |           |                     |     1     | -------------> |     1     |
+ * |           |                     +-----------+                +-----------+
+ * |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
+ * |           |                     +-----------+                   | | | | |
+ * |           |                     |     3     | ------------------+ | | | |
+ * |           |                     +-----------+                     | | | |
+ * |           |                     |     4     | --------------------+ | | |
+ * |    PMD    |                     +-----------+                       | | |
+ * |   level   |                     |     5     | ----------------------+ | |
+ * |  mapping  |                     +-----------+                         | |
+ * |           |                     |     6     | ------------------------+ |
+ * |           |                     +-----------+                           |
+ * |           |                     |     7     | --------------------------+
+ * |           |                     +-----------+
+ * |           |
+ * |           |
+ * |           |
+ * +-----------+
+ *
+ * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
+ * vmemmap pages and restore the previous mapping relationship.
+ *
+ * For the HugeTLB page of the pud level mapping. It is similar to the former.
+ * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
+ *
+ * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
+ * (e.g. aarch64) provides a contiguous bit in the translation table entries
+ * that hints to the MMU to indicate that it is one of a contiguous set of
+ * entries that can be cached in a single TLB entry.
+ *
+ * The contiguous bit is used to increase the mapping size at the pmd and pte
+ * (last) level. So this type of HugeTLB page can be optimized only when its
+ * size of the struct page structs is greater than 2 pages.
+ */
+#include "hugetlb_vmemmap.h"
+
+/*
+ * There are a lot of struct page structures associated with each HugeTLB page.
+ * For tail pages, the value of compound_head is the same. So we can reuse first
+ * page of tail page structures. We map the virtual addresses of the remaining
+ * pages of tail page structures to the first tail page struct, and then free
+ * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
+ */
+#define RESERVE_VMEMMAP_NR		2U
+#define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
+
+/*
+ * How many vmemmap pages associated with a HugeTLB page that can be freed
+ * to the buddy allocator.
+ *
+ * Todo: Returns zero for now, which means the feature is disabled. We will
+ * enable it once all the infrastructure is there.
+ */
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+	return 0;
+}
+
+static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
+{
+	return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
+}
+
+void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+	unsigned long vmemmap_addr = (unsigned long)head;
+
+	if (!free_vmemmap_pages_per_hpage(h))
+		return;
+
+	vmemmap_remap_free(vmemmap_addr + RESERVE_VMEMMAP_SIZE,
+			   free_vmemmap_pages_size_per_hpage(h));
+}
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
new file mode 100644
index 000000000000..6923f03534d5
--- /dev/null
+++ b/mm/hugetlb_vmemmap.h
@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Free some vmemmap pages of HugeTLB
+ *
+ * Copyright (c) 2020, Bytedance. All rights reserved.
+ *
+ *     Author: Muchun Song <songmuchun@bytedance.com>
+ */
+#ifndef _LINUX_HUGETLB_VMEMMAP_H
+#define _LINUX_HUGETLB_VMEMMAP_H
+#include <linux/hugetlb.h>
+
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+#else
+static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+}
+#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
+#endif /* _LINUX_HUGETLB_VMEMMAP_H */
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 16183d85a7d5..6cf2fdfb81e9 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -27,8 +27,185 @@
 #include <linux/spinlock.h>
 #include <linux/vmalloc.h>
 #include <linux/sched.h>
+#include <linux/pgtable.h>
+#include <linux/bootmem_info.h>
+
 #include <asm/dma.h>
 #include <asm/pgalloc.h>
+#include <asm/tlbflush.h>
+
+/*
+ * vmemmap_remap_walk - walk vmemmap page table
+ *
+ * @remap_pte:		called for each non-empty PTE (lowest-level) entry.
+ * @reuse_page:		the page which is reused for the tail vmemmap pages.
+ * @reuse_addr:		the virtual address of the @reuse_page page.
+ * @vmemmap_pages:	the list head of the vmemmap pages that can be freed.
+ */
+struct vmemmap_remap_walk {
+	void (*remap_pte)(pte_t *pte, unsigned long addr,
+			  struct vmemmap_remap_walk *walk);
+	struct page *reuse_page;
+	unsigned long reuse_addr;
+	struct list_head *vmemmap_pages;
+};
+
+static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
+			      unsigned long end,
+			      struct vmemmap_remap_walk *walk)
+{
+	pte_t *pte;
+
+	pte = pte_offset_kernel(pmd, addr);
+
+	if (walk->reuse_addr == addr) {
+		BUG_ON(pte_none(*pte));
+		walk->reuse_page = pte_page(*pte++);
+		addr += PAGE_SIZE;
+	}
+
+	for (; addr != end; addr += PAGE_SIZE, pte++) {
+		BUG_ON(pte_none(*pte));
+
+		walk->remap_pte(pte, addr, walk);
+	}
+}
+
+static void vmemmap_pmd_range(pud_t *pud, unsigned long addr,
+			      unsigned long end,
+			      struct vmemmap_remap_walk *walk)
+{
+	pmd_t *pmd;
+	unsigned long next;
+
+	pmd = pmd_offset(pud, addr);
+	do {
+		BUG_ON(pmd_none(*pmd));
+
+		next = pmd_addr_end(addr, end);
+		vmemmap_pte_range(pmd, addr, next, walk);
+	} while (pmd++, addr = next, addr != end);
+}
+
+static void vmemmap_pud_range(p4d_t *p4d, unsigned long addr,
+			      unsigned long end,
+			      struct vmemmap_remap_walk *walk)
+{
+	pud_t *pud;
+	unsigned long next;
+
+	pud = pud_offset(p4d, addr);
+	do {
+		BUG_ON(pud_none(*pud));
+
+		next = pud_addr_end(addr, end);
+		vmemmap_pmd_range(pud, addr, next, walk);
+	} while (pud++, addr = next, addr != end);
+}
+
+static void vmemmap_p4d_range(pgd_t *pgd, unsigned long addr,
+			      unsigned long end,
+			      struct vmemmap_remap_walk *walk)
+{
+	p4d_t *p4d;
+	unsigned long next;
+
+	p4d = p4d_offset(pgd, addr);
+	do {
+		BUG_ON(p4d_none(*p4d));
+
+		next = p4d_addr_end(addr, end);
+		vmemmap_pud_range(p4d, addr, next, walk);
+	} while (p4d++, addr = next, addr != end);
+}
+
+static void vmemmap_remap_range(unsigned long start, unsigned long end,
+				struct vmemmap_remap_walk *walk)
+{
+	unsigned long addr = start - PAGE_SIZE;
+	unsigned long next;
+	pgd_t *pgd;
+
+	VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
+	VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
+
+	walk->reuse_page = NULL;
+	walk->reuse_addr = addr;
+
+	pgd = pgd_offset_k(addr);
+	do {
+		BUG_ON(pgd_none(*pgd));
+
+		next = pgd_addr_end(addr, end);
+		vmemmap_p4d_range(pgd, addr, next, walk);
+	} while (pgd++, addr = next, addr != end);
+
+	flush_tlb_kernel_range(start, end);
+}
+
+/*
+ * Free a vmemmap page. A vmemmap page can be allocated from the memblock
+ * allocator or buddy allocator. If the PG_reserved flag is set, it means
+ * that it allocated from the memblock allocator, just free it via the
+ * free_bootmem_page(). Otherwise, use __free_page().
+ */
+static inline void free_vmemmap_page(struct page *page)
+{
+	if (PageReserved(page))
+		free_bootmem_page(page);
+	else
+		__free_page(page);
+}
+
+/* Free a list of the vmemmap pages */
+static void free_vmemmap_page_list(struct list_head *list)
+{
+	struct page *page, *next;
+
+	list_for_each_entry_safe(page, next, list, lru) {
+		list_del(&page->lru);
+		free_vmemmap_page(page);
+	}
+}
+
+static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
+			      struct vmemmap_remap_walk *walk)
+{
+	/*
+	 * Make the tail pages are mapped with read-only to catch
+	 * illegal write operation to the tail pages.
+	 */
+	pgprot_t pgprot = PAGE_KERNEL_RO;
+	pte_t entry = mk_pte(walk->reuse_page, pgprot);
+	struct page *page;
+
+	page = pte_page(*pte);
+	list_add(&page->lru, walk->vmemmap_pages);
+
+	set_pte_at(&init_mm, addr, pte, entry);
+}
+
+/**
+ * vmemmap_remap_free - remap the vmemmap virtual address range
+ *                      [start, start + size) to the page which
+ *                      [start - PAGE_SIZE, start) is mapped,
+ *                      then free vmemmap pages.
+ * @start:	start address of the vmemmap virtual address range
+ * @size:	size of the vmemmap virtual address range
+ */
+void vmemmap_remap_free(unsigned long start, unsigned long size)
+{
+	unsigned long end = start + size;
+	LIST_HEAD(vmemmap_pages);
+
+	struct vmemmap_remap_walk walk = {
+		.remap_pte	= vmemmap_remap_pte,
+		.vmemmap_pages	= &vmemmap_pages,
+	};
+
+	vmemmap_remap_range(start, end, &walk);
+	free_vmemmap_page_list(&vmemmap_pages);
+}
 
 /*
  * Allocate a block of memory to be used to back the virtual memory map
-- 
2.11.0

[PATCH v10 04/11] mm/hugetlb: Defer freeing of HugeTLB pages

[Muchun Song]

In the subsequent patch, we will allocate the vmemmap pages when free
HugeTLB pages. But update_and_free_page() is called from a non-task
context(and hold hugetlb_lock), so we can defer the actual freeing in
a workqueue to prevent from using GFP_ATOMIC to allocate the vmemmap
pages.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/hugetlb.c         | 80 ++++++++++++++++++++++++++++++++++++++++++++++++----
 mm/hugetlb_vmemmap.c | 12 --------
 mm/hugetlb_vmemmap.h | 17 +++++++++++
 3 files changed, 91 insertions(+), 18 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 140135fc8113..9f35f34d3195 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1292,15 +1292,79 @@ static inline void destroy_compound_gigantic_page(struct page *page,
 						unsigned int order) { }
 #endif
 
-static void update_and_free_page(struct hstate *h, struct page *page)
+static void __free_hugepage(struct hstate *h, struct page *page);
+
+/*
+ * As update_and_free_page() is be called from a non-task context(and hold
+ * hugetlb_lock), we can defer the actual freeing in a workqueue to prevent
+ * use GFP_ATOMIC to allocate a lot of vmemmap pages.
+ *
+ * update_hpage_vmemmap_workfn() locklessly retrieves the linked list of
+ * pages to be freed and frees them one-by-one. As the page->mapping pointer
+ * is going to be cleared in update_hpage_vmemmap_workfn() anyway, it is
+ * reused as the llist_node structure of a lockless linked list of huge
+ * pages to be freed.
+ */
+static LLIST_HEAD(hpage_update_freelist);
+
+static void update_hpage_vmemmap_workfn(struct work_struct *work)
 {
-	int i;
+	struct llist_node *node;
+	struct page *page;
 
