[PATCH v6 00/16] Free some vmemmap pages of hugetlb page

[PATCH v6 00/16] 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:
  1. 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.
  2. Support for other architectures (e.g. aarch64).
  3. Enable PMD/huge page mapping of vmemmap even if this feature was enabled.

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

  Changelog in v5:
  1. 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 v4:
  1. Move all the vmemmap functions to hugetlb_vmemmap.c.
  2. 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.
  3. Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions.
  4. Initialize page table lock for vmemmap through core_initcall mechanism.

  Thanks for Mike and Oscar's suggestions.

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

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

Muchun Song (16):
  mm/memory_hotplug: Move bootmem info registration API to
    bootmem_info.c
  mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c
  mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP
  mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate
  mm/bootmem_info: Introduce {free,prepare}_vmemmap_page()
  mm/hugetlb: Disable freeing vmemmap if struct page size is not power
    of two
  x86/mm/64: Disable PMD page mapping of 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: Introduce remap_huge_page_pmd_vmemmap helper
  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: Gather discrete indexes of tail page
  mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct
    page

 Documentation/admin-guide/kernel-parameters.txt |   9 +
 Documentation/admin-guide/mm/hugetlbpage.rst    |   3 +
 arch/x86/include/asm/pgtable_64_types.h         |   8 +
 arch/x86/mm/init_64.c                           |  12 +-
 fs/Kconfig                                      |  14 +
 include/linux/bootmem_info.h                    |  64 +++++
 include/linux/hugetlb.h                         |  16 ++
 include/linux/hugetlb_cgroup.h                  |  15 +-
 include/linux/memory_hotplug.h                  |  27 --
 mm/Makefile                                     |   2 +
 mm/bootmem_info.c                               | 124 ++++++++
 mm/hugetlb.c                                    | 144 ++++++++--
 mm/hugetlb_vmemmap.c                            | 365 ++++++++++++++++++++++++
 mm/hugetlb_vmemmap.h                            |  79 +++++
 mm/memory_hotplug.c                             | 116 --------
 mm/sparse.c                                     |   1 +
 16 files changed, 820 insertions(+), 179 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 v6 01/16] mm/memory_hotplug: Move bootmem info registration API to bootmem_info.c

[Muchun Song]

Move bootmem info registration common API to individual bootmem_info.c
for later patch use. 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>
---
 arch/x86/mm/init_64.c          |  1 +
 include/linux/bootmem_info.h   | 27 ++++++++++++
 include/linux/memory_hotplug.h | 23 ----------
 mm/Makefile                    |  1 +
 mm/bootmem_info.c              | 99 ++++++++++++++++++++++++++++++++++++++++++
 mm/memory_hotplug.c            | 91 +-------------------------------------
 6 files changed, 129 insertions(+), 113 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..c7f7ad55b625 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>
diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
new file mode 100644
index 000000000000..65bb9b23140f
--- /dev/null
+++ b/include/linux/bootmem_info.h
@@ -0,0 +1,27 @@
+/* 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);
+#else
+static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
+{
+}
+#endif
+
+#endif /* __LINUX_BOOTMEM_INFO_H */
diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 51a877fec8da..19e5d067294c 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. */
@@ -209,13 +197,6 @@ 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);
@@ -254,10 +235,6 @@ static inline int mhp_notimplemented(const char *func)
 	return -ENOSYS;
 }
 
-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 d5649f1c12c0..752111587c99 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -82,6 +82,7 @@ obj-$(CONFIG_SLAB) += slab.o
 obj-$(CONFIG_SLUB) += slub.o
 obj-$(CONFIG_KASAN)	+= kasan/
 obj-$(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..39fa8fc120bc
--- /dev/null
+++ b/mm/bootmem_info.c
@@ -0,0 +1,99 @@
+// 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>
+
+#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 baded53b9ff9..2da4ad071456 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -21,6 +21,7 @@
 #include <linux/memory.h>
 #include <linux/memremap.h>
 #include <linux/memory_hotplug.h>
+#include <linux/bootmem_info.h>
 #include <linux/highmem.h>
 #include <linux/vmalloc.h>
 #include <linux/ioport.h>
@@ -167,96 +168,6 @@ void put_page_bootmem(struct 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)
 {
-- 
2.11.0

[PATCH v6 02/16] mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c

[Muchun Song]

In the later patch, we will use {get,put}_page_bootmem() to initialize
the page for vmemmap or free vmemmap page to buddy. 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>
---
 arch/x86/mm/init_64.c          |  2 +-
 include/linux/bootmem_info.h   | 13 +++++++++++++
 include/linux/memory_hotplug.h |  4 ----
 mm/bootmem_info.c              | 25 +++++++++++++++++++++++++
 mm/memory_hotplug.c            | 27 ---------------------------
 mm/sparse.c                    |  1 +
 6 files changed, 40 insertions(+), 32 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index c7f7ad55b625..0a45f062826e 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1572,7 +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
index 65bb9b23140f..4ed6dee1adc9 100644
--- a/include/linux/bootmem_info.h
+++ b/include/linux/bootmem_info.h
@@ -18,10 +18,23 @@ enum {
 
 #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 19e5d067294c..c9f3361fe84b 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -197,10 +197,6 @@ static inline void arch_refresh_nodedata(int nid, pg_data_t *pgdat)
 #endif /* CONFIG_NUMA */
 #endif /* CONFIG_HAVE_ARCH_NODEDATA_EXTENSION */
 
-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);
 
diff --git a/mm/bootmem_info.c b/mm/bootmem_info.c
index 39fa8fc120bc..fcab5a3f8cc0 100644
--- a/mm/bootmem_info.c
+++ b/mm/bootmem_info.c
@@ -10,6 +10,31 @@
 #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)
 {
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 2da4ad071456..ae57eedc341f 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -21,7 +21,6 @@
 #include <linux/memory.h>
 #include <linux/memremap.h>
 #include <linux/memory_hotplug.h>
-#include <linux/bootmem_info.h>
 #include <linux/highmem.h>
 #include <linux/vmalloc.h>
 #include <linux/ioport.h>
@@ -142,32 +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);
-	}
-}
-
 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 b25ad8e64839..a4138410d890 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 v6 03/16] mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP

[Muchun Song]

The purpose of introducing HUGETLB_PAGE_FREE_VMEMMAP is to configure
whether to enable the feature of freeing unused vmemmap associated
with HugeTLB pages. And this is just for dependency check. Now only
support x86.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 arch/x86/mm/init_64.c |  2 +-
 fs/Kconfig            | 14 ++++++++++++++
 2 files changed, 15 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..4961dd488444 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -245,6 +245,20 @@ config HUGETLBFS
 config HUGETLB_PAGE
 	def_bool HUGETLBFS
 
+config HUGETLB_PAGE_FREE_VMEMMAP
+	def_bool HUGETLB_PAGE
+	depends on X86
+	depends on SPARSEMEM_VMEMMAP
+	depends on HAVE_BOOTMEM_INFO_NODE
+	help
+	  When using HUGETLB_PAGE_FREE_VMEMMAP, the system can save up some
+	  memory from pre-allocated HugeTLB pages when they are not used.
+	  6 pages per 2MB HugeTLB page and 4094 per 1GB HugeTLB page.
+
+	  When the pages are going to be used or freed up, the vmemmap array
+	  representing that range needs to be remapped again and the pages
+	  we discarded earlier need to be rellocated again.
+
 config MEMFD_CREATE
 	def_bool TMPFS || HUGETLBFS
 
-- 
2.11.0

[PATCH v6 04/16] mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate

[Muchun Song]

Every HugeTLB has more than one struct page structure. The 2M HugeTLB
has 512 struct page structure and 1G HugeTLB has 4096 struct page
structures. 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(8 page frames for 2MB HugeTLB
page and 4096 page frames for 1GB HugeTLB page) 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.