+	node = llist_del_all(&hpage_update_freelist);
+
+	while (node) {
+		page = container_of((struct address_space **)node,
+				     struct page, mapping);
+		node = node->next;
+		page->mapping = NULL;
+		__free_hugepage(page_hstate(page), page);
+
+		cond_resched();
+	}
+}
+static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);
+
+static inline void __update_and_free_page(struct hstate *h, struct page *page)
+{
+	/* No need to allocate vmemmap pages */
+	if (!free_vmemmap_pages_per_hpage(h)) {
+		__free_hugepage(h, page);
+		return;
+	}
+
+	/*
+	 * Defer freeing to avoid using GFP_ATOMIC to allocate vmemmap
+	 * pages.
+	 *
+	 * Only call schedule_work() if hpage_update_freelist is previously
+	 * empty. Otherwise, schedule_work() had been called but the workfn
+	 * hasn't retrieved the list yet.
+	 */
+	if (llist_add((struct llist_node *)&page->mapping,
+		      &hpage_update_freelist))
+		schedule_work(&hpage_update_work);
+}
+
+static void update_and_free_page(struct hstate *h, struct page *page)
+{
 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
 		return;
 
 	h->nr_huge_pages--;
 	h->nr_huge_pages_node[page_to_nid(page)]--;
+
+	__update_and_free_page(h, page);
+}
+
+/*
+ * This is where the call to allocate vmemmmap pages will be inserted.
+ */
+static void __free_hugepage(struct hstate *h, struct page *page)
+{
+	int i;
+
 	for (i = 0; i < pages_per_huge_page(h); i++) {
 		page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
 				1 << PG_referenced | 1 << PG_dirty |
@@ -1313,13 +1377,17 @@ static void update_and_free_page(struct hstate *h, struct page *page)
 	set_page_refcounted(page);
 	if (hstate_is_gigantic(h)) {
 		/*
-		 * Temporarily drop the hugetlb_lock, because
-		 * we might block in free_gigantic_page().
+		 * Temporarily drop the hugetlb_lock only when this type of
+		 * HugeTLB page does not support vmemmap optimization (which
+		 * context do not hold the hugetlb_lock), because we might
+		 * block in free_gigantic_page().
 		 */
-		spin_unlock(&hugetlb_lock);
+		if (!free_vmemmap_pages_per_hpage(h))
+			spin_unlock(&hugetlb_lock);
 		destroy_compound_gigantic_page(page, huge_page_order(h));
 		free_gigantic_page(page, huge_page_order(h));
-		spin_lock(&hugetlb_lock);
+		if (!free_vmemmap_pages_per_hpage(h))
+			spin_lock(&hugetlb_lock);
 	} else {
 		__free_pages(page, huge_page_order(h));
 	}
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 5cf7b6122c86..c4bbca270453 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -178,18 +178,6 @@
 #define RESERVE_VMEMMAP_NR		2U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-/*
- * How many vmemmap pages associated with a HugeTLB page that can be freed
- * to the buddy allocator.
- *
- * Todo: Returns zero for now, which means the feature is disabled. We will
- * enable it once all the infrastructure is there.
- */
-static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
-{
-	return 0;
-}
-
 static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
 {
 	return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 6923f03534d5..01f8637adbe0 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -12,9 +12,26 @@
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
 void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+
+/*
+ * How many vmemmap pages associated with a HugeTLB page that can be freed
+ * to the buddy allocator.
+ *
+ * Todo: Returns zero for now, which means the feature is disabled. We will
+ * enable it once all the infrastructure is there.
+ */
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+	return 0;
+}
 #else
 static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 {
 }
+
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+	return 0;
+}
 #endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
 #endif /* _LINUX_HUGETLB_VMEMMAP_H */
-- 
2.11.0

[PATCH v10 05/11] mm/hugetlb: Allocate the vmemmap pages associated with each HugeTLB page

[Muchun Song]

When we free a HugeTLB page to the buddy allocator, we should allocate the
vmemmap pages associated with it. We can do that in the __free_hugepage()
before freeing it to buddy.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/mm.h   |  1 +
 mm/hugetlb.c         |  2 ++
 mm/hugetlb_vmemmap.c | 11 ++++++++
 mm/hugetlb_vmemmap.h |  5 ++++
 mm/sparse-vmemmap.c  | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 5 files changed, 91 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 0ecad1a41190..e7d022b67ee1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3006,6 +3006,7 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
 #endif
 
 void vmemmap_remap_free(unsigned long start, unsigned long size);
+void vmemmap_remap_alloc(unsigned long start, unsigned long size);
 
 void *sparse_buffer_alloc(unsigned long size);
 struct page * __populate_section_memmap(unsigned long pfn,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9f35f34d3195..329f473b929e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1365,6 +1365,8 @@ static void __free_hugepage(struct hstate *h, struct page *page)
 {
 	int i;
 
+	alloc_huge_page_vmemmap(h, page);
+
 	for (i = 0; i < pages_per_huge_page(h); i++) {
 		page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
 				1 << PG_referenced | 1 << PG_dirty |
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index c4bbca270453..273816dd95b6 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -183,6 +183,17 @@ static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
 	return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
 }
 
+void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+	unsigned long vmemmap_addr = (unsigned long)head;
+
+	if (!free_vmemmap_pages_per_hpage(h))
+		return;
+
+	vmemmap_remap_alloc(vmemmap_addr + RESERVE_VMEMMAP_SIZE,
+			    free_vmemmap_pages_size_per_hpage(h));
+}
+
 void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 {
 	unsigned long vmemmap_addr = (unsigned long)head;
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 01f8637adbe0..b2c8d2f11d48 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -11,6 +11,7 @@
 #include <linux/hugetlb.h>
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+void alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
 void free_huge_page_vmemmap(struct hstate *h, struct page *head);
 
 /*
@@ -25,6 +26,10 @@ static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 	return 0;
 }
 #else
+static inline void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+}
+
 static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 {
 }
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 6cf2fdfb81e9..01228065b95d 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -29,6 +29,7 @@
 #include <linux/sched.h>
 #include <linux/pgtable.h>
 #include <linux/bootmem_info.h>
+#include <linux/delay.h>
 
 #include <asm/dma.h>
 #include <asm/pgalloc.h>
@@ -40,7 +41,8 @@
  * @remap_pte:		called for each non-empty PTE (lowest-level) entry.
  * @reuse_page:		the page which is reused for the tail vmemmap pages.
  * @reuse_addr:		the virtual address of the @reuse_page page.
- * @vmemmap_pages:	the list head of the vmemmap pages that can be freed.
+ * @vmemmap_pages:	the list head of the vmemmap pages that can be freed
+ *			or is mapped from.
  */
 struct vmemmap_remap_walk {
 	void (*remap_pte)(pte_t *pte, unsigned long addr,
@@ -50,6 +52,10 @@ struct vmemmap_remap_walk {
 	struct list_head *vmemmap_pages;
 };
 
+/* The gfp mask of allocating vmemmap page */
+#define GFP_VMEMMAP_PAGE		\
+	(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN | __GFP_THISNODE)
+
 static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
 			      unsigned long end,
 			      struct vmemmap_remap_walk *walk)
@@ -207,6 +213,71 @@ void vmemmap_remap_free(unsigned long start, unsigned long size)
 	free_vmemmap_page_list(&vmemmap_pages);
 }
 
+static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
+				struct vmemmap_remap_walk *walk)
+{
+	pgprot_t pgprot = PAGE_KERNEL;
+	struct page *page;
+	void *to;
+
+	BUG_ON(pte_page(*pte) != walk->reuse_page);
+
+	page = list_first_entry(walk->vmemmap_pages, struct page, lru);
+	list_del(&page->lru);
+	to = page_to_virt(page);
+	copy_page(to, (void *)walk->reuse_addr);
+
+	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
+}
+
+static void alloc_vmemmap_page_list(struct list_head *list,
+				    unsigned long start, unsigned long end)
+{
+	unsigned long addr;
+	int nid = page_to_nid((const void *)start);
+
+	for (addr = start; addr < end; addr += PAGE_SIZE) {
+		struct page *page;
+
+retry:
+		page = alloc_pages_node(nid, GFP_VMEMMAP_PAGE, 0);
+		if (unlikely(!page)) {
+			msleep(100);
+			/*
+			 * We should retry infinitely, because we cannot
+			 * handle allocation failures. Once we allocate
+			 * vmemmap pages successfully, then we can free
+			 * a HugeTLB page.
+			 */
+			goto retry;
+		}
+		list_add_tail(&page->lru, list);
+	}
+}
+
+/**
+ * vmemmap_remap_alloc - remap the vmemmap virtual address range
+ *                       [start, start + size) to the page respectively
+ *                       which from the @vmemmap_pages
+ * @start:	start address of the vmemmap virtual address range
+ * @size:	size of the vmemmap virtual address range
+ */
+void vmemmap_remap_alloc(unsigned long start, unsigned long size)
+{
+	LIST_HEAD(vmemmap_pages);
+	unsigned long end = start + size;
+
+	struct vmemmap_remap_walk walk = {
+		.remap_pte	= vmemmap_restore_pte,
+		.vmemmap_pages	= &vmemmap_pages,
+	};
+
+	might_sleep();
+
+	alloc_vmemmap_page_list(&vmemmap_pages, start, end);
+	vmemmap_remap_range(start, end, &walk);
+}
+
 /*
  * Allocate a block of memory to be used to back the virtual memory map
  * or to back the page tables that are used to create the mapping.
-- 
2.11.0

[PATCH v10 06/11] mm/hugetlb: Set the PageHWPoison to the raw error page

[Muchun Song]

Because we reuse the first tail vmemmap page frame and remap it
with read-only, we cannot set the PageHWPosion on a tail page.
So we can use the head[4].private to record the real error page
index and set the raw error page PageHWPoison later.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
 mm/hugetlb.c | 48 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 40 insertions(+), 8 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 329f473b929e..f15aa9b19b6e 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1347,6 +1347,43 @@ static inline void __update_and_free_page(struct hstate *h, struct page *page)
 		schedule_work(&hpage_update_work);
 }
 
+static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
+{
+	struct page *page;
+
+	if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
+		return;
+
+	page = head + page_private(head + 4);
+
+	/*
+	 * Move PageHWPoison flag from head page to the raw error page,
+	 * which makes any subpages rather than the error page reusable.
+	 */
+	if (page != head) {
+		SetPageHWPoison(page);
+		ClearPageHWPoison(head);
+	}
+}
+
+static inline void hwpoison_subpage_set(struct hstate *h, struct page *head,
+					struct page *page)
+{
+	if (!PageHWPoison(head))
+		return;
+
+	if (free_vmemmap_pages_per_hpage(h)) {
+		set_page_private(head + 4, page - head);
+	} else if (page != head) {
+		/*
+		 * Move PageHWPoison flag from head page to the raw error page,
+		 * which makes any subpages rather than the error page reusable.
+		 */
+		SetPageHWPoison(page);
+		ClearPageHWPoison(head);
+	}
+}
+
 static void update_and_free_page(struct hstate *h, struct page *page)
 {
 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
@@ -1366,6 +1403,7 @@ static void __free_hugepage(struct hstate *h, struct page *page)
 	int i;
 
 	alloc_huge_page_vmemmap(h, page);
+	hwpoison_subpage_deliver(h, page);
 
 	for (i = 0; i < pages_per_huge_page(h); i++) {
 		page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
@@ -1843,14 +1881,8 @@ int dissolve_free_huge_page(struct page *page)
 		int nid = page_to_nid(head);
 		if (h->free_huge_pages - h->resv_huge_pages == 0)
 			goto out;
-		/*
-		 * Move PageHWPoison flag from head page to the raw error page,
-		 * which makes any subpages rather than the error page reusable.
-		 */
-		if (PageHWPoison(head) && page != head) {
-			SetPageHWPoison(page);
-			ClearPageHWPoison(head);
-		}
+
+		hwpoison_subpage_set(h, head, page);
 		list_del(&head->lru);
 		h->free_huge_pages--;
 		h->free_huge_pages_node[nid]--;
-- 
2.11.0

[PATCH v10 07/11] mm/hugetlb: Flush work when dissolving hugetlb page

[Muchun Song]

We should flush work when dissolving a hugetlb page to make sure that
the hugetlb page is freed to the buddy.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f15aa9b19b6e..fea8a96dd718 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1326,6 +1326,12 @@ static void update_hpage_vmemmap_workfn(struct work_struct *work)
 }
 static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);
 
+static inline void flush_hpage_update_work(struct hstate *h)
+{
+	if (free_vmemmap_pages_per_hpage(h))
+		flush_work(&hpage_update_work);
+}
+
 static inline void __update_and_free_page(struct hstate *h, struct page *page)
 {
 	/* No need to allocate vmemmap pages */
@@ -1864,6 +1870,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
 int dissolve_free_huge_page(struct page *page)
 {
 	int rc = -EBUSY;
+	struct hstate *h = NULL;
 
 	/* Not to disrupt normal path by vainly holding hugetlb_lock */
 	if (!PageHuge(page))
@@ -1877,8 +1884,9 @@ int dissolve_free_huge_page(struct page *page)
 
 	if (!page_count(page)) {
 		struct page *head = compound_head(page);
-		struct hstate *h = page_hstate(head);
 		int nid = page_to_nid(head);
+
+		h = page_hstate(head);
 		if (h->free_huge_pages - h->resv_huge_pages == 0)
 			goto out;
 
@@ -1892,6 +1900,14 @@ int dissolve_free_huge_page(struct page *page)
 	}
 out:
 	spin_unlock(&hugetlb_lock);
+
+	/*
+	 * We should flush work before return to make sure that
+	 * the HugeTLB page is freed to the buddy.
+	 */
+	if (!rc && h)
+		flush_hpage_update_work(h);
+
 	return rc;
 }
 
-- 
2.11.0

[PATCH v10 08/11] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap

[Muchun Song]

Add a kernel parameter hugetlb_free_vmemmap to enable the feature of
freeing unused vmemmap pages associated with each hugetlb page on boot.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
 Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
 arch/x86/mm/init_64.c                           |  8 ++++++--
 include/linux/hugetlb.h                         | 19 +++++++++++++++++++
 mm/hugetlb_vmemmap.c                            | 16 ++++++++++++++++
 5 files changed, 58 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 3ae25630a223..44dde9be7e00 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1551,6 +1551,20 @@
 			Documentation/admin-guide/mm/hugetlbpage.rst.
 			Format: size[KMG]
 
+	hugetlb_free_vmemmap=
+			[KNL] When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set,
+			this controls freeing unused vmemmap pages associated
+			with each HugeTLB page. When this option is enabled,
+			we disable PMD/huge page mapping of vmemmap pages which
+			increase page table pages. So if a user/sysadmin only
+			uses a small number of HugeTLB pages (as a percentage
+			of system memory), they could end up using more memory
+			with hugetlb_free_vmemmap on as opposed to off.
+			Format: { on | off (default) }
+
+			on:  enable the feature
+			off: disable the feature
+
 	hung_task_panic=
 			[KNL] Should the hung task detector generate panics.
 			Format: 0 | 1
diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
index f7b1c7462991..3a23c2377acc 100644
--- a/Documentation/admin-guide/mm/hugetlbpage.rst
+++ b/Documentation/admin-guide/mm/hugetlbpage.rst
@@ -145,6 +145,9 @@ default_hugepagesz
 
 	will all result in 256 2M huge pages being allocated.  Valid default
 	huge page size is architecture dependent.
+hugetlb_free_vmemmap
+	When CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set, this enables freeing
+	unused vmemmap pages associated with each HugeTLB page.
 
 When multiple huge page sizes are supported, ``/proc/sys/vm/nr_hugepages``
 indicates the current number of pre-allocated huge pages of the default size.
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0435bee2e172..1bce5f20e6ca 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -34,6 +34,7 @@
 #include <linux/gfp.h>
 #include <linux/kcore.h>
 #include <linux/bootmem_info.h>
+#include <linux/hugetlb.h>
 
 #include <asm/processor.h>
 #include <asm/bios_ebda.h>
@@ -1557,7 +1558,8 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
 {
 	int err;
 
-	if (end - start < PAGES_PER_SECTION * sizeof(struct page))
+	if (is_hugetlb_free_vmemmap_enabled() ||
+	    end - start < PAGES_PER_SECTION * sizeof(struct page))
 		err = vmemmap_populate_basepages(start, end, node, NULL);
 	else if (boot_cpu_has(X86_FEATURE_PSE))
 		err = vmemmap_populate_hugepages(start, end, node, altmap);
@@ -1585,6 +1587,8 @@ void register_page_bootmem_memmap(unsigned long section_nr,
 	pmd_t *pmd;
 	unsigned int nr_pmd_pages;
 	struct page *page;
+	bool base_mapping = !boot_cpu_has(X86_FEATURE_PSE) ||
+			    is_hugetlb_free_vmemmap_enabled();
 
 	for (; addr < end; addr = next) {
 		pte_t *pte = NULL;
@@ -1610,7 +1614,7 @@ void register_page_bootmem_memmap(unsigned long section_nr,
 		}
 		get_page_bootmem(section_nr, pud_page(*pud), MIX_SECTION_INFO);
 
-		if (!boot_cpu_has(X86_FEATURE_PSE)) {
+		if (base_mapping) {
 			next = (addr + PAGE_SIZE) & PAGE_MASK;
 			pmd = pmd_offset(pud, addr);
 			if (pmd_none(*pmd))
diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index ebca2ef02212..7f47f0eeca3b 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -770,6 +770,20 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
 }
 #endif
 
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+extern bool hugetlb_free_vmemmap_enabled;
+
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+	return hugetlb_free_vmemmap_enabled;
+}
+#else
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+	return false;
+}
+#endif
+
 #else	/* CONFIG_HUGETLB_PAGE */
 struct hstate {};
 
@@ -923,6 +937,11 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
 					pte_t *ptep, pte_t pte, unsigned long sz)
 {
 }
+
+static inline bool is_hugetlb_free_vmemmap_enabled(void)
+{
+	return false;
+}
 #endif	/* CONFIG_HUGETLB_PAGE */
 
 static inline spinlock_t *huge_pte_lock(struct hstate *h,
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 273816dd95b6..9e9bd458a3f5 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -178,6 +178,22 @@
 #define RESERVE_VMEMMAP_NR		2U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
+bool hugetlb_free_vmemmap_enabled;
+
+static int __init early_hugetlb_free_vmemmap_param(char *buf)
+{
+	if (!buf)
+		return -EINVAL;
+
+	if (!strcmp(buf, "on"))
+		hugetlb_free_vmemmap_enabled = true;
+	else if (strcmp(buf, "off"))
+		return -EINVAL;
+
+	return 0;
+}
+early_param("hugetlb_free_vmemmap", early_hugetlb_free_vmemmap_param);
+
 static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
 {
 	return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
-- 
2.11.0

[PATCH v10 09/11] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Muchun Song]

All the infrastructure is ready, so we introduce nr_free_vmemmap_pages
field in the hstate to indicate how many vmemmap pages associated with
a HugeTLB page that can be freed to buddy allocator. And initialize it
in the hugetlb_vmemmap_init(). This patch is actual enablement of the
feature.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
 include/linux/hugetlb.h |  3 +++
 mm/hugetlb.c            |  1 +
 mm/hugetlb_vmemmap.c    | 33 +++++++++++++++++++++++++++++++++
 mm/hugetlb_vmemmap.h    | 10 ++++++----
 4 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 7f47f0eeca3b..66d82ae7b712 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -492,6 +492,9 @@ struct hstate {
 	unsigned int nr_huge_pages_node[MAX_NUMNODES];
 	unsigned int free_huge_pages_node[MAX_NUMNODES];
 	unsigned int surplus_huge_pages_node[MAX_NUMNODES];
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+	unsigned int nr_free_vmemmap_pages;
+#endif
 #ifdef CONFIG_CGROUP_HUGETLB
 	/* cgroup control files */
 	struct cftype cgroup_files_dfl[7];
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index fea8a96dd718..6c02f49959fd 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3326,6 +3326,7 @@ void __init hugetlb_add_hstate(unsigned int order)
 	h->next_nid_to_free = first_memory_node;
 	snprintf(h->name, HSTATE_NAME_LEN, "hugepages-%lukB",
 					huge_page_size(h)/1024);
+	hugetlb_vmemmap_init(h);
 
 	parsed_hstate = h;
 }
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 9e9bd458a3f5..3ebfe1706c77 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -166,6 +166,8 @@
  * (last) level. So this type of HugeTLB page can be optimized only when its
  * size of the struct page structs is greater than 2 pages.
  */
+#define pr_fmt(fmt)	"HugeTLB: " fmt
+
 #include "hugetlb_vmemmap.h"
 