So we introduce a new nr_free_vmemmap_pages field in the hstate to
indicate how many vmemmap pages associated with a HugeTLB page that we
can free to buddy system.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 include/linux/hugetlb.h |   3 ++
 mm/Makefile             |   1 +
 mm/hugetlb.c            |   3 ++
 mm/hugetlb_vmemmap.c    | 129 ++++++++++++++++++++++++++++++++++++++++++++++++
 mm/hugetlb_vmemmap.h    |  20 ++++++++
 5 files changed, 156 insertions(+)
 create mode 100644 mm/hugetlb_vmemmap.c
 create mode 100644 mm/hugetlb_vmemmap.h

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index d5cc5f802dd4..eed3dd3bd626 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/Makefile b/mm/Makefile
index 752111587c99..2a734576bbc0 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 81a41aa080a5..f88032c24667 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;
@@ -3285,6 +3286,8 @@ void __init hugetlb_add_hstate(unsigned int order)
 	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
new file mode 100644
index 000000000000..fad760483e01
--- /dev/null
+++ b/mm/hugetlb_vmemmap.c
@@ -0,0 +1,129 @@
+// 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 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.
+ */
+#define pr_fmt(fmt)	"HugeTLB Vmemmap: " fmt
+
+#include "hugetlb_vmemmap.h"
+
+/*
+ * There are a lot of struct page structures(8 page frames for 2MB HugeTLB page
+ * and 4096 page frames for 1GB HugeTLB page) 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
+
+void __init hugetlb_vmemmap_init(struct hstate *h)
+{
+	unsigned int order = huge_page_order(h);
+	unsigned int vmemmap_pages;
+
+	vmemmap_pages = ((1 << order) * sizeof(struct page)) >> PAGE_SHIFT;
+	/*
+	 * The head page and the first tail page are not to be freed to buddy
+	 * system, the others page will map to the first tail page. So there
+	 * are the remaining pages that can be freed.
+	 *
+	 * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? This is
+	 * not expected to happen unless the system is corrupted. So on the
+	 * safe side, it is only a safety net.
+	 */
+	if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
+		h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
+
+	pr_debug("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
new file mode 100644
index 000000000000..40c0c7dfb60d
--- /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 __init hugetlb_vmemmap_init(struct hstate *h);
+#else
+static inline void hugetlb_vmemmap_init(struct hstate *h)
+{
+}
+#endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
+#endif /* _LINUX_HUGETLB_VMEMMAP_H */
-- 
2.11.0

[PATCH v6 05/16] mm/bootmem_info: Introduce {free,prepare}_vmemmap_page()

[Muchun Song]

In the later patch, we can use the free_vmemmap_page() to free the
unused vmemmap pages and initialize a page for vmemmap page using
via prepare_vmemmap_page().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/bootmem_info.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/bootmem_info.h b/include/linux/bootmem_info.h
index 4ed6dee1adc9..239e3cc8f86c 100644
--- a/include/linux/bootmem_info.h
+++ b/include/linux/bootmem_info.h
@@ -3,6 +3,7 @@
 #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 +23,29 @@ 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);
+
+static inline void free_vmemmap_page(struct page *page)
+{
+	VM_WARN_ON(!PageReserved(page) || page_ref_count(page) != 2);
+
+	/* bootmem page has reserved flag in the reserve_bootmem_region */
+	if (PageReserved(page)) {
+		unsigned long magic = (unsigned long)page->freelist;
+
+		if (magic == SECTION_INFO || magic == MIX_SECTION_INFO)
+			put_page_bootmem(page);
+		else
+			WARN_ON(1);
+	}
+}
+
+static inline void prepare_vmemmap_page(struct page *page)
+{
+	unsigned long section_nr = pfn_to_section_nr(page_to_pfn(page));
+
+	get_page_bootmem(section_nr, page, SECTION_INFO);
+	mark_page_reserved(page);
+}
 #else
 static inline void register_page_bootmem_info_node(struct pglist_data *pgdat)
 {
-- 
2.11.0

[PATCH v6 06/16] mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two

[Muchun Song]

We only can free the tail vmemmap pages of HugeTLB to the buddy allocator
when the size of struct page is a power of two.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb_vmemmap.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index fad760483e01..fd60cfdf3d40 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -111,6 +111,11 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	unsigned int order = huge_page_order(h);
 	unsigned int vmemmap_pages;
 
+	if (!is_power_of_2(sizeof(struct page))) {
+		pr_info("disable freeing vmemmap pages for %s\n", h->name);
+		return;
+	}
+
 	vmemmap_pages = ((1 << order) * sizeof(struct page)) >> PAGE_SHIFT;
 	/*
 	 * The head page and the first tail page are not to be freed to buddy
-- 
2.11.0

[PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of vmemmap

[Muchun Song]

If we enable the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, we can just
disbale PMD page mapping of vmemmap to simplify the code. In this
case, we do not need complex code doing vmemmap page table
manipulation. This is a way to simply the first version of this
patch series. In the future, we can add some code doing page table
manipulation.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 arch/x86/mm/init_64.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 0435bee2e172..155cb06a6961 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1557,7 +1557,9 @@ 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_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP))
+		err = vmemmap_populate_basepages(start, end, node, NULL);
+	else if (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);
@@ -1610,7 +1612,8 @@ 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 (!boot_cpu_has(X86_FEATURE_PSE) ||
+		    IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP)) {
 			next = (addr + PAGE_SIZE) & PAGE_MASK;
 			pmd = pmd_offset(pud, addr);
 			if (pmd_none(*pmd))
-- 
2.11.0

[PATCH v6 08/16] mm/hugetlb: Free the vmemmap pages associated with each hugetlb page

[Muchun Song]