 /*
@@ -182,6 +184,12 @@ bool hugetlb_free_vmemmap_enabled;
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
+	/* We cannot optimize if a "struct page" crosses page boundaries. */
+	if ((!is_power_of_2(sizeof(struct page)))) {
+		pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n");
+		return 0;
+	}
+
 	if (!buf)
 		return -EINVAL;
 
@@ -220,3 +228,28 @@ void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 	vmemmap_remap_free(vmemmap_addr + RESERVE_VMEMMAP_SIZE,
 			   free_vmemmap_pages_size_per_hpage(h));
 }
+
+void __init hugetlb_vmemmap_init(struct hstate *h)
+{
+	unsigned int nr_pages = pages_per_huge_page(h);
+	unsigned int vmemmap_pages;
+
+	if (!hugetlb_free_vmemmap_enabled)
+		return;
+
+	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
+	/*
+	 * The head page and the first tail page are not to be freed to buddy
+	 * allocator, the other pages will map to the first tail page, so they
+	 * can be freed.
+	 *
+	 * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
+	 * on some architectures (e.g. aarch64). See Documentation/arm64/
+	 * hugetlbpage.rst for more details.
+	 */
+	if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
+		h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
+
+	pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages,
+		h->name);
+}
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index b2c8d2f11d48..8fd9ae113dbd 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -13,17 +13,15 @@
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
 void alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
 void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+void hugetlb_vmemmap_init(struct hstate *h);
 
 /*
  * How many vmemmap pages associated with a HugeTLB page that can be freed
  * to the buddy allocator.
- *
- * Todo: Returns zero for now, which means the feature is disabled. We will
- * enable it once all the infrastructure is there.
  */
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
-	return 0;
+	return h->nr_free_vmemmap_pages;
 }
 #else
 static inline void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
@@ -38,5 +36,9 @@ static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
 	return 0;
 }
+
+static inline void hugetlb_vmemmap_init(struct hstate *h)
+{
+}
 #endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
 #endif /* _LINUX_HUGETLB_VMEMMAP_H */
-- 
2.11.0

[PATCH v10 10/11] mm/hugetlb: Gather discrete indexes of tail page

[Muchun Song]

For HugeTLB page, there are more metadata to save in the struct page.
But the head struct page cannot meet our needs, so we have to abuse
other tail struct page to store the metadata. In order to avoid
conflicts caused by subsequent use of more tail struct pages, we can
gather these discrete indexes of tail struct page. In this case, it
will be easier to add a new tail page index later.

There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
---
 include/linux/hugetlb.h        | 13 +++++++++++++
 include/linux/hugetlb_cgroup.h | 15 +++++++++------
 mm/hugetlb.c                   | 16 ++++++++--------
 mm/hugetlb_vmemmap.c           |  8 ++++++++
 4 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 66d82ae7b712..7295f6b3d55e 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -28,6 +28,19 @@ typedef struct { unsigned long pd; } hugepd_t;
 #include <linux/shm.h>
 #include <asm/tlbflush.h>
 
+enum {
+	SUBPAGE_INDEX_ACTIVE = 1,	/* reuse page flags of PG_private */
+	SUBPAGE_INDEX_TEMPORARY,	/* reuse page->mapping */
+#ifdef CONFIG_CGROUP_HUGETLB
+	SUBPAGE_INDEX_CGROUP = SUBPAGE_INDEX_TEMPORARY,/* reuse page->private */
+	SUBPAGE_INDEX_CGROUP_RSVD,	/* reuse page->private */
+#endif
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+	SUBPAGE_INDEX_HWPOISON,		/* reuse page->private */
+#endif
+	NR_USED_SUBPAGE,
+};
+
 struct hugepage_subpool {
 	spinlock_t lock;
 	long count;
diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
index 2ad6e92f124a..3d3c1c49efe4 100644
--- a/include/linux/hugetlb_cgroup.h
+++ b/include/linux/hugetlb_cgroup.h
@@ -24,8 +24,9 @@ struct file_region;
 /*
  * Minimum page order trackable by hugetlb cgroup.
  * At least 4 pages are necessary for all the tracking information.
- * The second tail page (hpage[2]) is the fault usage cgroup.
- * The third tail page (hpage[3]) is the reservation usage cgroup.
+ * The second tail page (hpage[SUBPAGE_INDEX_CGROUP]) is the fault
+ * usage cgroup. The third tail page (hpage[SUBPAGE_INDEX_CGROUP_RSVD])
+ * is the reservation usage cgroup.
  */
 #define HUGETLB_CGROUP_MIN_ORDER	2
 
@@ -66,9 +67,9 @@ __hugetlb_cgroup_from_page(struct page *page, bool rsvd)
 	if (compound_order(page) < HUGETLB_CGROUP_MIN_ORDER)
 		return NULL;
 	if (rsvd)
-		return (struct hugetlb_cgroup *)page[3].private;
+		return (void *)page_private(page + SUBPAGE_INDEX_CGROUP_RSVD);
 	else
-		return (struct hugetlb_cgroup *)page[2].private;
+		return (void *)page_private(page + SUBPAGE_INDEX_CGROUP);
 }
 
 static inline struct hugetlb_cgroup *hugetlb_cgroup_from_page(struct page *page)
@@ -90,9 +91,11 @@ static inline int __set_hugetlb_cgroup(struct page *page,
 	if (compound_order(page) < HUGETLB_CGROUP_MIN_ORDER)
 		return -1;
 	if (rsvd)
-		page[3].private = (unsigned long)h_cg;
+		set_page_private(page + SUBPAGE_INDEX_CGROUP_RSVD,
+				 (unsigned long)h_cg);
 	else
-		page[2].private = (unsigned long)h_cg;
+		set_page_private(page + SUBPAGE_INDEX_CGROUP,
+				 (unsigned long)h_cg);
 	return 0;
 }
 
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6c02f49959fd..78dd88dda857 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1360,7 +1360,7 @@ static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
 	if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
 		return;
 
-	page = head + page_private(head + 4);
+	page = head + page_private(head + SUBPAGE_INDEX_HWPOISON);
 
 	/*
 	 * Move PageHWPoison flag from head page to the raw error page,
@@ -1379,7 +1379,7 @@ static inline void hwpoison_subpage_set(struct hstate *h, struct page *head,
 		return;
 
 	if (free_vmemmap_pages_per_hpage(h)) {
-		set_page_private(head + 4, page - head);
+		set_page_private(head + SUBPAGE_INDEX_HWPOISON, page - head);
 	} else if (page != head) {
 		/*
 		 * Move PageHWPoison flag from head page to the raw error page,
@@ -1459,20 +1459,20 @@ struct hstate *size_to_hstate(unsigned long size)
 bool page_huge_active(struct page *page)
 {
 	VM_BUG_ON_PAGE(!PageHuge(page), page);
-	return PageHead(page) && PagePrivate(&page[1]);
+	return PageHead(page) && PagePrivate(&page[SUBPAGE_INDEX_ACTIVE]);
 }
 
 /* never called for tail page */
 static void set_page_huge_active(struct page *page)
 {
 	VM_BUG_ON_PAGE(!PageHeadHuge(page), page);
-	SetPagePrivate(&page[1]);
+	SetPagePrivate(&page[SUBPAGE_INDEX_ACTIVE]);
 }
 
 static void clear_page_huge_active(struct page *page)
 {
 	VM_BUG_ON_PAGE(!PageHeadHuge(page), page);
-	ClearPagePrivate(&page[1]);
+	ClearPagePrivate(&page[SUBPAGE_INDEX_ACTIVE]);
 }
 
 /*
@@ -1484,17 +1484,17 @@ static inline bool PageHugeTemporary(struct page *page)
 	if (!PageHuge(page))
 		return false;
 
-	return (unsigned long)page[2].mapping == -1U;
+	return (unsigned long)page[SUBPAGE_INDEX_TEMPORARY].mapping == -1U;
 }
 
 static inline void SetPageHugeTemporary(struct page *page)
 {
-	page[2].mapping = (void *)-1U;
+	page[SUBPAGE_INDEX_TEMPORARY].mapping = (void *)-1U;
 }
 
 static inline void ClearPageHugeTemporary(struct page *page)
 {
-	page[2].mapping = NULL;
+	page[SUBPAGE_INDEX_TEMPORARY].mapping = NULL;
 }
 
 static void __free_huge_page(struct page *page)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 3ebfe1706c77..ad123b760245 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -234,6 +234,14 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	unsigned int nr_pages = pages_per_huge_page(h);
 	unsigned int vmemmap_pages;
 
+	/*
+	 * There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
+	 * page structs that can be used when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP,
+	 * so add a BUILD_BUG_ON to catch invalid usage of the tail struct page.
+	 */
+	BUILD_BUG_ON(NR_USED_SUBPAGE >=
+		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
+
 	if (!hugetlb_free_vmemmap_enabled)
 		return;
 
-- 
2.11.0

[PATCH v10 11/11] mm/hugetlb: Optimize the code with the help of the compiler

[Muchun Song]

We cannot optimize if a "struct page" crosses page boundaries. If
it is true, we can optimize the code with the help of a compiler.
When free_vmemmap_pages_per_hpage() returns zero, most functions are
optimized by the compiler.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/hugetlb.h | 3 ++-
 mm/hugetlb_vmemmap.c    | 7 +++++++
 mm/hugetlb_vmemmap.h    | 5 +++--
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 7295f6b3d55e..adc17765e0e9 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -791,7 +791,8 @@ extern bool hugetlb_free_vmemmap_enabled;
 
 static inline bool is_hugetlb_free_vmemmap_enabled(void)
 {
-	return hugetlb_free_vmemmap_enabled;
+	return hugetlb_free_vmemmap_enabled &&
+	       is_power_of_2(sizeof(struct page));
 }
 #else
 static inline bool is_hugetlb_free_vmemmap_enabled(void)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index ad123b760245..987248a004f0 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -242,6 +242,13 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	BUILD_BUG_ON(NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 
+	/*
+	 * The compiler can help us to optimize this function to null
+	 * when the size of the struct page is not power of 2.
+	 */
+	if (!is_power_of_2(sizeof(struct page)))
+		return;
+
 	if (!hugetlb_free_vmemmap_enabled)
 		return;
 
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 8fd9ae113dbd..e8de41295d4d 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -17,11 +17,12 @@ void hugetlb_vmemmap_init(struct hstate *h);
 
 /*
  * How many vmemmap pages associated with a HugeTLB page that can be freed
- * to the buddy allocator.
+ * to the buddy allocator. The checking of the is_power_of_2() aims to let
+ * the compiler help us optimize the code as much as possible.
  */
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
-	return h->nr_free_vmemmap_pages;
+	return is_power_of_2(sizeof(struct page)) ? h->nr_free_vmemmap_pages : 0;
 }
 #else
 static inline void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
-- 
2.11.0

Re: [PATCH v10 00/11] Free some vmemmap pages of HugeTLB page

[David Hildenbrand]

On 17.12.20 13:12, Muchun Song wrote:
> Hi all,
> 
> This patch series will free some vmemmap pages(struct page structures)
> associated with each hugetlbpage when preallocated to save memory.
> 
> In order to reduce the difficulty of the first version of code review.
> From this version, we disable PMD/huge page mapping of vmemmap if this
> feature was enabled. This accutualy eliminate a bunch of the complex code
> doing page table manipulation. When this patch series is solid, we cam add
> the code of vmemmap page table manipulation in the future.
> 
> The struct page structures (page structs) are used to describe a physical
> page frame. By default, there is a one-to-one mapping from a page frame to
> it's corresponding page struct.
> 
> The HugeTLB pages consist of multiple base page size pages and is supported
> by many architectures. See hugetlbpage.rst in the Documentation directory
> for more details. On the x86 architecture, HugeTLB pages of size 2MB and 1GB
> are currently supported. Since the base page size on x86 is 4KB, a 2MB
> HugeTLB page consists of 512 base pages and a 1GB HugeTLB page consists of
> 4096 base pages. For each base page, there is a corresponding page struct.
> 
> Within the HugeTLB subsystem, only the first 4 page structs are used to
> contain unique information about a HugeTLB page. HUGETLB_CGROUP_MIN_ORDER
> provides this upper limit. The only 'useful' information in the remaining
> page structs is the compound_head field, and this field is the same for all
> tail pages.
> 
> By removing redundant page structs for HugeTLB pages, memory can returned to
> the buddy allocator for other uses.
> 
> When the system boot up, every 2M HugeTLB has 512 struct page structs which
> size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE).
> 
>     HugeTLB                  struct pages(8 pages)         page frame(8 pages)
>  +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
>  |           |                     |     0     | -------------> |     0     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     1     | -------------> |     1     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     2     | -------------> |     2     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     3     | -------------> |     3     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     4     | -------------> |     4     |
>  |    2MB    |                     +-----------+                +-----------+
>  |           |                     |     5     | -------------> |     5     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     6     | -------------> |     6     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     7     | -------------> |     7     |
>  |           |                     +-----------+                +-----------+
>  |           |
>  |           |
>  |           |
>  +-----------+
> 
> The value of page->compound_head is the same for all tail pages. The first
> page of page structs (page 0) associated with the HugeTLB page contains the 4
> page structs necessary to describe the HugeTLB. The only use of the remaining
> pages of page structs (page 1 to page 7) is to point to page->compound_head.
> Therefore, we can remap pages 2 to 7 to page 1. Only 2 pages of page structs
> will be used for each HugeTLB page. This will allow us to free the remaining
> 6 pages to the buddy allocator.
> 
> Here is how things look after remapping.
> 
>     HugeTLB                  struct pages(8 pages)         page frame(8 pages)
>  +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
>  |           |                     |     0     | -------------> |     0     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     1     | -------------> |     1     |
>  |           |                     +-----------+                +-----------+
>  |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
>  |           |                     +-----------+                   | | | | |
>  |           |                     |     3     | ------------------+ | | | |
>  |           |                     +-----------+                     | | | |
>  |           |                     |     4     | --------------------+ | | |
>  |    2MB    |                     +-----------+                       | | |
>  |           |                     |     5     | ----------------------+ | |
>  |           |                     +-----------+                         | |
>  |           |                     |     6     | ------------------------+ |
>  |           |                     +-----------+                           |
>  |           |                     |     7     | --------------------------+
>  |           |                     +-----------+
>  |           |
>  |           |
>  |           |
>  +-----------+
> 
> When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> vmemmap pages and restore the previous mapping relationship.
> 
> Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar
> to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
> pages.
> 
> In this case, for the 1GB HugeTLB page, we can save 4088 pages(There are
> 4096 pages for struct page structs, we reserve 2 pages for vmemmap and 8
> pages for page tables. So we can save 4088 pages). This is a very substantial
> gain. On our server, run some SPDK/QEMU applications which will use 1024GB
> hugetlbpage. With this feature enabled, we can save ~16GB(1G hugepage)/~11GB
> (2MB hugepage, the worst case is 10GB while the best is 12GB) memory.
> 
> Because there are vmemmap page tables reconstruction on the freeing/allocating
> path, it increases some overhead. Here are some overhead analysis.
> 
> 1) Allocating 10240 2MB hugetlb pages.
> 
>    a) With this patch series applied:
>    # time echo 10240 > /proc/sys/vm/nr_hugepages
> 
>    real     0m0.166s
>    user     0m0.000s
>    sys      0m0.166s
> 
>    # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
>    Attaching 2 probes...
> 
>    @latency:
>    [8K, 16K)           8360 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
>    [16K, 32K)          1868 |@@@@@@@@@@@                                         |
>    [32K, 64K)            10 |                                                    |
>    [64K, 128K)            2 |                                                    |
> 
>    b) Without this patch series:
>    # time echo 10240 > /proc/sys/vm/nr_hugepages
> 
>    real     0m0.066s
>    user     0m0.000s
>    sys      0m0.066s
> 
>    # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
>    Attaching 2 probes...
> 
>    @latency:
>    [4K, 8K)           10176 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
>    [8K, 16K)             62 |                                                    |
>    [16K, 32K)             2 |                                                    |
> 
>    Summarize: this feature is about ~2x slower than before.
> 
> 2) Freeing 10240 2MB hugetlb pages.
> 
>    a) With this patch series applied:
>    # time echo 0 > /proc/sys/vm/nr_hugepages
> 
>    real     0m0.004s
>    user     0m0.000s
>    sys      0m0.002s
> 
>    # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
>    Attaching 2 probes...
> 
>    @latency:
>    [16K, 32K)         10240 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> 
>    b) Without this patch series:
>    # time echo 0 > /proc/sys/vm/nr_hugepages
> 
>    real     0m0.077s
>    user     0m0.001s
>    sys      0m0.075s
> 
>    # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }'
>    Attaching 2 probes...
> 
>    @latency:
>    [4K, 8K)            9950 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
>    [8K, 16K)            287 |@                                                   |
>    [16K, 32K)             3 |                                                    |
> 
>    Summarize: The overhead of __free_hugepage is about ~2-4x slower than before.
>               But according to the allocation test above, I think that here is
> 	      also ~2x slower than before.
> 
>               But why the 'real' time of patched is smaller than before? Because
> 	      In this patch series, the freeing hugetlb is asynchronous(through
> 	      kwoker).
> 
> Although the overhead has increased, the overhead is not significant. Like Mike
> said, "However, remember that the majority of use cases create hugetlb pages at
> or shortly after boot time and add them to the pool. So, additional overhead is
> at pool creation time. There is no change to 'normal run time' operations of
> getting a page from or returning a page to the pool (think page fault/unmap)".
> 

Just FYI, I'll be offline until first week of January. I'm planning on
reviewing when I'm back.


-- 
Thanks,

David / dhildenb

Re: [PATCH v10 00/11] Free some vmemmap pages of HugeTLB page

[Oscar Salvador]

On Thu, Dec 17, 2020 at 08:12:52PM +0800, Muchun Song wrote:
> In this case, for the 1GB HugeTLB page, we can save 4088 pages(There are
> 4096 pages for struct page structs, we reserve 2 pages for vmemmap and 8
> pages for page tables. So we can save 4088 pages). This is a very substantial
> gain. On our server, run some SPDK/QEMU applications which will use 1024GB
> hugetlbpage. With this feature enabled, we can save ~16GB(1G hugepage)/~11GB
> (2MB hugepage, the worst case is 10GB while the best is 12GB) memory.

Is the above really true?
We no longer need to allocate pagetables, so the savings go up to 4094, right?

I will be off for a few days but I expect to get back to this and review the
missing bits when I am back.

-- 
Oscar Salvador
SUSE L3

Re: [External] Re: [PATCH v10 00/11] Free some vmemmap pages of HugeTLB page

[Muchun Song]

On Thu, Dec 17, 2020 at 11:00 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Dec 17, 2020 at 08:12:52PM +0800, Muchun Song wrote:
> > In this case, for the 1GB HugeTLB page, we can save 4088 pages(There are
> > 4096 pages for struct page structs, we reserve 2 pages for vmemmap and 8
> > pages for page tables. So we can save 4088 pages). This is a very substantial
> > gain. On our server, run some SPDK/QEMU applications which will use 1024GB
> > hugetlbpage. With this feature enabled, we can save ~16GB(1G hugepage)/~11GB
> > (2MB hugepage, the worst case is 10GB while the best is 12GB) memory.
>
> Is the above really true?
> We no longer need to allocate pagetables, so the savings go up to 4094, right?

Yeah, you are right. I forget to update this.

>
> I will be off for a few days but I expect to get back to this and review the
> missing bits when I am back.
>

Thanks.

> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [PATCH v10 10/11] mm/hugetlb: Gather discrete indexes of tail page

[Oscar Salvador]

On Thu, Dec 17, 2020 at 08:13:02PM +0800, Muchun Song wrote:
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6c02f49959fd..78dd88dda857 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1360,7 +1360,7 @@ static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
>  	if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
>  		return;
>  
> -	page = head + page_private(head + 4);
> +	page = head + page_private(head + SUBPAGE_INDEX_HWPOISON);
>  
>  	/*
>  	 * Move PageHWPoison flag from head page to the raw error page,
> @@ -1379,7 +1379,7 @@ static inline void hwpoison_subpage_set(struct hstate *h, struct page *head,
>  		return;
>  
>  	if (free_vmemmap_pages_per_hpage(h)) {
> -		set_page_private(head + 4, page - head);
> +		set_page_private(head + SUBPAGE_INDEX_HWPOISON, page - head);

Ok, I was too eager here.

If CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is not set for whatever reason
(e.g: CONFIG_MEMORY_HOTREMOVE is disabled), when you convert "+4"
to its index (SUBPAGE_INDEX_HWPOISON), this will no longer build
since we only define SUBPAGE_INDEX_HWPOISON when the config
option CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set.