When we allocate a hugetlb page from the buddy, we should free the
unused vmemmap pages associated with it. We can do that in the
prep_new_huge_page().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 arch/x86/include/asm/pgtable_64_types.h |   8 ++
 mm/hugetlb.c                            |   2 +
 mm/hugetlb_vmemmap.c                    | 133 +++++++++++++++++++++++++++++++-
 mm/hugetlb_vmemmap.h                    |   5 ++
 4 files changed, 147 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable_64_types.h b/arch/x86/include/asm/pgtable_64_types.h
index 52e5f5f2240d..bedbd2e7d06c 100644
--- a/arch/x86/include/asm/pgtable_64_types.h
+++ b/arch/x86/include/asm/pgtable_64_types.h
@@ -139,6 +139,14 @@ extern unsigned int ptrs_per_p4d;
 # define VMEMMAP_START		__VMEMMAP_BASE_L4
 #endif /* CONFIG_DYNAMIC_MEMORY_LAYOUT */
 
+/*
+ * VMEMMAP_SIZE - allows the whole linear region to be covered by
+ *                a struct page array.
+ */
+#define VMEMMAP_SIZE		(1UL << (__VIRTUAL_MASK_SHIFT - PAGE_SHIFT - \
+					 1 + ilog2(sizeof(struct page))))
+#define VMEMMAP_END		(VMEMMAP_START + VMEMMAP_SIZE)
+
 #define VMALLOC_END		(VMALLOC_START + (VMALLOC_SIZE_TB << 40) - 1)
 
 #define MODULES_VADDR		(__START_KERNEL_map + KERNEL_IMAGE_SIZE)
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f88032c24667..9662b5535f3a 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1499,6 +1499,8 @@ 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
index fd60cfdf3d40..1576f69bd1d3 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -92,8 +92,9 @@
  * to the 2MB HugeTLB page. We also can use this approach to free the vmemmap
  * pages.
  */
-#define pr_fmt(fmt)	"HugeTLB Vmemmap: " fmt
+#define pr_fmt(fmt)	"HugeTLB vmemmap: " fmt
 
+#include <linux/bootmem_info.h>
 #include "hugetlb_vmemmap.h"
 
 /*
@@ -105,6 +106,136 @@
  * 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)
+#define TAIL_PAGE_REUSE			-1
+
+#ifndef VMEMMAP_HPAGE_SHIFT
+#define VMEMMAP_HPAGE_SHIFT		HPAGE_SHIFT
+#endif
+#define VMEMMAP_HPAGE_ORDER		(VMEMMAP_HPAGE_SHIFT - PAGE_SHIFT)
+#define VMEMMAP_HPAGE_NR		(1 << VMEMMAP_HPAGE_ORDER)
+#define VMEMMAP_HPAGE_SIZE		((1UL) << VMEMMAP_HPAGE_SHIFT)
+#define VMEMMAP_HPAGE_MASK		(~(VMEMMAP_HPAGE_SIZE - 1))
+
+#define vmemmap_hpage_addr_end(addr, end)				 \
+({									 \
+	unsigned long __boundary;					 \
+	__boundary = ((addr) + VMEMMAP_HPAGE_SIZE) & VMEMMAP_HPAGE_MASK; \
+	(__boundary - 1 < (end) - 1) ? __boundary : (end);		 \
+})
+
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+	return h->nr_free_vmemmap_pages;
+}
+
+static inline unsigned int vmemmap_pages_per_hpage(struct hstate *h)
+{
+	return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
+}
+
+static inline unsigned long vmemmap_pages_size_per_hpage(struct hstate *h)
+{
+	return (unsigned long)vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
+}
+
+/*
+ * Walk a vmemmap address to the pmd it maps.
+ */
+static pmd_t *vmemmap_to_pmd(unsigned long page)
+{
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+
+	if (page < VMEMMAP_START || page >= VMEMMAP_END)
+		return NULL;
+
+	pgd = pgd_offset_k(page);
+	if (pgd_none(*pgd))
+		return NULL;
+	p4d = p4d_offset(pgd, page);
+	if (p4d_none(*p4d))
+		return NULL;
+	pud = pud_offset(p4d, page);
+	if (pud_none(*pud) || pud_bad(*pud))
+		return NULL;
+
+	return pmd_offset(pud, page);
+}
+
+static inline 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 __free_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
+					 unsigned long start,
+					 unsigned long end,
+					 struct list_head *free_pages)
+{
+	/* Make the tail pages are mapped read-only. */
+	pgprot_t pgprot = PAGE_KERNEL_RO;
+	pte_t entry = mk_pte(reuse, pgprot);
+	unsigned long addr;
+
+	for (addr = start; addr < end; addr += PAGE_SIZE, ptep++) {
+		struct page *page;
+		pte_t old = *ptep;
+
+		VM_WARN_ON(!pte_present(old));
+		page = pte_page(old);
+		list_add(&page->lru, free_pages);
+
+		set_pte_at(&init_mm, addr, ptep, entry);
+	}
+}
+
+static void __free_huge_page_pmd_vmemmap(pmd_t *pmd, unsigned long start,
+					 unsigned long end,
+					 struct list_head *vmemmap_pages)
+{
+	unsigned long next, addr = start;
+	struct page *reuse = NULL;
+
+	do {
+		pte_t *ptep;
+
+		ptep = pte_offset_kernel(pmd, addr);
+		if (!reuse)
+			reuse = pte_page(ptep[TAIL_PAGE_REUSE]);
+
+		next = vmemmap_hpage_addr_end(addr, end);
+		__free_huge_page_pte_vmemmap(reuse, ptep, addr, next,
+					     vmemmap_pages);
+	} while (pmd++, addr = next, addr != end);
+
+	flush_tlb_kernel_range(start, end);
+}
+
+void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+	pmd_t *pmd;
+	unsigned long start, end;
+	unsigned long vmemmap_addr = (unsigned long)head;
+	LIST_HEAD(free_pages);
+
+	if (!free_vmemmap_pages_per_hpage(h))
+		return;
+
+	pmd = vmemmap_to_pmd(vmemmap_addr);
+	BUG_ON(!pmd);
+
+	start = vmemmap_addr + RESERVE_VMEMMAP_SIZE;
+	end = vmemmap_addr + vmemmap_pages_size_per_hpage(h);
+	__free_huge_page_pmd_vmemmap(pmd, start, end, &free_pages);
+	free_vmemmap_page_list(&free_pages);
+}
 
 void __init hugetlb_vmemmap_init(struct hstate *h)
 {
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 40c0c7dfb60d..67113b67495f 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -12,9 +12,14 @@
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
 void __init hugetlb_vmemmap_init(struct hstate *h);
+void free_huge_page_vmemmap(struct hstate *h, struct page *head);
 #else
 static inline void hugetlb_vmemmap_init(struct hstate *h)
 {
 }
+
+static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+}
 #endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
 #endif /* _LINUX_HUGETLB_VMEMMAP_H */
-- 
2.11.0