Different things can be done to fix this:

e.g:

 - Define a two different hwpoison_subpage_{deliver,set}
   and have them under
   #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
   ...
   #else
   ...
   #endif

 - Work it around as is with IS_ENABLED(CONFIG_HUGETLB_...
 - Have a common entry and decide depending on whether
   the config is enabled.

I guess option #1 might be cleaner.

-- 
Oscar Salvador
SUSE L3

Re: [External] Re: [PATCH v10 10/11] mm/hugetlb: Gather discrete indexes of tail page

[Muchun Song]

On Fri, Dec 18, 2020 at 5:06 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Dec 17, 2020 at 08:13:02PM +0800, Muchun Song wrote:
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 6c02f49959fd..78dd88dda857 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1360,7 +1360,7 @@ static inline void hwpoison_subpage_deliver(struct hstate *h, struct page *head)
> >       if (!PageHWPoison(head) || !free_vmemmap_pages_per_hpage(h))
> >               return;
> >
> > -     page = head + page_private(head + 4);
> > +     page = head + page_private(head + SUBPAGE_INDEX_HWPOISON);
> >
> >       /*
> >        * Move PageHWPoison flag from head page to the raw error page,
> > @@ -1379,7 +1379,7 @@ static inline void hwpoison_subpage_set(struct hstate *h, struct page *head,
> >               return;
> >
> >       if (free_vmemmap_pages_per_hpage(h)) {
> > -             set_page_private(head + 4, page - head);
> > +             set_page_private(head + SUBPAGE_INDEX_HWPOISON, page - head);
>
> Ok, I was too eager here.
>
> If CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is not set for whatever reason
> (e.g: CONFIG_MEMORY_HOTREMOVE is disabled), when you convert "+4"
> to its index (SUBPAGE_INDEX_HWPOISON), this will no longer build
> since we only define SUBPAGE_INDEX_HWPOISON when the config
> option CONFIG_HUGETLB_PAGE_FREE_VMEMMAP is set.

Yeah, it is my mistake. Thanks for pointing that out.

>
> Different things can be done to fix this:
>
> e.g:
>
>  - Define a two different hwpoison_subpage_{deliver,set}
>    and have them under
>    #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
>    ...
>    #else
>    ...
>    #endif
>
>  - Work it around as is with IS_ENABLED(CONFIG_HUGETLB_...
>  - Have a common entry and decide depending on whether
>    the config is enabled.
>
> I guess option #1 might be cleaner.

Thanks for your suggestion. I also prefer option #1.

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [PATCH v10 02/11] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Oscar Salvador]

On Thu, Dec 17, 2020 at 08:12:54PM +0800, Muchun Song wrote:
> The HUGETLB_PAGE_FREE_VMEMMAP option is used to enable the freeing
> of unnecessary vmemmap associated with HugeTLB pages. The config
> option is introduced early so that supporting code can be written
> to depend on the option. The initial version of the code only
> provides support for x86-64.
> 
> Like other code which frees vmemmap, this config option depends on
> HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
> used to register bootmem info. Therefore, make sure
> register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
> is defined.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Ok, this commit message is much more clear, and the same goes for the
config description.

Reviewed-by: Oscar Salvador <osalvador@suse.de>

-- 
Oscar Salvador
SUSE L3

Re: [PATCH v10 09/11] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Oscar Salvador]

On Thu, Dec 17, 2020 at 08:13:01PM +0800, Muchun Song wrote:
> @@ -182,6 +184,12 @@ bool hugetlb_free_vmemmap_enabled;
>  
>  static int __init early_hugetlb_free_vmemmap_param(char *buf)
>  {
> +	/* We cannot optimize if a "struct page" crosses page boundaries. */
> +	if ((!is_power_of_2(sizeof(struct page)))) {
> +		pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n");
> +		return 0;
> +	}

Unless there is a strong reason behind, I would move this to the previous patch,
where early_hugetlb_free_vmemmap_param is introduced.

-- 
Oscar Salvador
SUSE L3

Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Oscar Salvador]

On Thu, Dec 17, 2020 at 08:12:55PM +0800, Muchun Song wrote:
> +static inline void free_bootmem_page(struct page *page)
> +{
> +	unsigned long magic = (unsigned long)page->freelist;
> +
> +	/*
> +	 * The reserve_bootmem_region sets the reserved flag on bootmem
> +	 * pages.
> +	 */
> +	VM_WARN_ON(page_ref_count(page) != 2);
> +
> +	if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> +		put_page_bootmem(page);
> +	else
> +		VM_WARN_ON(1);

Ideally, I think we want to see what how the page looks since its state
is not what we expected, so maybe join both conditions and use dump_page().

> + * By removing redundant page structs for HugeTLB pages, memory can returned to
                                                                     ^^ be
> + * the buddy allocator for other uses.

[...]

> +void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> +{
> +	unsigned long vmemmap_addr = (unsigned long)head;
> +
> +	if (!free_vmemmap_pages_per_hpage(h))
> +		return;
> +
> +	vmemmap_remap_free(vmemmap_addr + RESERVE_VMEMMAP_SIZE,
> +			   free_vmemmap_pages_size_per_hpage(h));

I am not sure what others think, but I would like to see vmemmap_remap_free taking
three arguments: start, end, and reuse addr, e.g:

 void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 {
      unsigned long vmemmap_addr = (unsigned long)head;
      unsigned long vmemmap_end, vmemmap_reuse;
      
      if (!free_vmemmap_pages_per_hpage(h))
              return;

      vmemmap_addr += RESERVE_MEMMAP_SIZE;
      vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
      vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
 
      vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
 }

The reason for me to do this is to let the callers of vmemmap_remap_free decide
__what__ they want to remap.

More on this below.


> +static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> +			      unsigned long end,
> +			      struct vmemmap_remap_walk *walk)
> +{
> +	pte_t *pte;
> +
> +	pte = pte_offset_kernel(pmd, addr);
> +
> +	if (walk->reuse_addr == addr) {
> +		BUG_ON(pte_none(*pte));
> +		walk->reuse_page = pte_page(*pte++);
> +		addr += PAGE_SIZE;
> +	}

Although it is quite obvious, a brief comment here pointing out what are we
doing and that this is meant to be set only once would be nice.


> +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> +				struct vmemmap_remap_walk *walk)
> +{
> +	unsigned long addr = start - PAGE_SIZE;
> +	unsigned long next;
> +	pgd_t *pgd;
> +
> +	VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> +	VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> +
> +	walk->reuse_page = NULL;
> +	walk->reuse_addr = addr;

With the change I suggested above, struct vmemmap_remap_walk should be
initialitzed at once in vmemmap_remap_free, so this should not longer be needed.
(And btw, you do not need to set reuse_page to NULL, the way you init the struct
in vmemmap_remap_free makes sure to null any field you do not explicitly set).


> +static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> +			      struct vmemmap_remap_walk *walk)
> +{
> +	/*
> +	 * Make the tail pages are mapped with read-only to catch
> +	 * illegal write operation to the tail pages.
        "Remap the tail pages as read-only to ..."

> +	 */
> +	pgprot_t pgprot = PAGE_KERNEL_RO;
> +	pte_t entry = mk_pte(walk->reuse_page, pgprot);
> +	struct page *page;
> +
> +	page = pte_page(*pte);

 struct page *page = pte_page(*pte);

since you did the same for the other two.

> +	list_add(&page->lru, walk->vmemmap_pages);
> +
> +	set_pte_at(&init_mm, addr, pte, entry);
> +}
> +
> +/**
> + * vmemmap_remap_free - remap the vmemmap virtual address range
> + *                      [start, start + size) to the page which
> + *                      [start - PAGE_SIZE, start) is mapped,
> + *                      then free vmemmap pages.
> + * @start:	start address of the vmemmap virtual address range
> + * @size:	size of the vmemmap virtual address range
> + */
> +void vmemmap_remap_free(unsigned long start, unsigned long size)
> +{
> +	unsigned long end = start + size;
> +	LIST_HEAD(vmemmap_pages);
> +
> +	struct vmemmap_remap_walk walk = {
> +		.remap_pte	= vmemmap_remap_pte,
> +		.vmemmap_pages	= &vmemmap_pages,
> +	};

As stated above, this would become:

 void vmemmap_remap_free(unsigned long start, unsigned long end,
                         usigned long reuse)
 {
       LIST_HEAD(vmemmap_pages);
       struct vmemmap_remap_walk walk = {
               .reuse_addr = reuse,
               .remap_pte = vmemmap_remap_pte,
               .vmemmap_pages = &vmemmap_pages,
       };

  You might have had your reasons to do this way, but this looks more natural
  to me, with the plus that callers of vmemmap_remap_free can specify
  what they want to remap.


-- 
Oscar Salvador
SUSE L3

Re: [External] Re: [PATCH v10 09/11] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Muchun Song]

On Mon, Dec 21, 2020 at 4:17 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Dec 17, 2020 at 08:13:01PM +0800, Muchun Song wrote:
> > @@ -182,6 +184,12 @@ bool hugetlb_free_vmemmap_enabled;
> >
> >  static int __init early_hugetlb_free_vmemmap_param(char *buf)
> >  {
> > +     /* We cannot optimize if a "struct page" crosses page boundaries. */
> > +     if ((!is_power_of_2(sizeof(struct page)))) {
> > +             pr_warn("cannot free vmemmap pages because \"struct page\" crosses page boundaries\n");
> > +             return 0;
> > +     }
>
> Unless there is a strong reason behind, I would move this to the previous patch,
> where early_hugetlb_free_vmemmap_param is introduced.

OK. Will do. Thanks.

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [PATCH v10 04/11] mm/hugetlb: Defer freeing of HugeTLB pages

[Oscar Salvador]

On Thu, Dec 17, 2020 at 08:12:56PM +0800, Muchun Song wrote:
> In the subsequent patch, we will allocate the vmemmap pages when free
> HugeTLB pages. But update_and_free_page() is called from a non-task
> context(and hold hugetlb_lock), so we can defer the actual freeing in
> a workqueue to prevent from using GFP_ATOMIC to allocate the vmemmap
> pages.

I think we would benefit from a more complete changelog, at least I had
to stare at the code for a while in order to grasp what are we trying
to do and the reasons behind.

> +static void __free_hugepage(struct hstate *h, struct page *page);
> +
> +/*
> + * As update_and_free_page() is be called from a non-task context(and hold
> + * hugetlb_lock), we can defer the actual freeing in a workqueue to prevent
> + * use GFP_ATOMIC to allocate a lot of vmemmap pages.

The above implies that update_and_free_page() is __always__ called from a 
non-task context, but that is not always the case?

> +static void update_hpage_vmemmap_workfn(struct work_struct *work)
>  {
> -	int i;
> +	struct llist_node *node;
> +	struct page *page;
>  
> +	node = llist_del_all(&hpage_update_freelist);
> +
> +	while (node) {
> +		page = container_of((struct address_space **)node,
> +				     struct page, mapping);
> +		node = node->next;
> +		page->mapping = NULL;
> +		__free_hugepage(page_hstate(page), page);
> +
> +		cond_resched();
> +	}
> +}
> +static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);

I wonder if this should be moved to hugetlb_vmemmap.c

> +/*
> + * This is where the call to allocate vmemmmap pages will be inserted.
> + */

I think this should go in the changelog.

> +static void __free_hugepage(struct hstate *h, struct page *page)
> +{
> +	int i;
> +
>  	for (i = 0; i < pages_per_huge_page(h); i++) {
>  		page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
>  				1 << PG_referenced | 1 << PG_dirty |
> @@ -1313,13 +1377,17 @@ static void update_and_free_page(struct hstate *h, struct page *page)
>  	set_page_refcounted(page);
>  	if (hstate_is_gigantic(h)) {
>  		/*
> -		 * Temporarily drop the hugetlb_lock, because
> -		 * we might block in free_gigantic_page().
> +		 * Temporarily drop the hugetlb_lock only when this type of
> +		 * HugeTLB page does not support vmemmap optimization (which
> +		 * context do not hold the hugetlb_lock), because we might
> +		 * block in free_gigantic_page().

"
 /*
  * Temporarily drop the hugetlb_lock, because we might block
  * in free_gigantic_page(). Only drop it in case the vmemmap
  * optimization is disabled, since that context does not hold
  * the lock.
  */
" ?

 
Oscar Salvador
SUSE L3

Re: [PATCH v10 07/11] mm/hugetlb: Flush work when dissolving hugetlb page

[Oscar Salvador]

On Thu, Dec 17, 2020 at 08:12:59PM +0800, Muchun Song wrote:
> We should flush work when dissolving a hugetlb page to make sure that
> the hugetlb page is freed to the buddy.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Reviewed-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE L3

Re: [External] Re: [PATCH v10 04/11] mm/hugetlb: Defer freeing of HugeTLB pages

[Muchun Song]

On Mon, Dec 21, 2020 at 6:27 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Dec 17, 2020 at 08:12:56PM +0800, Muchun Song wrote:
> > In the subsequent patch, we will allocate the vmemmap pages when free
> > HugeTLB pages. But update_and_free_page() is called from a non-task
> > context(and hold hugetlb_lock), so we can defer the actual freeing in
> > a workqueue to prevent from using GFP_ATOMIC to allocate the vmemmap
> > pages.
>
> I think we would benefit from a more complete changelog, at least I had
> to stare at the code for a while in order to grasp what are we trying
> to do and the reasons behind.

OK. Will do.

>
> > +static void __free_hugepage(struct hstate *h, struct page *page);
> > +
> > +/*
> > + * As update_and_free_page() is be called from a non-task context(and hold
> > + * hugetlb_lock), we can defer the actual freeing in a workqueue to prevent
> > + * use GFP_ATOMIC to allocate a lot of vmemmap pages.
>
> The above implies that update_and_free_page() is __always__ called from a
> non-task context, but that is not always the case?

IIUC, here is always the case.

>
> > +static void update_hpage_vmemmap_workfn(struct work_struct *work)
> >  {
> > -     int i;
> > +     struct llist_node *node;
> > +     struct page *page;
> >
> > +     node = llist_del_all(&hpage_update_freelist);
> > +
> > +     while (node) {
> > +             page = container_of((struct address_space **)node,
> > +                                  struct page, mapping);
> > +             node = node->next;
> > +             page->mapping = NULL;
> > +             __free_hugepage(page_hstate(page), page);
> > +
> > +             cond_resched();
> > +     }
> > +}
> > +static DECLARE_WORK(hpage_update_work, update_hpage_vmemmap_workfn);
>
> I wonder if this should be moved to hugetlb_vmemmap.c

Maybe I can do a try.

>
> > +/*
> > + * This is where the call to allocate vmemmmap pages will be inserted.
> > + */
>
> I think this should go in the changelog.

OK. Will do.

>
> > +static void __free_hugepage(struct hstate *h, struct page *page)
> > +{
> > +     int i;
> > +
> >       for (i = 0; i < pages_per_huge_page(h); i++) {
> >               page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
> >                               1 << PG_referenced | 1 << PG_dirty |
> > @@ -1313,13 +1377,17 @@ static void update_and_free_page(struct hstate *h, struct page *page)
> >       set_page_refcounted(page);
> >       if (hstate_is_gigantic(h)) {
> >               /*
> > -              * Temporarily drop the hugetlb_lock, because
> > -              * we might block in free_gigantic_page().
> > +              * Temporarily drop the hugetlb_lock only when this type of
> > +              * HugeTLB page does not support vmemmap optimization (which
> > +              * context do not hold the hugetlb_lock), because we might
> > +              * block in free_gigantic_page().
>
> "
>  /*
>   * Temporarily drop the hugetlb_lock, because we might block
>   * in free_gigantic_page(). Only drop it in case the vmemmap
>   * optimization is disabled, since that context does not hold
>   * the lock.
>   */
> " ?

Thanks a lot.

>
>
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [External] Re: [PATCH v10 07/11] mm/hugetlb: Flush work when dissolving hugetlb page

[Muchun Song]

On Mon, Dec 21, 2020 at 6:40 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Dec 17, 2020 at 08:12:59PM +0800, Muchun Song wrote:
> > We should flush work when dissolving a hugetlb page to make sure that
> > the hugetlb page is freed to the buddy.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>

Thanks.
>
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Mon, Dec 21, 2020 at 5:11 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Thu, Dec 17, 2020 at 08:12:55PM +0800, Muchun Song wrote:
> > +static inline void free_bootmem_page(struct page *page)
> > +{
> > +     unsigned long magic = (unsigned long)page->freelist;
> > +
> > +     /*
> > +      * The reserve_bootmem_region sets the reserved flag on bootmem
> > +      * pages.
> > +      */
> > +     VM_WARN_ON(page_ref_count(page) != 2);
> > +
> > +     if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
> > +             put_page_bootmem(page);
> > +     else
> > +             VM_WARN_ON(1);
>
> Ideally, I think we want to see what how the page looks since its state
> is not what we expected, so maybe join both conditions and use dump_page().

Agree. Will do. Thanks.

>
> > + * By removing redundant page structs for HugeTLB pages, memory can returned to
>                                                                      ^^ be

Thanks.

> > + * the buddy allocator for other uses.
>
> [...]
>
> > +void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > +     unsigned long vmemmap_addr = (unsigned long)head;
> > +
> > +     if (!free_vmemmap_pages_per_hpage(h))
> > +             return;
> > +
> > +     vmemmap_remap_free(vmemmap_addr + RESERVE_VMEMMAP_SIZE,
> > +                        free_vmemmap_pages_size_per_hpage(h));
>
> I am not sure what others think, but I would like to see vmemmap_remap_free taking
> three arguments: start, end, and reuse addr, e.g:
>
>  void free_huge_page_vmemmap(struct hstate *h, struct page *head)
>  {
>       unsigned long vmemmap_addr = (unsigned long)head;
>       unsigned long vmemmap_end, vmemmap_reuse;
>
>       if (!free_vmemmap_pages_per_hpage(h))
>               return;
>
>       vmemmap_addr += RESERVE_MEMMAP_SIZE;
>       vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
>       vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
>
>       vmemmap_remap_free(vmemmap_addr, vmemmap_end, vmemmap_reuse);
>  }
>
> The reason for me to do this is to let the callers of vmemmap_remap_free decide
> __what__ they want to remap.
>
> More on this below.
>
>
> > +static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
> > +                           unsigned long end,
> > +                           struct vmemmap_remap_walk *walk)
> > +{
> > +     pte_t *pte;
> > +
> > +     pte = pte_offset_kernel(pmd, addr);
> > +
> > +     if (walk->reuse_addr == addr) {
> > +             BUG_ON(pte_none(*pte));
> > +             walk->reuse_page = pte_page(*pte++);
> > +             addr += PAGE_SIZE;
> > +     }
>
> Although it is quite obvious, a brief comment here pointing out what are we
> doing and that this is meant to be set only once would be nice.

OK. Will do.

>
>
> > +static void vmemmap_remap_range(unsigned long start, unsigned long end,
> > +                             struct vmemmap_remap_walk *walk)
> > +{
> > +     unsigned long addr = start - PAGE_SIZE;
> > +     unsigned long next;
> > +     pgd_t *pgd;
> > +
> > +     VM_BUG_ON(!IS_ALIGNED(start, PAGE_SIZE));
> > +     VM_BUG_ON(!IS_ALIGNED(end, PAGE_SIZE));
> > +
> > +     walk->reuse_page = NULL;
> > +     walk->reuse_addr = addr;
>
> With the change I suggested above, struct vmemmap_remap_walk should be
> initialitzed at once in vmemmap_remap_free, so this should not longer be needed.

You are right.

> (And btw, you do not need to set reuse_page to NULL, the way you init the struct
> in vmemmap_remap_free makes sure to null any field you do not explicitly set).
>
>
> > +static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> > +                           struct vmemmap_remap_walk *walk)
> > +{
> > +     /*
> > +      * Make the tail pages are mapped with read-only to catch
> > +      * illegal write operation to the tail pages.
>         "Remap the tail pages as read-only to ..."

Thanks.

>
> > +      */
> > +     pgprot_t pgprot = PAGE_KERNEL_RO;
> > +     pte_t entry = mk_pte(walk->reuse_page, pgprot);
> > +     struct page *page;
> > +
> > +     page = pte_page(*pte);
>
>  struct page *page = pte_page(*pte);
>
> since you did the same for the other two.

Yeah. Will change to this.

>
> > +     list_add(&page->lru, walk->vmemmap_pages);
> > +
> > +     set_pte_at(&init_mm, addr, pte, entry);
> > +}
> > +
> > +/**
> > + * vmemmap_remap_free - remap the vmemmap virtual address range
> > + *                      [start, start + size) to the page which
> > + *                      [start - PAGE_SIZE, start) is mapped,
> > + *                      then free vmemmap pages.
> > + * @start:   start address of the vmemmap virtual address range
> > + * @size:    size of the vmemmap virtual address range
> > + */
> > +void vmemmap_remap_free(unsigned long start, unsigned long size)
> > +{
> > +     unsigned long end = start + size;
> > +     LIST_HEAD(vmemmap_pages);
> > +
> > +     struct vmemmap_remap_walk walk = {
> > +             .remap_pte      = vmemmap_remap_pte,
> > +             .vmemmap_pages  = &vmemmap_pages,
> > +     };
>
> As stated above, this would become:
>
>  void vmemmap_remap_free(unsigned long start, unsigned long end,
>                          usigned long reuse)
>  {
>        LIST_HEAD(vmemmap_pages);
>        struct vmemmap_remap_walk walk = {
>                .reuse_addr = reuse,
>                .remap_pte = vmemmap_remap_pte,
>                .vmemmap_pages = &vmemmap_pages,
>        };
>
>   You might have had your reasons to do this way, but this looks more natural
>   to me, with the plus that callers of vmemmap_remap_free can specify
>   what they want to remap.