[PATCH v6 09/16] 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 use GFP_ATOMIC to allocate the vmemmap pages.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb.c         | 96 ++++++++++++++++++++++++++++++++++++++++++++++------
 mm/hugetlb_vmemmap.c |  5 ---
 mm/hugetlb_vmemmap.h | 10 ++++++
 3 files changed, 95 insertions(+), 16 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9662b5535f3a..41056b4230f1 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1221,7 +1221,7 @@ static void destroy_compound_gigantic_page(struct page *page,
 	__ClearPageHead(page);
 }
 
-static void free_gigantic_page(struct page *page, unsigned int order)
+static void __free_gigantic_page(struct page *page, unsigned int order)
 {
 	/*
 	 * If the page isn't allocated using the cma allocator,
@@ -1288,20 +1288,100 @@ static struct page *alloc_gigantic_page(struct hstate *h, gfp_t gfp_mask,
 {
 	return NULL;
 }
-static inline void free_gigantic_page(struct page *page, unsigned int order) { }
+static inline void __free_gigantic_page(struct page *page,
+					unsigned int order) { }
 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);
+}
+
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+static inline void free_gigantic_page(struct hstate *h, struct page *page)
+{
+	__free_gigantic_page(page, huge_page_order(h));
+}
+#else
+static inline void free_gigantic_page(struct hstate *h, struct page *page)
+{
+	/*
+	 * Temporarily drop the hugetlb_lock, because
+	 * we might block in __free_gigantic_page().
+	 */
+	spin_unlock(&hugetlb_lock);
+	__free_gigantic_page(page, huge_page_order(h));
+	spin_lock(&hugetlb_lock);
+}
+#endif
+
+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);
+}
+
+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 |
@@ -1313,14 +1393,8 @@ static void update_and_free_page(struct hstate *h, struct page *page)
 	set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
 	set_page_refcounted(page);
 	if (hstate_is_gigantic(h)) {
-		/*
-		 * Temporarily drop the hugetlb_lock, because
-		 * we might block in free_gigantic_page().
-		 */
-		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);
+		free_gigantic_page(h, page);
 	} else {
 		__free_pages(page, huge_page_order(h));
 	}
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 1576f69bd1d3..f6ba288966d4 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -124,11 +124,6 @@
 	(__boundary - 1 < (end) - 1) ? __boundary : (end);		 \
 })
 
-static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
-{
-	return h->nr_free_vmemmap_pages;
-}
-
 static inline unsigned int vmemmap_pages_per_hpage(struct hstate *h)
 {
 	return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 67113b67495f..293897b9f1d8 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -13,6 +13,11 @@
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
 void __init hugetlb_vmemmap_init(struct hstate *h);
 void free_huge_page_vmemmap(struct hstate *h, struct page *head);
+
+static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
+{
+	return h->nr_free_vmemmap_pages;
+}
 #else
 static inline void hugetlb_vmemmap_init(struct hstate *h)
 {
@@ -21,5 +26,10 @@ static inline void hugetlb_vmemmap_init(struct hstate *h)
 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 v6 10/16] mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page

[Muchun Song]

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

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb.c         |   2 +
 mm/hugetlb_vmemmap.c | 102 +++++++++++++++++++++++++++++++++++++++++++++++++++
 mm/hugetlb_vmemmap.h |   5 +++
 3 files changed, 109 insertions(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 41056b4230f1..3fafa39fcac6 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1382,6 +1382,8 @@ 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 f6ba288966d4..d6a1b06c1322 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -95,6 +95,7 @@
 #define pr_fmt(fmt)	"HugeTLB vmemmap: " fmt
 
 #include <linux/bootmem_info.h>
+#include <linux/delay.h>
 #include "hugetlb_vmemmap.h"
 
 /*
@@ -108,6 +109,8 @@
 #define RESERVE_VMEMMAP_NR		2U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 #define TAIL_PAGE_REUSE			-1
+#define GFP_VMEMMAP_PAGE		\
+	(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_HIGH)
 
 #ifndef VMEMMAP_HPAGE_SHIFT
 #define VMEMMAP_HPAGE_SHIFT		HPAGE_SHIFT
@@ -159,6 +162,105 @@ static pmd_t *vmemmap_to_pmd(unsigned long page)
 	return pmd_offset(pud, page);
 }
 
+static void __remap_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
+					  unsigned long start,
+					  unsigned long end,
+					  struct list_head *remap_pages)
+{
+	pgprot_t pgprot = PAGE_KERNEL;
+	void *from = page_to_virt(reuse);
+	unsigned long addr;
+
+	for (addr = start; addr < end; addr += PAGE_SIZE) {
+		void *to;
+		struct page *page;
+		pte_t entry, old = *ptep;
+
+		page = list_first_entry(remap_pages, struct page, lru);
+		list_del(&page->lru);
+		to = page_to_virt(page);
+		copy_page(to, from);
+
+		/*
+		 * Make sure that any data that writes to the @to is made
+		 * visible to the physical page.
+		 */
+		flush_kernel_vmap_range(to, PAGE_SIZE);
+
+		prepare_vmemmap_page(page);
+
+		entry = mk_pte(page, pgprot);
+		set_pte_at(&init_mm, addr, ptep++, entry);
+
+		VM_BUG_ON(!pte_present(old) || pte_page(old) != reuse);
+	}
+}
+
+static void __remap_huge_page_pmd_vmemmap(pmd_t *pmd, unsigned long start,
+					  unsigned long end,
+					  struct list_head *vmemmap_pages)
+{
+	unsigned long next, addr = start;
+	struct page *reuse = NULL;
+
+	do {
+		pte_t *ptep;
+
+		ptep = pte_offset_kernel(pmd, addr);
+		if (!reuse)
+			reuse = pte_page(ptep[TAIL_PAGE_REUSE]);
+
+		next = vmemmap_hpage_addr_end(addr, end);
+		__remap_huge_page_pte_vmemmap(reuse, ptep, addr, next,
+					      vmemmap_pages);
+	} while (pmd++, addr = next, addr != end);
+
+	flush_tlb_kernel_range(start, end);
+}
+
+static inline void alloc_vmemmap_pages(struct hstate *h, struct list_head *list)
+{
+	unsigned int nr = free_vmemmap_pages_per_hpage(h);
+
+	while (nr--) {
+		struct page *page;
+
+retry:
+		page = alloc_page(GFP_VMEMMAP_PAGE);
+		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);
+	}
+}
+
+void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
+{
+	pmd_t *pmd;
+	unsigned long start, end;
+	unsigned long vmemmap_addr = (unsigned long)head;
+	LIST_HEAD(map_pages);
+
+	if (!free_vmemmap_pages_per_hpage(h))
+		return;
+
+	alloc_vmemmap_pages(h, &map_pages);
+
+	pmd = vmemmap_to_pmd(vmemmap_addr);
+	BUG_ON(!pmd);
+
+	start = vmemmap_addr + RESERVE_VMEMMAP_SIZE;
+	end = vmemmap_addr + vmemmap_pages_size_per_hpage(h);
+	__remap_huge_page_pmd_vmemmap(pmd, start, end, &map_pages);
+}
+
 static inline void free_vmemmap_page_list(struct list_head *list)
 {
 	struct page *page, *next;
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 293897b9f1d8..7887095488f4 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -12,6 +12,7 @@
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
 void __init hugetlb_vmemmap_init(struct hstate *h);
+void alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
 void free_huge_page_vmemmap(struct hstate *h, struct page *head);
 
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
@@ -23,6 +24,10 @@ static inline void hugetlb_vmemmap_init(struct hstate *h)
 {
 }
 
+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)
 {
 }
-- 
2.11.0

[PATCH v6 11/16] mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper

[Muchun Song]

The __free_huge_page_pmd_vmemmap and __remap_huge_page_pmd_vmemmap are
almost the same code. So introduce remap_free_huge_page_pmd_vmemmap
helper to simplify the code.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb_vmemmap.c | 87 +++++++++++++++++++++-------------------------------
 1 file changed, 35 insertions(+), 52 deletions(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index d6a1b06c1322..509ca451e232 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -127,6 +127,10 @@
 	(__boundary - 1 < (end) - 1) ? __boundary : (end);		 \
 })
 
+typedef void (*vmemmap_pte_remap_func_t)(struct page *reuse, pte_t *ptep,
+					 unsigned long start, unsigned long end,
+					 void *priv);
+
 static inline unsigned int vmemmap_pages_per_hpage(struct hstate *h)
 {
 	return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
@@ -162,21 +166,42 @@ static pmd_t *vmemmap_to_pmd(unsigned long page)
 	return pmd_offset(pud, page);
 }
 
+static void remap_huge_page_pmd_vmemmap(pmd_t *pmd, unsigned long start,
+					unsigned long end,
+					vmemmap_pte_remap_func_t fn, void *priv)
+{
+	unsigned long next, addr = start;
+	struct page *reuse = NULL;
+
+	do {
+		pte_t *ptep;
+
+		ptep = pte_offset_kernel(pmd, addr);
+		if (!reuse)
+			reuse = pte_page(ptep[TAIL_PAGE_REUSE]);
+
+		next = vmemmap_hpage_addr_end(addr, end);
+		fn(reuse, ptep, addr, next, priv);
+	} while (pmd++, addr = next, addr != end);
+
+	flush_tlb_kernel_range(start, end);
+}
+
 static void __remap_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
 					  unsigned long start,
-					  unsigned long end,
-					  struct list_head *remap_pages)
+					  unsigned long end, void *priv)
 {
 	pgprot_t pgprot = PAGE_KERNEL;
 	void *from = page_to_virt(reuse);
 	unsigned long addr;
+	struct list_head *pages = priv;
 
 	for (addr = start; addr < end; addr += PAGE_SIZE) {
 		void *to;
 		struct page *page;
 		pte_t entry, old = *ptep;
 
-		page = list_first_entry(remap_pages, struct page, lru);
+		page = list_first_entry(pages, struct page, lru);
 		list_del(&page->lru);
 		to = page_to_virt(page);
 		copy_page(to, from);
@@ -196,28 +221,6 @@ static void __remap_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
 	}
 }
 
-static void __remap_huge_page_pmd_vmemmap(pmd_t *pmd, unsigned long start,
-					  unsigned long end,
-					  struct list_head *vmemmap_pages)
-{
-	unsigned long next, addr = start;
-	struct page *reuse = NULL;
-
-	do {
-		pte_t *ptep;
-
-		ptep = pte_offset_kernel(pmd, addr);
-		if (!reuse)
-			reuse = pte_page(ptep[TAIL_PAGE_REUSE]);
-
-		next = vmemmap_hpage_addr_end(addr, end);
-		__remap_huge_page_pte_vmemmap(reuse, ptep, addr, next,
-					      vmemmap_pages);
-	} while (pmd++, addr = next, addr != end);
-
-	flush_tlb_kernel_range(start, end);
-}
-
 static inline void alloc_vmemmap_pages(struct hstate *h, struct list_head *list)
 {
 	unsigned int nr = free_vmemmap_pages_per_hpage(h);
@@ -258,7 +261,8 @@ void alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
 
 	start = vmemmap_addr + RESERVE_VMEMMAP_SIZE;
 	end = vmemmap_addr + vmemmap_pages_size_per_hpage(h);
-	__remap_huge_page_pmd_vmemmap(pmd, start, end, &map_pages);
+	remap_huge_page_pmd_vmemmap(pmd, start, end,
+				    __remap_huge_page_pte_vmemmap, &map_pages);
 }
 
 static inline void free_vmemmap_page_list(struct list_head *list)
@@ -273,13 +277,13 @@ static inline void free_vmemmap_page_list(struct list_head *list)
 
 static void __free_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
 					 unsigned long start,
-					 unsigned long end,
-					 struct list_head *free_pages)
+					 unsigned long end, void *priv)
 {
 	/* Make the tail pages are mapped read-only. */
 	pgprot_t pgprot = PAGE_KERNEL_RO;
 	pte_t entry = mk_pte(reuse, pgprot);
 	unsigned long addr;
+	struct list_head *pages = priv;
 
 	for (addr = start; addr < end; addr += PAGE_SIZE, ptep++) {
 		struct page *page;
@@ -287,34 +291,12 @@ static void __free_huge_page_pte_vmemmap(struct page *reuse, pte_t *ptep,
 
 		VM_WARN_ON(!pte_present(old));
 		page = pte_page(old);
-		list_add(&page->lru, free_pages);
+		list_add(&page->lru, pages);
 
 		set_pte_at(&init_mm, addr, ptep, entry);
 	}
 }
 
-static void __free_huge_page_pmd_vmemmap(pmd_t *pmd, unsigned long start,
-					 unsigned long end,
-					 struct list_head *vmemmap_pages)
-{
-	unsigned long next, addr = start;
-	struct page *reuse = NULL;
-
-	do {
-		pte_t *ptep;
-
-		ptep = pte_offset_kernel(pmd, addr);
-		if (!reuse)
-			reuse = pte_page(ptep[TAIL_PAGE_REUSE]);
-
-		next = vmemmap_hpage_addr_end(addr, end);
-		__free_huge_page_pte_vmemmap(reuse, ptep, addr, next,
-					     vmemmap_pages);
-	} while (pmd++, addr = next, addr != end);
-
-	flush_tlb_kernel_range(start, end);
-}
-
 void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 {
 	pmd_t *pmd;
@@ -330,7 +312,8 @@ void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 
 	start = vmemmap_addr + RESERVE_VMEMMAP_SIZE;
 	end = vmemmap_addr + vmemmap_pages_size_per_hpage(h);
-	__free_huge_page_pmd_vmemmap(pmd, start, end, &free_pages);
+	remap_huge_page_pmd_vmemmap(pmd, start, end,
+				    __free_huge_page_pte_vmemmap, &free_pages);
 	free_vmemmap_page_list(&free_pages);
 }
 
-- 
2.11.0

[PATCH v6 12/16] 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].mapping to record the real error page
index and set the raw error page PageHWPoison later.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb.c         | 11 +++--------
 mm/hugetlb_vmemmap.h | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3fafa39fcac6..ade20954eb81 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1383,6 +1383,7 @@ static void __free_hugepage(struct hstate *h, struct page *page)
 	int i;
 
 	alloc_huge_page_vmemmap(h, page);
+	subpage_hwpoison_deliver(page);
 
 	for (i = 0; i < pages_per_huge_page(h); i++) {
 		page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
@@ -1930,14 +1931,8 @@ 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);
-		}
+
+		set_subpage_hwpoison(head, page);
 		list_del(&head->lru);
 		h->free_huge_pages--;
 		h->free_huge_pages_node[nid]--;
diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
index 7887095488f4..4bb35d87ae10 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -15,6 +15,29 @@ void __init hugetlb_vmemmap_init(struct hstate *h);
 void alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
 void free_huge_page_vmemmap(struct hstate *h, struct page *head);
 
+static inline void subpage_hwpoison_deliver(struct page *head)
+{
+	struct page *page = head;
+
+	if (PageHWPoison(head))
+		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 set_subpage_hwpoison(struct page *head, struct page *page)
+{
+	if (PageHWPoison(head))
+		set_page_private(head + 4, page - head);
+}
+
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
 	return h->nr_free_vmemmap_pages;
@@ -32,6 +55,22 @@ static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
 {
 }
 
+static inline void subpage_hwpoison_deliver(struct page *head)
+{
+}
+
+static inline void set_subpage_hwpoison(struct page *head, struct page *page)
+{
+	/*
+	 * 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);
+	}
+}
+
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
 {
 	return 0;
-- 
2.11.0

[PATCH v6 13/16] 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 ade20954eb81..15e2c1dd32ea 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1328,6 +1328,12 @@ 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 */
@@ -1914,6 +1920,7 @@ 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))
@@ -1927,8 +1934,9 @@ 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;
 
@@ -1942,6 +1950,14 @@ 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 v6 14/16] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap

[Muchun Song]

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

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
 Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
 mm/hugetlb_vmemmap.c                            | 19 ++++++++++++++++++-
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 5debfe238027..d28c3acde965 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1551,6 +1551,15 @@
 			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.
+			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..6a8b57f6d3b7 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 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/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 509ca451e232..b2222f8d1245 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -131,6 +131,22 @@ typedef void (*vmemmap_pte_remap_func_t)(struct page *reuse, pte_t *ptep,
 					 unsigned long start, unsigned long end,
 					 void *priv);
 
+static bool hugetlb_free_vmemmap_enabled __initdata;
+
+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 int vmemmap_pages_per_hpage(struct hstate *h)
 {
 	return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
@@ -322,7 +338,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	unsigned int order = huge_page_order(h);
 	unsigned int vmemmap_pages;
 
-	if (!is_power_of_2(sizeof(struct page))) {
+	if (!is_power_of_2(sizeof(struct page)) ||
+	    !hugetlb_free_vmemmap_enabled) {
 		pr_info("disable freeing vmemmap pages for %s\n", h->name);
 		return;
 	}
-- 
2.11.0

[PATCH v6 15/16] 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.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/hugetlb.h        | 13 +++++++++++++
 include/linux/hugetlb_cgroup.h | 15 +++++++++------
 mm/hugetlb.c                   | 12 ++++++------
 mm/hugetlb_vmemmap.h           |  4 ++--
 4 files changed, 30 insertions(+), 14 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index eed3dd3bd626..8a615ae2d233 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 15e2c1dd32ea..7700da372716 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1429,20 +1429,20 @@ 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]);
 }
 
 /*
@@ -1454,17 +1454,17 @@ 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.h b/mm/hugetlb_vmemmap.h
index 4bb35d87ae10..54c2ca0e0dbe 100644
--- a/mm/hugetlb_vmemmap.h
+++ b/mm/hugetlb_vmemmap.h
@@ -20,7 +20,7 @@ static inline void subpage_hwpoison_deliver(struct page *head)
 	struct page *page = head;
 
 	if (PageHWPoison(head))
-		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,
@@ -35,7 +35,7 @@ static inline void subpage_hwpoison_deliver(struct page *head)
 static inline void set_subpage_hwpoison(struct page *head, struct page *page)
 {
 	if (PageHWPoison(head))
-		set_page_private(head + 4, page - head);
+		set_page_private(head + SUBPAGE_INDEX_HWPOISON, page - head);
 }
 
 static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
-- 
2.11.0

[PATCH v6 16/16] mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct page

[Muchun Song]

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

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb_vmemmap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index b2222f8d1245..d2c013582110 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -338,6 +338,9 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	unsigned int order = huge_page_order(h);
 	unsigned int vmemmap_pages;
 
+	BUILD_BUG_ON(NR_USED_SUBPAGE >=
+		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
+
 	if (!is_power_of_2(sizeof(struct page)) ||
 	    !hugetlb_free_vmemmap_enabled) {
 		pr_info("disable freeing vmemmap pages for %s\n", h->name);
-- 
2.11.0

Re: [PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of vmemmap

[Oscar Salvador]

On Tue, Nov 24, 2020 at 05:52:50PM +0800, Muchun Song wrote:
> If we enable the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, we can just
> disbale PMD page mapping of vmemmap to simplify the code. In this
> case, we do not need complex code doing vmemmap page table
> manipulation. This is a way to simply the first version of this
> patch series. In the future, we can add some code doing page table
> manipulation.

IIRC, CONFIG_HUGETLB_PAGE_FREE_VMEMMAP was supposed to be enabled by default,
right?
And we would control whether we __really__ want to this by a boot option,
which was disabled by default?

If you go for populating the memmap with basepages by checking
CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, would not everybody, even the ones that
did not enable this by the boot option be affected?

-- 
Oscar Salvador
SUSE L3

RE: [PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of vmemmap

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On
> Behalf Of Oscar Salvador
> Sent: Tuesday, November 24, 2020 11:25 PM
> To: Muchun Song <songmuchun@bytedance.com>
> Cc: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de;
> mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com;
> dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org;
> viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org;
> mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com;
> rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com;
> jroedel@suse.de; almasrymina@google.com; rientjes@google.com;
> willy@infradead.org; mhocko@suse.com; Song Bao Hua (Barry Song)
> <song.bao.hua@hisilicon.com>; duanxiongchun@bytedance.com;
> linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> linux-fsdevel@vger.kernel.org
> Subject: Re: [PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of
> vmemmap
> 
> On Tue, Nov 24, 2020 at 05:52:50PM +0800, Muchun Song wrote:
> > If we enable the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, we can just
> > disbale PMD page mapping of vmemmap to simplify the code. In this
> > case, we do not need complex code doing vmemmap page table
> > manipulation. This is a way to simply the first version of this
> > patch series. In the future, we can add some code doing page table
> > manipulation.
> 
> IIRC, CONFIG_HUGETLB_PAGE_FREE_VMEMMAP was supposed to be enabled
> by default,
> right?
> And we would control whether we __really__ want to this by a boot option,
> which was disabled by default?
> 
> If you go for populating the memmap with basepages by checking
> CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, would not everybody, even the
> ones that
> did not enable this by the boot option be affected?
> 

I would believe we could only bypass the pmd mapping of vmemmap while
free_vmemmap is explicitly enabled.
pmd mapping shouldn't be disabled in default. Would a cmdline of enabling
vmemmap_free be used for the first patchset?

> --
> Oscar Salvador
> SUSE L3

Thanks
Barry

RE: [PATCH v6 14/16] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Muchun Song [mailto:songmuchun@bytedance.com]
> Sent: Tuesday, November 24, 2020 10:53 PM
> To: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de;
> mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com;
> dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org;
> viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org;
> mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com;
> rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com;
> jroedel@suse.de; almasrymina@google.com; rientjes@google.com;
> willy@infradead.org; osalvador@suse.de; mhocko@suse.com; Song Bao Hua
> (Barry Song) <song.bao.hua@hisilicon.com>
> Cc: duanxiongchun@bytedance.com; linux-doc@vger.kernel.org;
> linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> linux-fsdevel@vger.kernel.org; Muchun Song <songmuchun@bytedance.com>
> Subject: [PATCH v6 14/16] mm/hugetlb: Add a kernel parameter
> hugetlb_free_vmemmap
> 
> Add a kernel parameter hugetlb_free_vmemmap to disable the feature of
> freeing unused vmemmap pages associated with each hugetlb page on boot.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
>  Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
>  mm/hugetlb_vmemmap.c                            | 19
> ++++++++++++++++++-
>  3 files changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt
> b/Documentation/admin-guide/kernel-parameters.txt
> index 5debfe238027..d28c3acde965 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1551,6 +1551,15 @@
>  			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.
> +			Format: { on | off (default) }
> +
> +			on:  enable the feature
> +			off: disable the feature
> +

We've a parameter here. but wouldn't it be applied to "x86/mm/64/:disable
Pmd page mapping of vmemmap" as well?
If (hugetlb_free_vmemmap_enabled)
	Do Basepage mapping?

>  	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..6a8b57f6d3b7 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 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/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 509ca451e232..b2222f8d1245 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -131,6 +131,22 @@ typedef void (*vmemmap_pte_remap_func_t)(struct
> page *reuse, pte_t *ptep,
>  					 unsigned long start, unsigned long end,
>  					 void *priv);
> 
> +static bool hugetlb_free_vmemmap_enabled __initdata;
> +
> +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 int vmemmap_pages_per_hpage(struct hstate *h)
>  {
>  	return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
> @@ -322,7 +338,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>  	unsigned int order = huge_page_order(h);
>  	unsigned int vmemmap_pages;
> 
> -	if (!is_power_of_2(sizeof(struct page))) {
> +	if (!is_power_of_2(sizeof(struct page)) ||
> +	    !hugetlb_free_vmemmap_enabled) {
>  		pr_info("disable freeing vmemmap pages for %s\n", h->name);
>  		return;
>  	}
> --
> 2.11.0

Thanks
Barry

Re: [External] RE: [PATCH v6 14/16] mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap

[Muchun Song]

On Tue, Nov 24, 2020 at 6:54 PM Song Bao Hua (Barry Song)
<song.bao.hua@hisilicon.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Muchun Song [mailto:songmuchun@bytedance.com]
> > Sent: Tuesday, November 24, 2020 10:53 PM
> > To: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de;
> > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com;
> > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org;
> > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org;
> > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com;
> > rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com;
> > jroedel@suse.de; almasrymina@google.com; rientjes@google.com;
> > willy@infradead.org; osalvador@suse.de; mhocko@suse.com; Song Bao Hua
> > (Barry Song) <song.bao.hua@hisilicon.com>
> > Cc: duanxiongchun@bytedance.com; linux-doc@vger.kernel.org;
> > linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > linux-fsdevel@vger.kernel.org; Muchun Song <songmuchun@bytedance.com>
> > Subject: [PATCH v6 14/16] mm/hugetlb: Add a kernel parameter
> > hugetlb_free_vmemmap
> >
> > Add a kernel parameter hugetlb_free_vmemmap to disable the feature of
> > freeing unused vmemmap pages associated with each hugetlb page on boot.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
> >  Documentation/admin-guide/mm/hugetlbpage.rst    |  3 +++
> >  mm/hugetlb_vmemmap.c                            | 19
> > ++++++++++++++++++-
> >  3 files changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt
> > b/Documentation/admin-guide/kernel-parameters.txt
> > index 5debfe238027..d28c3acde965 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1551,6 +1551,15 @@
> >                       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.
> > +                     Format: { on | off (default) }
> > +
> > +                     on:  enable the feature
> > +                     off: disable the feature
> > +
>
> We've a parameter here. but wouldn't it be applied to "x86/mm/64/:disable
> Pmd page mapping of vmemmap" as well?
> If (hugetlb_free_vmemmap_enabled)
>         Do Basepage mapping?

Oh, yeah, we can. Thanks.

>
> >       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..6a8b57f6d3b7 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 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/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 509ca451e232..b2222f8d1245 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -131,6 +131,22 @@ typedef void (*vmemmap_pte_remap_func_t)(struct
> > page *reuse, pte_t *ptep,
> >                                        unsigned long start, unsigned long end,
> >                                        void *priv);
> >
> > +static bool hugetlb_free_vmemmap_enabled __initdata;
> > +
> > +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 int vmemmap_pages_per_hpage(struct hstate *h)
> >  {
> >       return free_vmemmap_pages_per_hpage(h) + RESERVE_VMEMMAP_NR;
> > @@ -322,7 +338,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> >       unsigned int order = huge_page_order(h);
> >       unsigned int vmemmap_pages;
> >
> > -     if (!is_power_of_2(sizeof(struct page))) {
> > +     if (!is_power_of_2(sizeof(struct page)) ||
> > +         !hugetlb_free_vmemmap_enabled) {
> >               pr_info("disable freeing vmemmap pages for %s\n", h->name);
> >               return;
> >       }
> > --
> > 2.11.0
>
> Thanks
> Barry
>


-- 
Yours,
Muchun

Re: [External] Re: [PATCH v6 07/16] x86/mm/64: Disable PMD page mapping of vmemmap

[Muchun Song]

On Tue, Nov 24, 2020 at 6:24 PM Oscar Salvador <osalvador@suse.de> wrote:
>
> On Tue, Nov 24, 2020 at 05:52:50PM +0800, Muchun Song wrote:
> > If we enable the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, we can just
> > disbale PMD page mapping of vmemmap to simplify the code. In this
> > case, we do not need complex code doing vmemmap page table
> > manipulation. This is a way to simply the first version of this
> > patch series. In the future, we can add some code doing page table
> > manipulation.
>
> IIRC, CONFIG_HUGETLB_PAGE_FREE_VMEMMAP was supposed to be enabled by default,
> right?
> And we would control whether we __really__ want to this by a boot option,
> which was disabled by default?
>
> If you go for populating the memmap with basepages by checking
> CONFIG_HUGETLB_PAGE_FREE_VMEMMAP, would not everybody, even the ones that
> did not enable this by the boot option be affected?

Yeah, this should be improved. We should enable the basepage mapping
of vmemmap only when this feature is enabled via boot command line.
I will apply the suggestions mentioned by Barry. Thanks.

>
> --
> Oscar Salvador
> SUSE L3



-- 
Yours,
Muchun

Re: [PATCH v6 09/16] mm/hugetlb: Defer freeing of HugeTLB pages

[Michal Hocko]

On Tue 24-11-20 17:52:52, 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 use GFP_ATOMIC to allocate the vmemmap pages.

This has been brought up earlier without any satisfying answer. Do we
really have bother with the freeing from the pool and reconstructing the
vmemmap page tables? Do existing usecases really require such a dynamic
behavior? In other words, wouldn't it be much simpler to allow to use
hugetlb pages with sparse vmemmaps only for the boot time reservations
and never allow them to be freed back to the allocator. This is pretty
restrictive, no question about that, but it would drop quite some code
AFAICS and the resulting series would be much easier to review really
carefully. Additional enhancements can be done on top with specifics
about usecases which require more flexibility.

> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/hugetlb.c         | 96 ++++++++++++++++++++++++++++++++++++++++++++++------
>  mm/hugetlb_vmemmap.c |  5 ---
>  mm/hugetlb_vmemmap.h | 10 ++++++
>  3 files changed, 95 insertions(+), 16 deletions(-)
-- 
Michal Hocko
SUSE Labs

Re: [External] Re: [PATCH v6 09/16] mm/hugetlb: Defer freeing of HugeTLB pages

[Muchun Song]

On Tue, Nov 24, 2020 at 7:51 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 24-11-20 17:52:52, 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 use GFP_ATOMIC to allocate the vmemmap pages.
>
> This has been brought up earlier without any satisfying answer. Do we
> really have bother with the freeing from the pool and reconstructing the
> vmemmap page tables? Do existing usecases really require such a dynamic
> behavior? In other words, wouldn't it be much simpler to allow to use

If someone wants to free a HugeTLB page, there is no way to do that if we
do not allow this behavior. When do we need this? On our server, we will
allocate a lot of HugeTLB pages for SPDK or virtualization. Sometimes,
we want to debug some issues and want to apt install some debug tools,
but if the host has little memory and the install operation can be failed
because of no memory. In this time, we can try to free some HugeTLB
pages to buddy in order to continue debugging. So maybe we need this.

> hugetlb pages with sparse vmemmaps only for the boot time reservations
> and never allow them to be freed back to the allocator. This is pretty
> restrictive, no question about that, but it would drop quite some code

Yeah, if we do not allow freeing the HugeTLB page to buddy, it actually
can drop some code. But I think that it only drop this one and next one
patch. It seems not a lot. And if we drop this patch, we need to add some
another code to do the boot time reservations and other code to disallow
freeing HugeTLB pages. So why not support freeing now.

> AFAICS and the resulting series would be much easier to review really
> carefully. Additional enhancements can be done on top with specifics
> about usecases which require more flexibility.

The code of allocating vmemmap pages for the HugeTLB page is very
similar to the freeing vmemmap pages. The two operations are opposite.
I think that if someone can understand the freeing path, it is also easy
for him to understand the allcating path. If you look at close to this patch,
I believe that it is easy for you.

>
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  mm/hugetlb.c         | 96 ++++++++++++++++++++++++++++++++++++++++++++++------
> >  mm/hugetlb_vmemmap.c |  5 ---
> >  mm/hugetlb_vmemmap.h | 10 ++++++
> >  3 files changed, 95 insertions(+), 16 deletions(-)
> --
> Michal Hocko
> SUSE Labs



-- 
Yours,
Muchun

Re: [External] Re: [PATCH v6 09/16] mm/hugetlb: Defer freeing of HugeTLB pages

[Michal Hocko]

On Tue 24-11-20 20:45:30, Muchun Song wrote:
> On Tue, Nov 24, 2020 at 7:51 PM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Tue 24-11-20 17:52:52, 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 use GFP_ATOMIC to allocate the vmemmap pages.
> >
> > This has been brought up earlier without any satisfying answer. Do we
> > really have bother with the freeing from the pool and reconstructing the
> > vmemmap page tables? Do existing usecases really require such a dynamic
> > behavior? In other words, wouldn't it be much simpler to allow to use
> 
> If someone wants to free a HugeTLB page, there is no way to do that if we
> do not allow this behavior.

Right. The question is how much that matters for the _initial_ feature
submission. Is this restriction so important that it would render it
unsuable?

> When do we need this? On our server, we will
> allocate a lot of HugeTLB pages for SPDK or virtualization. Sometimes,
> we want to debug some issues and want to apt install some debug tools,
> but if the host has little memory and the install operation can be failed
> because of no memory. In this time, we can try to free some HugeTLB
> pages to buddy in order to continue debugging. So maybe we need this.

Or maybe you can still allocate hugetlb pages for debugging in runtime
and try to free those when you need to.

> > hugetlb pages with sparse vmemmaps only for the boot time reservations
> > and never allow them to be freed back to the allocator. This is pretty
> > restrictive, no question about that, but it would drop quite some code
> 
> Yeah, if we do not allow freeing the HugeTLB page to buddy, it actually
> can drop some code. But I think that it only drop this one and next one
> patch. It seems not a lot. And if we drop this patch, we need to add some
> another code to do the boot time reservations and other code to disallow
> freeing HugeTLB pages.

you need a per hugetlb page flag to note the sparse vmemmap anyway so
the freeing path should be a trivial check for the flag. Early boot
reservation. Special casing for the early boot reservation shouldn't be
that hard either. But I haven't checked closely.

> So why not support freeing now.

Because it adds some non trivial challenges which would be better to
deal with with a stable and tested and feature limited implementation.
The most obvious one is the problem with vmemmap allocations when
freeing hugetlb page. Others like vmemmap manipulation is quite some
code but no surprises. Btw. that should be implemented in vmemmap proper
and ready for other potential users. But this is a minor detail.

-- 
Michal Hocko
SUSE Labs