Should we add a BUG_ON in vmemmap_remap_free() for now?

        BUG_ON(reuse != start + PAGE_SIZE);

>
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Oscar Salvador]

On Mon, Dec 21, 2020 at 07:25:15PM +0800, Muchun Song wrote:

> Should we add a BUG_ON in vmemmap_remap_free() for now?
> 
>         BUG_ON(reuse != start + PAGE_SIZE);

I do not think we have to, plus we would be BUG_ing for some specific use
case in "generic" function.
Maybe others think different though.

-- 
Oscar Salvador
SUSE L3

Re: [External] Re: [PATCH v10 04/11] mm/hugetlb: Defer freeing of HugeTLB pages

[Oscar Salvador]

On Mon, Dec 21, 2020 at 07:07:18PM +0800, Muchun Song wrote:
> > The above implies that update_and_free_page() is __always__ called from a
> > non-task context, but that is not always the case?
> 
> IIUC, here is always the case.

I might be missing something obvious, so bear with me.

I guess you are refering to the call __free_huge_page()->update_and_free_page().
AFAICS, free_huge_page might call __free_huge_page right away when in task
context, and so, we would be calling update_and_free in a task context as well.

Or are you referring to the other callers?

-- 
Oscar Salvador
SUSE L3

Re: [External] Re: [PATCH v10 04/11] mm/hugetlb: Defer freeing of HugeTLB pages

[Muchun Song]

On Mon, Dec 21, 2020 at 10:14 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Mon, Dec 21, 2020 at 07:07:18PM +0800, Muchun Song wrote:
> > > The above implies that update_and_free_page() is __always__ called from a
> > > non-task context, but that is not always the case?
> >
> > IIUC, here is always the case.
>
> I might be missing something obvious, so bear with me.
>
> I guess you are refering to the call __free_huge_page()->update_and_free_page().
> AFAICS, free_huge_page might call __free_huge_page right away when in task
> context, and so, we would be calling update_and_free in a task context as well.

Yeah. You are right. I mean the call __free_huge_page()->update_and_free_page().
Because update_and_free_page is called under hugetlb_lock, it is
non-task context,
right?


>
> Or are you referring to the other callers?
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Mon, Dec 21, 2020 at 9:44 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Mon, Dec 21, 2020 at 07:25:15PM +0800, Muchun Song wrote:
>
> > Should we add a BUG_ON in vmemmap_remap_free() for now?
> >
> >         BUG_ON(reuse != start + PAGE_SIZE);
>
> I do not think we have to, plus we would be BUG_ing for some specific use
> case in "generic" function.

The vmemmap_remap_range() walks page table range [start, end),
if reuse is equal to (start + PAGE_SIZE), the range can adjust to
[start - PAGE_SIZE, end). But if not, we need some work to
implement the "generic" function.

  - adjust range to [min(start, reuse), end) and call
    vmemmap_remap_rangeand which skip the hole
    which is [reuse + PAGE_SIZE, start) or [end, reuse).
  - call vmemmap_remap_range(reuse, reuse + PAGE_SIZE)
    to get the reuse page.Then, call vmemmap_remap_range(start, end)
    again to remap.

Which one do you prefer?

> Maybe others think different though.
>
> --
> Oscar Salvador
> SUSE L3



--
Yours,
Muchun

Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Oscar Salvador]

On Mon, Dec 21, 2020 at 11:52:30PM +0800, Muchun Song wrote:
> On Mon, Dec 21, 2020 at 9:44 PM Oscar Salvador <osalvador@suse.de> wrote:
> >
> > On Mon, Dec 21, 2020 at 07:25:15PM +0800, Muchun Song wrote:
> >
> > > Should we add a BUG_ON in vmemmap_remap_free() for now?
> > >
> > >         BUG_ON(reuse != start + PAGE_SIZE);
> >
> > I do not think we have to, plus we would be BUG_ing for some specific use
> > case in "generic" function.
> 
> The vmemmap_remap_range() walks page table range [start, end),
> if reuse is equal to (start + PAGE_SIZE), the range can adjust to
> [start - PAGE_SIZE, end). But if not, we need some work to
> implement the "generic" function.
> 
>   - adjust range to [min(start, reuse), end) and call
>     vmemmap_remap_rangeand which skip the hole
>     which is [reuse + PAGE_SIZE, start) or [end, reuse).
>   - call vmemmap_remap_range(reuse, reuse + PAGE_SIZE)
>     to get the reuse page.Then, call vmemmap_remap_range(start, end)
>     again to remap.
> 
> Which one do you prefer?

I would not overcomplicate things at this stage.
Just follow my sugestion and add a BUG_ON as you said, that might be the
easier way now.
We can overthink this in the future when some other usecases come
around, right?

Thanks


-- 
Oscar Salvador
SUSE L3

Re: [PATCH v10 02/11] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Mike Kravetz]

On 12/17/20 4:12 AM, Muchun Song wrote:
> The HUGETLB_PAGE_FREE_VMEMMAP option is used to enable the freeing
> of unnecessary vmemmap associated with HugeTLB pages. The config
> option is introduced early so that supporting code can be written
> to depend on the option. The initial version of the code only
> provides support for x86-64.
> 
> Like other code which frees vmemmap, this config option depends on
> HAVE_BOOTMEM_INFO_NODE. The routine register_page_bootmem_info() is
> used to register bootmem info. Therefore, make sure
> register_page_bootmem_info is enabled if HUGETLB_PAGE_FREE_VMEMMAP
> is defined.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  arch/x86/mm/init_64.c |  2 +-
>  fs/Kconfig            | 18 ++++++++++++++++++
>  2 files changed, 19 insertions(+), 1 deletion(-)

Thanks for updating,

Acked-by: Mike Kravetz <mike.kravetz@oracle.com>

-- 
Mike Kravetz

Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Mike Kravetz]

On 12/21/20 10:00 AM, Oscar Salvador wrote:
> On Mon, Dec 21, 2020 at 11:52:30PM +0800, Muchun Song wrote:
>> On Mon, Dec 21, 2020 at 9:44 PM Oscar Salvador <osalvador@suse.de> wrote:
>>>
>>> On Mon, Dec 21, 2020 at 07:25:15PM +0800, Muchun Song wrote:
>>>
>>>> Should we add a BUG_ON in vmemmap_remap_free() for now?
>>>>
>>>>         BUG_ON(reuse != start + PAGE_SIZE);
>>>
>>> I do not think we have to, plus we would be BUG_ing for some specific use
>>> case in "generic" function.
>>
>> The vmemmap_remap_range() walks page table range [start, end),
>> if reuse is equal to (start + PAGE_SIZE), the range can adjust to
>> [start - PAGE_SIZE, end). But if not, we need some work to
>> implement the "generic" function.
>>
>>   - adjust range to [min(start, reuse), end) and call
>>     vmemmap_remap_rangeand which skip the hole
>>     which is [reuse + PAGE_SIZE, start) or [end, reuse).
>>   - call vmemmap_remap_range(reuse, reuse + PAGE_SIZE)
>>     to get the reuse page.Then, call vmemmap_remap_range(start, end)
>>     again to remap.
>>
>> Which one do you prefer?
> 
> I would not overcomplicate things at this stage.
> Just follow my sugestion and add a BUG_ON as you said, that might be the
> easier way now.
> We can overthink this in the future when some other usecases come
> around, right?

I too like the suggestion of specifying the reuse address.  It is better
than than relying on 'start + PAGE_SIZE' or even 'start - PAGE_SIZE' as
in the previous version.

However, if we do allow this then we can not allow just any reuse address
without complicating the code.  Why?  Because the code would also need to
do a page table walk to validate reuse addr.  In the current code, that is
handled as long as reuse address is part of the range we are walking.

I see two assumptions in the current code:
1) reuse address is part of the range
2) remap_pte is found 'first' in table walk before we start remapping.

In the current use case, the 'reuse addr' is always going to be the start
of the page table range we walk.  Correct?  If so, perhaps it would just
be simpler for now to have range be
[reuse addr, last page mapped to reuse addr].  IOW, always have the range
start with reuse addr and all subsequent pages in the range are mapped to
reuse addr.  I know it is not very generic or flexible.  But, it might be
easier to understand than the adjustments (+- PAGE_SIZE) currently being
made in the code.

Just a thought.
-- 
Mike Kravetz

Re: [External] Re: [PATCH v10 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Tue, Dec 22, 2020 at 2:00 AM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Mon, Dec 21, 2020 at 11:52:30PM +0800, Muchun Song wrote:
> > On Mon, Dec 21, 2020 at 9:44 PM Oscar Salvador <osalvador@suse.de> wrote:
> > >
> > > On Mon, Dec 21, 2020 at 07:25:15PM +0800, Muchun Song wrote:
> > >
> > > > Should we add a BUG_ON in vmemmap_remap_free() for now?
> > > >
> > > >         BUG_ON(reuse != start + PAGE_SIZE);
> > >
> > > I do not think we have to, plus we would be BUG_ing for some specific use
> > > case in "generic" function.
> >
> > The vmemmap_remap_range() walks page table range [start, end),
> > if reuse is equal to (start + PAGE_SIZE), the range can adjust to
> > [start - PAGE_SIZE, end). But if not, we need some work to
> > implement the "generic" function.
> >
> >   - adjust range to [min(start, reuse), end) and call
> >     vmemmap_remap_rangeand which skip the hole
> >     which is [reuse + PAGE_SIZE, start) or [end, reuse).
> >   - call vmemmap_remap_range(reuse, reuse + PAGE_SIZE)
> >     to get the reuse page.Then, call vmemmap_remap_range(start, end)
> >     again to remap.
> >
> > Which one do you prefer?
>
> I would not overcomplicate things at this stage.
> Just follow my sugestion and add a BUG_ON as you said, that might be the
> easier way now.
> We can overthink this in the future when some other usecases come
> around, right?

You are right. Will do this. Thanks for your suggestions.

>
> Thanks
>
>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun