[PATCH v3 0/4] Free the 2nd vmemmap page associated with each HugeTLB page

[PATCH v3 0/4] Free the 2nd vmemmap page associated with each HugeTLB page

[Muchun Song]

This series can minimize the overhead of struct page for 2MB HugeTLB pages
significantly, comments and reviews are welcome. Thanks.

After the feature of "Free sonme vmemmap pages of HugeTLB page" is enabled,
the mapping of the vmemmap addresses associated with a 2MB HugeTLB page
becomes the figure below.

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

As we can see, the 2nd vmemmap page frame (indexed by 1) is reused and remaped.
However, the 2nd vmemmap page frame is also can be freed to the buddy allocator,
then we can change the mapping from the figure above to the figure below.

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

After we do this, all tail vmemmap pages (1-7) are mapped to the head vmemmap
page frame (0). In other words, there are more than one page struct with
PG_head associated with each HugeTLB page.  We __know__ that there is only one
head page struct, the tail page structs with PG_head are fake head page structs.
We need an approach to distinguish between those two different types of page
structs so that compound_head(), PageHead() and PageTail() can work properly
if the parameter is the tail page struct but with PG_head.

The following code snippet describes how to distinguish between real and fake
head page struct.

	if (test_bit(PG_head, &page->flags)) {
		unsigned long head = READ_ONCE(page[1].compound_head);

		if (head & 1) {
			if (head == (unsigned long)page + 1)
				==> head page struct
			else
				==> tail page struct
		} else
			==> head page struct
	}

We can safely access the field of the @page[1] with PG_head because the @page
is a compound page composed with at least two contiguous pages. The main
implementation is in the patch 1.

In our server, we can save extra 2GB memory with this patchset applied if there
are 1 TB HugeTLB (2 MB) pages. If the size of the HugeTLB page is 1 GB, it only
can save 4MB. For 2 MB HugeTLB page, it is a nice gain.

Changlogs in v3:
  1. Rename page_head_if_fake() to page_fixed_fake_head().
  2. Introducing a new helper page_is_fake_head() to make code more readable.
  3. Update commit log of patch 3 to add more judgements.
  4. Add some comments in check_page_flags() in the patch 4.

  Thanks Barry for his suggestions and reviews.

Changlogs in v2:
  1. Drop two patches of introducing PAGEFLAGS_MASK from this series.
  2. Let page_head_if_fake() return page instead of NULL.
  3. Add a selftest to check if PageHead or PageTail work well.

Muchun Song (4):
  mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB
    page
  mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key
  mm: sparsemem: use page table lock to protect kernel pmd operations
  selftests: vm: add a hugetlb test case

 Documentation/admin-guide/kernel-parameters.txt |   2 +-
 include/linux/hugetlb.h                         |  10 +-
 include/linux/page-flags.h                      |  79 ++++++++++++-
 mm/hugetlb_vmemmap.c                            |  66 ++++++-----
 mm/memory_hotplug.c                             |   2 +-
 mm/ptdump.c                                     |  16 ++-
 mm/sparse-vmemmap.c                             |  70 +++++++++---
 tools/testing/selftests/vm/vmemmap_hugetlb.c    | 144 ++++++++++++++++++++++++
 8 files changed, 332 insertions(+), 57 deletions(-)
 create mode 100644 tools/testing/selftests/vm/vmemmap_hugetlb.c

-- 
2.11.0

[PATCH v3 1/4] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Muchun Song]

Currently, we only free 6 vmemmap pages associated with a 2MB HugeTLB
page. However, we can remap all tail vmemmap pages to the page frame
mapped to with the head vmemmap page. Finally, we can free 7 vmemmap
pages for a 2MB HugeTLB page. It is a fine gain (e.g. we can save
extra 2GB memory when there is 1TB HugeTLB pages in the system
compared with the current implementation).

But the head vmemmap page is not freed to the buddy allocator and all
tail vmemmap pages are mapped to the head vmemmap page frame. So we
can see more than one struct page struct with PG_head (e.g. 8 per 2 MB
HugeTLB page) associated with each HugeTLB page. We should adjust
compound_head() to make it returns the real head struct page when the
parameter is the tail struct page but with PG_head flag.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  2 +-
 include/linux/page-flags.h                      | 77 +++++++++++++++++++++++--
 mm/hugetlb_vmemmap.c                            | 60 ++++++++++---------
 mm/sparse-vmemmap.c                             | 21 +++++++
 4 files changed, 128 insertions(+), 32 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 91ba391f9b32..5aaf2f271980 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1617,7 +1617,7 @@
 			[KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
 			enabled.
 			Allows heavy hugetlb users to free up some more
-			memory (6 * PAGE_SIZE for each 2MB hugetlb page).
+			memory (7 * PAGE_SIZE for each 2MB hugetlb page).
 			Format: { on | off (default) }
 
 			on:  enable the feature
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index a68af80649a4..b47a7f51d2c3 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -184,13 +184,68 @@ enum pageflags {
 
 #ifndef __GENERATING_BOUNDS_H
 
+#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
+extern bool hugetlb_free_vmemmap_enabled;
+
+/*
+ * If the feature of freeing some vmemmap pages associated with each HugeTLB
+ * page is enabled, the head vmemmap page frame is reused and all of the tail
+ * vmemmap addresses map to the head vmemmap page frame (furture details can
+ * refer to the figure at the head of the mm/hugetlb_vmemmap.c).  In other
+ * words, there are more than one page struct with PG_head associated with each
+ * HugeTLB page.  We __know__ that there is only one head page struct, the tail
+ * page structs with PG_head are fake head page structs.  We need an approach
+ * to distinguish between those two different types of page structs so that
+ * compound_head() can return the real head page struct when the parameter is
+ * the tail page struct but with PG_head.
+ *
+ * The page_fixed_fake_head() returns the real head page struct if the @page is
+ * fake page head, otherwise, returns @page which can either be a true page
+ * head or tail.
+ */
+static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
+{
+	if (!hugetlb_free_vmemmap_enabled)
+		return page;
+	/*
+	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
+	 * struct page. The alignment check aims to avoid access the fields (
+	 * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
+	 * cold cacheline in some cases.
+	 */
+	if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
+	    test_bit(PG_head, &page->flags)) {
+		/*
+		 * We can safely access the field of the @page[1] with PG_head
+		 * because the @page is a compound page composed with at least
+		 * two contiguous pages.
+		 */
+		unsigned long head = READ_ONCE(page[1].compound_head);
+
+		if (likely(head & 1))
+			return (const struct page *)(head - 1);
+	}
+	return page;
+}
+#else
+static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
+{
+	return page;
+}
+#endif
+
+static __always_inline int page_is_fake_head(struct page *page)
+{
+	return page_fixed_fake_head(page) != page;
+}
+
 static inline unsigned long _compound_head(const struct page *page)
 {
 	unsigned long head = READ_ONCE(page->compound_head);
 
 	if (unlikely(head & 1))
 		return head - 1;
-	return (unsigned long)page;
+	return (unsigned long)page_fixed_fake_head(page);
 }
 
 #define compound_head(page)	((typeof(page))_compound_head(page))
@@ -225,12 +280,13 @@ static inline unsigned long _compound_head(const struct page *page)
 
 static __always_inline int PageTail(struct page *page)
 {
-	return READ_ONCE(page->compound_head) & 1;
+	return READ_ONCE(page->compound_head) & 1 || page_is_fake_head(page);
 }
 
 static __always_inline int PageCompound(struct page *page)
 {
-	return test_bit(PG_head, &page->flags) || PageTail(page);
+	return test_bit(PG_head, &page->flags) ||
+	       READ_ONCE(page->compound_head) & 1;
 }
 
 #define	PAGE_POISON_PATTERN	-1l
@@ -675,7 +731,20 @@ static inline bool test_set_page_writeback(struct page *page)
 	return set_page_writeback(page);
 }
 
-__PAGEFLAG(Head, head, PF_ANY) CLEARPAGEFLAG(Head, head, PF_ANY)
+static __always_inline bool folio_test_head(struct folio *folio)
+{
+	return test_bit(PG_head, folio_flags(folio, FOLIO_PF_ANY));
+}
+
+static __always_inline int PageHead(struct page *page)
+{
+	PF_POISONED_CHECK(page);
+	return test_bit(PG_head, &page->flags) && !page_is_fake_head(page);
+}
+
+__SETPAGEFLAG(Head, head, PF_ANY)
+__CLEARPAGEFLAG(Head, head, PF_ANY)
+CLEARPAGEFLAG(Head, head, PF_ANY)
 
 /* Whether there are one or multiple pages in a folio */
 static inline bool folio_test_single(struct folio *folio)
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index c540c21e26f5..527bcaa44a48 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -124,9 +124,9 @@
  * 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
+ * Therefore, we can remap pages 1 to 7 to page 0. Only 1 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.
+ * 7 pages to the buddy allocator.
  *
  * Here is how things look after remapping.
  *
@@ -134,30 +134,30 @@
  * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
  * |           |                     |     0     | -------------> |     0     |
  * |           |                     +-----------+                +-----------+
- * |           |                     |     1     | -------------> |     1     |
- * |           |                     +-----------+                +-----------+
- * |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
- * |           |                     +-----------+                   | | | | |
- * |           |                     |     3     | ------------------+ | | | |
- * |           |                     +-----------+                     | | | |
- * |           |                     |     4     | --------------------+ | | |
- * |    PMD    |                     +-----------+                       | | |
- * |   level   |                     |     5     | ----------------------+ | |
- * |  mapping  |                     +-----------+                         | |
- * |           |                     |     6     | ------------------------+ |
- * |           |                     +-----------+                           |
- * |           |                     |     7     | --------------------------+
+ * |           |                     |     1     | ---------------^ ^ ^ ^ ^ ^ ^
+ * |           |                     +-----------+                  | | | | | |
+ * |           |                     |     2     | -----------------+ | | | | |
+ * |           |                     +-----------+                    | | | | |
+ * |           |                     |     3     | -------------------+ | | | |
+ * |           |                     +-----------+                      | | | |
+ * |           |                     |     4     | ---------------------+ | | |
+ * |    PMD    |                     +-----------+                        | | |
+ * |   level   |                     |     5     | -----------------------+ | |
+ * |  mapping  |                     +-----------+                          | |
+ * |           |                     |     6     | -------------------------+ |
+ * |           |                     +-----------+                            |
+ * |           |                     |     7     | ---------------------------+
  * |           |                     +-----------+
  * |           |
  * |           |
  * |           |
  * +-----------+
  *
- * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
+ * When a HugeTLB is freed to the buddy system, we should allocate 7 pages for
  * vmemmap pages and restore the previous mapping relationship.
  *
  * For the HugeTLB page of the pud level mapping. It is similar to the former.
- * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
+ * We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages.
  *
  * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
  * (e.g. aarch64) provides a contiguous bit in the translation table entries
@@ -166,7 +166,13 @@
  *
  * The contiguous bit is used to increase the mapping size at the pmd and pte
  * (last) level. So this type of HugeTLB page can be optimized only when its
- * size of the struct page structs is greater than 2 pages.
+ * size of the struct page structs is greater than 1 pages.
+ *
+ * Notice: The head vmemmap page is not freed to the buddy allocator and all
+ * tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
+ * more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page)
+ * associated with each HugeTLB page. The compound_head() can handle this
+ * correctly (more details refer to the comment above compound_head()).
  */
 #define pr_fmt(fmt)	"HugeTLB: " fmt
 
@@ -175,14 +181,16 @@
 /*
  * There are a lot of struct page structures associated with each HugeTLB page.
  * For tail pages, the value of compound_head is the same. So we can reuse first
- * page of tail page structures. We map the virtual addresses of the remaining
- * pages of tail page structures to the first tail page struct, and then free
- * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
+ * page of head page structures. We map the virtual addresses of all the pages
+ * of tail page structures to the head page struct, and then free these page
+ * frames. Therefore, we need to reserve one pages as vmemmap areas.
  */
-#define RESERVE_VMEMMAP_NR		2U
+#define RESERVE_VMEMMAP_NR		1U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-bool hugetlb_free_vmemmap_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
+bool hugetlb_free_vmemmap_enabled __read_mostly =
+	IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
+EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
@@ -236,7 +244,6 @@ int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
 	 */
 	ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
 				  GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
-
 	if (!ret)
 		ClearHPageVmemmapOptimized(head);
 
@@ -282,9 +289,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 
 	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
 	/*
-	 * The head page and the first tail page are not to be freed to buddy
-	 * allocator, the other pages will map to the first tail page, so they
-	 * can be freed.
+	 * The head page is not to be freed to buddy allocator, the other tail
+	 * pages will map to the head page, so they can be freed.
 	 *
 	 * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
 	 * on some architectures (e.g. aarch64). See Documentation/arm64/
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index bdce883f9286..62e3d20648ce 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -53,6 +53,17 @@ struct vmemmap_remap_walk {
 	struct list_head *vmemmap_pages;
 };
 
+/*
+ * How many struct page structs need to be reset. When we reuse the head
+ * struct page, the special metadata (e.g. page->flags or page->mapping)
+ * cannot copy to the tail struct page structs. The invalid value will be
+ * checked in the free_tail_pages_check(). In order to avoid the message
+ * of "corrupted mapping in tail page". We need to reset at least 3 (one
+ * head struct page struct and two tail struct page structs) struct page
+ * structs.
+ */
+#define NR_RESET_STRUCT_PAGE		3
+
 static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
 				  struct vmemmap_remap_walk *walk)
 {
@@ -245,6 +256,15 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
 	set_pte_at(&init_mm, addr, pte, entry);
 }
 
+static inline void reset_struct_pages(struct page *start)
+{
+	int i;
+	struct page *from = start + NR_RESET_STRUCT_PAGE;
+
+	for (i = 0; i < NR_RESET_STRUCT_PAGE; i++)
+		memcpy(start + i, from, sizeof(*from));
+}
+
 static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
 				struct vmemmap_remap_walk *walk)
 {
@@ -258,6 +278,7 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
 	list_del(&page->lru);
 	to = page_to_virt(page);
 	copy_page(to, (void *)walk->reuse_addr);
+	reset_struct_pages(to);
 
 	set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
 }
-- 
2.11.0

[PATCH v3 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Muchun Song]

The page_head_if_fake() is used throughout memory management and the
conditional check requires checking a global variable, although the
overhead of this check may be small, it increases when the memory
cache comes under pressure. Also, the global variable will not be
modified after system boot, so it is very appropriate to use static
key machanism.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 include/linux/hugetlb.h    | 10 ++++++++--
 include/linux/page-flags.h |  6 ++++--
 mm/hugetlb_vmemmap.c       | 12 ++++++------
 mm/memory_hotplug.c        |  2 +-
 4 files changed, 19 insertions(+), 11 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 1faebe1cd0ed..4cc647a5dbf8 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1066,9 +1066,15 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
 #endif	/* CONFIG_HUGETLB_PAGE */
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
+static inline bool hugetlb_free_vmemmap_enabled(void)
+{
+	return static_key_enabled(&hugetlb_free_vmemmap_enabled_key);
+}
 #else
-#define hugetlb_free_vmemmap_enabled	false
+static inline bool hugetlb_free_vmemmap_enabled(void)
+{
+	return false;
+}
 #endif
 
 static inline spinlock_t *huge_pte_lock(struct hstate *h,
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index b47a7f51d2c3..54e119e44496 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -185,7 +185,8 @@ enum pageflags {
 #ifndef __GENERATING_BOUNDS_H
 
 #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
-extern bool hugetlb_free_vmemmap_enabled;
+DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			 hugetlb_free_vmemmap_enabled_key);
 
 /*
  * If the feature of freeing some vmemmap pages associated with each HugeTLB
@@ -205,7 +206,8 @@ extern bool hugetlb_free_vmemmap_enabled;
  */
 static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
 {
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+				 &hugetlb_free_vmemmap_enabled_key))
 		return page;
 	/*
 	 * Only addresses aligned with PAGE_SIZE of struct page may be fake head
diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 527bcaa44a48..47517e878ed5 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -188,9 +188,9 @@
 #define RESERVE_VMEMMAP_NR		1U
 #define RESERVE_VMEMMAP_SIZE		(RESERVE_VMEMMAP_NR << PAGE_SHIFT)
 
-bool hugetlb_free_vmemmap_enabled __read_mostly =
-	IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
-EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
+DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
+			hugetlb_free_vmemmap_enabled_key);
+EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
 
 static int __init early_hugetlb_free_vmemmap_param(char *buf)
 {
@@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
 		return -EINVAL;
 
 	if (!strcmp(buf, "on"))
-		hugetlb_free_vmemmap_enabled = true;
+		static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
 	else if (!strcmp(buf, "off"))
-		hugetlb_free_vmemmap_enabled = false;
+		static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
 	else
 		return -EINVAL;
 
@@ -284,7 +284,7 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
 	BUILD_BUG_ON(__NR_USED_SUBPAGE >=
 		     RESERVE_VMEMMAP_SIZE / sizeof(struct page));
 
-	if (!hugetlb_free_vmemmap_enabled)
+	if (!hugetlb_free_vmemmap_enabled())
 		return;
 
 	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 0488eed3327c..89c1fde02162 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1341,7 +1341,7 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
 	 *       populate a single PMD.
 	 */
 	return memmap_on_memory &&
-	       !hugetlb_free_vmemmap_enabled &&
+	       !hugetlb_free_vmemmap_enabled() &&
 	       IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
 	       size == memory_block_size_bytes() &&
 	       IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
-- 
2.11.0

[PATCH v3 3/4] mm: sparsemem: use page table lock to protect kernel pmd operations

[Muchun Song]

The init_mm.page_table_lock is used to protect kernel page tables, we
can use it to serialize splitting vmemmap PMD mappings instead of mmap
write lock, which can increase the concurrency of vmemmap_remap_free().

Actually, It increase the concurrency between allocations of HugeTLB
pages. But it is not the only benefit. There are a lot of users of
mmap read lock of init_mm. The mmap write lock is holding through
vmemmap_remap_free(), removing mmap write lock usage to make it does
not affect other users of mmap read lock. It is not making anything
worse and always a win to move.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/ptdump.c         | 16 ++++++++++++----
 mm/sparse-vmemmap.c | 49 ++++++++++++++++++++++++++++++++++---------------
 2 files changed, 46 insertions(+), 19 deletions(-)

diff --git a/mm/ptdump.c b/mm/ptdump.c
index da751448d0e4..eea3d28d173c 100644
--- a/mm/ptdump.c
+++ b/mm/ptdump.c
@@ -40,8 +40,10 @@ static int ptdump_pgd_entry(pgd_t *pgd, unsigned long addr,
 	if (st->effective_prot)
 		st->effective_prot(st, 0, pgd_val(val));
 
-	if (pgd_leaf(val))
+	if (pgd_leaf(val)) {
 		st->note_page(st, addr, 0, pgd_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
@@ -61,8 +63,10 @@ static int ptdump_p4d_entry(p4d_t *p4d, unsigned long addr,
 	if (st->effective_prot)
 		st->effective_prot(st, 1, p4d_val(val));
 
-	if (p4d_leaf(val))
+	if (p4d_leaf(val)) {
 		st->note_page(st, addr, 1, p4d_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
@@ -82,8 +86,10 @@ static int ptdump_pud_entry(pud_t *pud, unsigned long addr,
 	if (st->effective_prot)
 		st->effective_prot(st, 2, pud_val(val));
 
-	if (pud_leaf(val))
+	if (pud_leaf(val)) {
 		st->note_page(st, addr, 2, pud_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
@@ -101,8 +107,10 @@ static int ptdump_pmd_entry(pmd_t *pmd, unsigned long addr,
 
 	if (st->effective_prot)
 		st->effective_prot(st, 3, pmd_val(val));
-	if (pmd_leaf(val))
+	if (pmd_leaf(val)) {
 		st->note_page(st, addr, 3, pmd_val(val));
+		walk->action = ACTION_CONTINUE;
+	}
 
 	return 0;
 }
diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
index 62e3d20648ce..e636943ccfc4 100644
--- a/mm/sparse-vmemmap.c
+++ b/mm/sparse-vmemmap.c
@@ -64,8 +64,8 @@ struct vmemmap_remap_walk {
  */
 #define NR_RESET_STRUCT_PAGE		3
 
-static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
-				  struct vmemmap_remap_walk *walk)
+static int __split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
+				    struct vmemmap_remap_walk *walk)
 {
 	pmd_t __pmd;
 	int i;
@@ -87,15 +87,37 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
 		set_pte_at(&init_mm, addr, pte, entry);
 	}
 
-	/* Make pte visible before pmd. See comment in __pte_alloc(). */
-	smp_wmb();
-	pmd_populate_kernel(&init_mm, pmd, pgtable);
+	spin_lock(&init_mm.page_table_lock);
+	if (likely(pmd_leaf(*pmd))) {
+		/* Make pte visible before pmd. See comment in __pte_alloc(). */
+		smp_wmb();
+		pmd_populate_kernel(&init_mm, pmd, pgtable);
+		flush_tlb_kernel_range(start, start + PMD_SIZE);
+		spin_unlock(&init_mm.page_table_lock);
 
-	flush_tlb_kernel_range(start, start + PMD_SIZE);
+		return 0;
+	}
+	spin_unlock(&init_mm.page_table_lock);
+	pte_free_kernel(&init_mm, pgtable);
 
 	return 0;
 }
 
+static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
+				  struct vmemmap_remap_walk *walk)
+{
+	int ret;
+
+	spin_lock(&init_mm.page_table_lock);
+	ret = pmd_leaf(*pmd);
+	spin_unlock(&init_mm.page_table_lock);
+
+	if (ret)
+		ret = __split_vmemmap_huge_pmd(pmd, start, walk);
+
+	return ret;
+}
+
 static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
 			      unsigned long end,
 			      struct vmemmap_remap_walk *walk)
@@ -132,13 +154,12 @@ static int vmemmap_pmd_range(pud_t *pud, unsigned long addr,
 
 	pmd = pmd_offset(pud, addr);
 	do {
-		if (pmd_leaf(*pmd)) {
-			int ret;
+		int ret;
+
+		ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, walk);
+		if (ret)
+			return ret;
 
-			ret = split_vmemmap_huge_pmd(pmd, addr & PMD_MASK, walk);
-			if (ret)
-				return ret;
-		}
 		next = pmd_addr_end(addr, end);
 		vmemmap_pte_range(pmd, addr, next, walk);
 	} while (pmd++, addr = next, addr != end);
@@ -321,10 +342,8 @@ int vmemmap_remap_free(unsigned long start, unsigned long end,
 	 */
 	BUG_ON(start - reuse != PAGE_SIZE);
 
-	mmap_write_lock(&init_mm);
+	mmap_read_lock(&init_mm);
 	ret = vmemmap_remap_range(reuse, end, &walk);
-	mmap_write_downgrade(&init_mm);
-
 	if (ret && walk.nr_walked) {
 		end = reuse + walk.nr_walked * PAGE_SIZE;
 		/*
-- 
2.11.0

[PATCH v3 4/4] selftests: vm: add a hugetlb test case

[Muchun Song]

Since the head vmemmap page frame associated with each HugeTLB page is
reused, we should hide the PG_head flag of tail struct page from the
user. Add a tese case to check whether it is work properly. The test
steps are as follows.

  1) alloc 2MB hugeTLB
  2) get each page frame
  3) apply those APIs in each page frame
  4) Those APIs work completely the same as before.

Reading the flags of a page by /proc/kpageflags is done in
stable_page_flags(), which has invoked PageHead(), PageTail(),
PageCompound() and compound_head(). If those APIs work properly, the
head page must have 15 and 17 bits set. And tail pages must have 16
and 17 bits set but 15 bit unset. Those flags are checked in
check_page_flags().

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 tools/testing/selftests/vm/vmemmap_hugetlb.c | 144 +++++++++++++++++++++++++++
 1 file changed, 144 insertions(+)
 create mode 100644 tools/testing/selftests/vm/vmemmap_hugetlb.c

diff --git a/tools/testing/selftests/vm/vmemmap_hugetlb.c b/tools/testing/selftests/vm/vmemmap_hugetlb.c
new file mode 100644
index 000000000000..4cc74dd4c333
--- /dev/null
+++ b/tools/testing/selftests/vm/vmemmap_hugetlb.c
@@ -0,0 +1,144 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A test case of using hugepage memory in a user application using the
+ * mmap system call with MAP_HUGETLB flag.  Before running this program
+ * make sure the administrator has allocated enough default sized huge
+ * pages to cover the 2 MB allocation.
+ *
+ * For ia64 architecture, Linux kernel reserves Region number 4 for hugepages.
+ * That means the addresses starting with 0x800000... will need to be
+ * specified.  Specifying a fixed address is not required on ppc64, i386
+ * or x86_64.
+ */
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+
+#define MAP_LENGTH		(2UL * 1024 * 1024)
+
+#ifndef MAP_HUGETLB
+#define MAP_HUGETLB		0x40000	/* arch specific */
+#endif
+
+#define PAGE_SIZE		4096
+
+#define PAGE_COMPOUND_HEAD	(1UL << 15)
+#define PAGE_COMPOUND_TAIL	(1UL << 16)
+#define PAGE_HUGE		(1UL << 17)
+
+#define HEAD_PAGE_FLAGS		(PAGE_COMPOUND_HEAD | PAGE_HUGE)
+#define TAIL_PAGE_FLAGS		(PAGE_COMPOUND_TAIL | PAGE_HUGE)
+
+#define PM_PFRAME_BITS		55
+#define PM_PFRAME_MASK		~((1UL << PM_PFRAME_BITS) - 1)
+
+/* Only ia64 requires this */
+#ifdef __ia64__
+#define MAP_ADDR		(void *)(0x8000000000000000UL)
+#define MAP_FLAGS		(MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB | MAP_FIXED)
+#else
+#define MAP_ADDR		NULL
+#define MAP_FLAGS		(MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB)
+#endif
+
+static void write_bytes(char *addr, size_t length)
+{
+	unsigned long i;
+
+	for (i = 0; i < length; i++)
+		*(addr + i) = (char)i;
+}
+
+static unsigned long virt_to_pfn(void *addr)
+{
+	int fd;
+	unsigned long pagemap;
+
+	fd = open("/proc/self/pagemap", O_RDONLY);
+	if (fd < 0)
+		return -1UL;
+
+	lseek(fd, (unsigned long)addr / PAGE_SIZE * sizeof(pagemap), SEEK_SET);
+	read(fd, &pagemap, sizeof(pagemap));
+	close(fd);
+
+	return pagemap & ~PM_PFRAME_MASK;
+}
+
+static int check_page_flags(unsigned long pfn)
+{
+	int fd, i;
+	unsigned long pageflags;
+
+	fd = open("/proc/kpageflags", O_RDONLY);
+	if (fd < 0)
+		return -1;
+
+	lseek(fd, pfn * sizeof(pageflags), SEEK_SET);
+
+	read(fd, &pageflags, sizeof(pageflags));
+	if ((pageflags & HEAD_PAGE_FLAGS) != HEAD_PAGE_FLAGS) {
+		close(fd);
+		printf("Head page flags (%lx) is invalid\n", pageflags);
+		return -1;
+	}
+
+	/*
+	 * pages other than the first page must be tail and shouldn't be head;
+	 * this also verifies kernel has correctly set the fake page_head to tail
+	 * while hugetlb_free_vmemmap is enabled.
+	 */
+	for (i = 1; i < MAP_LENGTH / PAGE_SIZE; i++) {
+		read(fd, &pageflags, sizeof(pageflags));
+		if ((pageflags & TAIL_PAGE_FLAGS) != TAIL_PAGE_FLAGS ||
+		    (pageflags & HEAD_PAGE_FLAGS) == HEAD_PAGE_FLAGS) {
+			close(fd);
+			printf("Tail page flags (%lx) is invalid\n", pageflags);
+			return -1;
+		}
+	}
+
+	close(fd);
+
+	return 0;
+}
+
+int main(int argc, char **argv)
+{
+	void *addr;
+	unsigned long pfn;
+
+	addr = mmap(MAP_ADDR, MAP_LENGTH, PROT_READ | PROT_WRITE, MAP_FLAGS, -1, 0);
+	if (addr == MAP_FAILED) {
+		perror("mmap");
+		exit(1);
+	}
+
+	/* Trigger allocation of HugeTLB page. */
+	write_bytes(addr, MAP_LENGTH);
+
+	pfn = virt_to_pfn(addr);
+	if (pfn == -1UL) {
+		munmap(addr, MAP_LENGTH);
+		perror("virt_to_pfn");
+		exit(1);
+	}
+
+	printf("Returned address is %p whose pfn is %lx\n", addr, pfn);
+
+	if (check_page_flags(pfn) < 0) {
+		munmap(addr, MAP_LENGTH);
+		perror("check_page_flags");
+		exit(1);
+	}
+
+	/* munmap() length of MAP_HUGETLB memory must be hugepage aligned */
+	if (munmap(addr, MAP_LENGTH)) {
+		perror("munmap");
+		exit(1);
+	}
+
+	return 0;
+}
-- 
2.11.0

Re: [PATCH v3 1/4] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Barry Song]

On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> Currently, we only free 6 vmemmap pages associated with a 2MB HugeTLB
> page. However, we can remap all tail vmemmap pages to the page frame
> mapped to with the head vmemmap page. Finally, we can free 7 vmemmap
> pages for a 2MB HugeTLB page. It is a fine gain (e.g. we can save
> extra 2GB memory when there is 1TB HugeTLB pages in the system
> compared with the current implementation).
>
> But the head vmemmap page is not freed to the buddy allocator and all
> tail vmemmap pages are mapped to the head vmemmap page frame. So we
> can see more than one struct page struct with PG_head (e.g. 8 per 2 MB
> HugeTLB page) associated with each HugeTLB page. We should adjust
> compound_head() to make it returns the real head struct page when the
> parameter is the tail struct page but with PG_head flag.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |  2 +-
>  include/linux/page-flags.h                      | 77 +++++++++++++++++++++++--
>  mm/hugetlb_vmemmap.c                            | 60 ++++++++++---------
>  mm/sparse-vmemmap.c                             | 21 +++++++
>  4 files changed, 128 insertions(+), 32 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 91ba391f9b32..5aaf2f271980 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1617,7 +1617,7 @@
>                         [KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
>                         enabled.
>                         Allows heavy hugetlb users to free up some more
> -                       memory (6 * PAGE_SIZE for each 2MB hugetlb page).
> +                       memory (7 * PAGE_SIZE for each 2MB hugetlb page).
>                         Format: { on | off (default) }
>
>                         on:  enable the feature
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index a68af80649a4..b47a7f51d2c3 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -184,13 +184,68 @@ enum pageflags {
>
>  #ifndef __GENERATING_BOUNDS_H
>
> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +extern bool hugetlb_free_vmemmap_enabled;
> +
> +/*
> + * If the feature of freeing some vmemmap pages associated with each HugeTLB
> + * page is enabled, the head vmemmap page frame is reused and all of the tail
> + * vmemmap addresses map to the head vmemmap page frame (furture details can
> + * refer to the figure at the head of the mm/hugetlb_vmemmap.c).  In other
> + * words, there are more than one page struct with PG_head associated with each
> + * HugeTLB page.  We __know__ that there is only one head page struct, the tail
> + * page structs with PG_head are fake head page structs.  We need an approach
> + * to distinguish between those two different types of page structs so that
> + * compound_head() can return the real head page struct when the parameter is
> + * the tail page struct but with PG_head.
> + *
> + * The page_fixed_fake_head() returns the real head page struct if the @page is
> + * fake page head, otherwise, returns @page which can either be a true page
> + * head or tail.
> + */
> +static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> +{
> +       if (!hugetlb_free_vmemmap_enabled)
> +               return page;
> +       /*
> +        * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> +        * struct page. The alignment check aims to avoid access the fields (
> +        * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
> +        * cold cacheline in some cases.
> +        */
> +       if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> +           test_bit(PG_head, &page->flags)) {
> +               /*
> +                * We can safely access the field of the @page[1] with PG_head
> +                * because the @page is a compound page composed with at least
> +                * two contiguous pages.
> +                */
> +               unsigned long head = READ_ONCE(page[1].compound_head);
> +
> +               if (likely(head & 1))
> +                       return (const struct page *)(head - 1);
> +       }
> +       return page;
> +}
> +#else
> +static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> +{
> +       return page;
> +}
> +#endif
> +
> +static __always_inline int page_is_fake_head(struct page *page)
> +{
> +       return page_fixed_fake_head(page) != page;
> +}
> +
>  static inline unsigned long _compound_head(const struct page *page)
>  {
>         unsigned long head = READ_ONCE(page->compound_head);
>
>         if (unlikely(head & 1))
>                 return head - 1;
> -       return (unsigned long)page;
> +       return (unsigned long)page_fixed_fake_head(page);
>  }
>
>  #define compound_head(page)    ((typeof(page))_compound_head(page))
> @@ -225,12 +280,13 @@ static inline unsigned long _compound_head(const struct page *page)
>
>  static __always_inline int PageTail(struct page *page)
>  {
> -       return READ_ONCE(page->compound_head) & 1;
> +       return READ_ONCE(page->compound_head) & 1 || page_is_fake_head(page);
>  }
>
>  static __always_inline int PageCompound(struct page *page)
>  {
> -       return test_bit(PG_head, &page->flags) || PageTail(page);
> +       return test_bit(PG_head, &page->flags) ||
> +              READ_ONCE(page->compound_head) & 1;
>  }
>
>  #define        PAGE_POISON_PATTERN     -1l
> @@ -675,7 +731,20 @@ static inline bool test_set_page_writeback(struct page *page)
>         return set_page_writeback(page);
>  }
>
> -__PAGEFLAG(Head, head, PF_ANY) CLEARPAGEFLAG(Head, head, PF_ANY)
> +static __always_inline bool folio_test_head(struct folio *folio)
> +{
> +       return test_bit(PG_head, folio_flags(folio, FOLIO_PF_ANY));
> +}
> +
> +static __always_inline int PageHead(struct page *page)
> +{
> +       PF_POISONED_CHECK(page);
> +       return test_bit(PG_head, &page->flags) && !page_is_fake_head(page);
> +}
> +
> +__SETPAGEFLAG(Head, head, PF_ANY)
> +__CLEARPAGEFLAG(Head, head, PF_ANY)
> +CLEARPAGEFLAG(Head, head, PF_ANY)
>
>  /* Whether there are one or multiple pages in a folio */
>  static inline bool folio_test_single(struct folio *folio)
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index c540c21e26f5..527bcaa44a48 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -124,9 +124,9 @@
>   * 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
> + * Therefore, we can remap pages 1 to 7 to page 0. Only 1 pages of page structs

1 page.

>   * will be used for each HugeTLB page. This will allow us to free the remaining
> - * 6 pages to the buddy allocator.
> + * 7 pages to the buddy allocator.
>   *
>   * Here is how things look after remapping.
>   *
> @@ -134,30 +134,30 @@
>   * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
>   * |           |                     |     0     | -------------> |     0     |
>   * |           |                     +-----------+                +-----------+
> - * |           |                     |     1     | -------------> |     1     |
> - * |           |                     +-----------+                +-----------+
> - * |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
> - * |           |                     +-----------+                   | | | | |
> - * |           |                     |     3     | ------------------+ | | | |
> - * |           |                     +-----------+                     | | | |
> - * |           |                     |     4     | --------------------+ | | |
> - * |    PMD    |                     +-----------+                       | | |
> - * |   level   |                     |     5     | ----------------------+ | |
> - * |  mapping  |                     +-----------+                         | |
> - * |           |                     |     6     | ------------------------+ |
> - * |           |                     +-----------+                           |
> - * |           |                     |     7     | --------------------------+
> + * |           |                     |     1     | ---------------^ ^ ^ ^ ^ ^ ^
> + * |           |                     +-----------+                  | | | | | |
> + * |           |                     |     2     | -----------------+ | | | | |
> + * |           |                     +-----------+                    | | | | |
> + * |           |                     |     3     | -------------------+ | | | |
> + * |           |                     +-----------+                      | | | |
> + * |           |                     |     4     | ---------------------+ | | |
> + * |    PMD    |                     +-----------+                        | | |
> + * |   level   |                     |     5     | -----------------------+ | |
> + * |  mapping  |                     +-----------+                          | |
> + * |           |                     |     6     | -------------------------+ |
> + * |           |                     +-----------+                            |
> + * |           |                     |     7     | ---------------------------+
>   * |           |                     +-----------+
>   * |           |
>   * |           |
>   * |           |
>   * +-----------+
>   *
> - * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> + * When a HugeTLB is freed to the buddy system, we should allocate 7 pages for
>   * vmemmap pages and restore the previous mapping relationship.
>   *
>   * For the HugeTLB page of the pud level mapping. It is similar to the former.
> - * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
> + * We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages.
>   *
>   * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
>   * (e.g. aarch64) provides a contiguous bit in the translation table entries
> @@ -166,7 +166,13 @@
>   *
>   * The contiguous bit is used to increase the mapping size at the pmd and pte
>   * (last) level. So this type of HugeTLB page can be optimized only when its
> - * size of the struct page structs is greater than 2 pages.
> + * size of the struct page structs is greater than 1 pages.

1 page.

> + *
> + * Notice: The head vmemmap page is not freed to the buddy allocator and all
> + * tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
> + * more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page)
> + * associated with each HugeTLB page. The compound_head() can handle this
> + * correctly (more details refer to the comment above compound_head()).
>   */
>  #define pr_fmt(fmt)    "HugeTLB: " fmt
>
> @@ -175,14 +181,16 @@
>  /*
>   * There are a lot of struct page structures associated with each HugeTLB page.
>   * For tail pages, the value of compound_head is the same. So we can reuse first
> - * page of tail page structures. We map the virtual addresses of the remaining
> - * pages of tail page structures to the first tail page struct, and then free
> - * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
> + * page of head page structures. We map the virtual addresses of all the pages
> + * of tail page structures to the head page struct, and then free these page
> + * frames. Therefore, we need to reserve one pages as vmemmap areas.
>   */
> -#define RESERVE_VMEMMAP_NR             2U
> +#define RESERVE_VMEMMAP_NR             1U
>  #define RESERVE_VMEMMAP_SIZE           (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -bool hugetlb_free_vmemmap_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> +bool hugetlb_free_vmemmap_enabled __read_mostly =
> +       IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> +EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
>
>  static int __init early_hugetlb_free_vmemmap_param(char *buf)
>  {
> @@ -236,7 +244,6 @@ int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
>          */
>         ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
>                                   GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> -
>         if (!ret)
>                 ClearHPageVmemmapOptimized(head);
>
> @@ -282,9 +289,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>
>         vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
>         /*
> -        * The head page and the first tail page are not to be freed to buddy
> -        * allocator, the other pages will map to the first tail page, so they
> -        * can be freed.
> +        * The head page is not to be freed to buddy allocator, the other tail
> +        * pages will map to the head page, so they can be freed.
>          *
>          * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
>          * on some architectures (e.g. aarch64). See Documentation/arm64/
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index bdce883f9286..62e3d20648ce 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -53,6 +53,17 @@ struct vmemmap_remap_walk {
>         struct list_head *vmemmap_pages;
>  };
>
> +/*
> + * How many struct page structs need to be reset. When we reuse the head
> + * struct page, the special metadata (e.g. page->flags or page->mapping)
> + * cannot copy to the tail struct page structs. The invalid value will be
> + * checked in the free_tail_pages_check(). In order to avoid the message
> + * of "corrupted mapping in tail page". We need to reset at least 3 (one
> + * head struct page struct and two tail struct page structs) struct page
> + * structs.
> + */
> +#define NR_RESET_STRUCT_PAGE           3
> +
>  static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
>                                   struct vmemmap_remap_walk *walk)
>  {
> @@ -245,6 +256,15 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
>         set_pte_at(&init_mm, addr, pte, entry);
>  }
>
> +static inline void reset_struct_pages(struct page *start)
> +{
> +       int i;
> +       struct page *from = start + NR_RESET_STRUCT_PAGE;
> +
> +       for (i = 0; i < NR_RESET_STRUCT_PAGE; i++)
> +               memcpy(start + i, from, sizeof(*from));
> +}
> +
>  static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
>                                 struct vmemmap_remap_walk *walk)
>  {
> @@ -258,6 +278,7 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
>         list_del(&page->lru);
>         to = page_to_virt(page);
>         copy_page(to, (void *)walk->reuse_addr);
> +       reset_struct_pages(to);

Is this done unconditionally? Are we going here while
hugetlb_free_vmemmap_enabled=false?

>
>         set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
>  }
> --
> 2.11.0
>

Thanks
barry

Re: [PATCH v3 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Barry Song]

On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> The page_head_if_fake() is used throughout memory management and the
> conditional check requires checking a global variable, although the
> overhead of this check may be small, it increases when the memory
> cache comes under pressure. Also, the global variable will not be
> modified after system boot, so it is very appropriate to use static
> key machanism.
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  include/linux/hugetlb.h    | 10 ++++++++--
>  include/linux/page-flags.h |  6 ++++--
>  mm/hugetlb_vmemmap.c       | 12 ++++++------
>  mm/memory_hotplug.c        |  2 +-
>  4 files changed, 19 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 1faebe1cd0ed..4cc647a5dbf8 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -1066,9 +1066,15 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
>  #endif /* CONFIG_HUGETLB_PAGE */
>
>  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> -extern bool hugetlb_free_vmemmap_enabled;
> +static inline bool hugetlb_free_vmemmap_enabled(void)
> +{
> +       return static_key_enabled(&hugetlb_free_vmemmap_enabled_key);

could it be
       if (static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
                                &hugetlb_free_vmemmap_enabled_key))

then we are able to remove the duplication in page_fixed_fake_head()?

> +}
>  #else
> -#define hugetlb_free_vmemmap_enabled   false
> +static inline bool hugetlb_free_vmemmap_enabled(void)
> +{
> +       return false;
> +}
>  #endif
>
>  static inline spinlock_t *huge_pte_lock(struct hstate *h,
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index b47a7f51d2c3..54e119e44496 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -185,7 +185,8 @@ enum pageflags {
>  #ifndef __GENERATING_BOUNDS_H
>
>  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> -extern bool hugetlb_free_vmemmap_enabled;
> +DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> +                        hugetlb_free_vmemmap_enabled_key);
>
>  /*
>   * If the feature of freeing some vmemmap pages associated with each HugeTLB
> @@ -205,7 +206,8 @@ extern bool hugetlb_free_vmemmap_enabled;
>   */
>  static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
>  {
> -       if (!hugetlb_free_vmemmap_enabled)
> +       if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> +                                &hugetlb_free_vmemmap_enabled_key))
>                 return page;
>         /*
>          * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 527bcaa44a48..47517e878ed5 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -188,9 +188,9 @@
>  #define RESERVE_VMEMMAP_NR             1U
>  #define RESERVE_VMEMMAP_SIZE           (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -bool hugetlb_free_vmemmap_enabled __read_mostly =
> -       IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> -EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
> +DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> +                       hugetlb_free_vmemmap_enabled_key);
> +EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
>
>  static int __init early_hugetlb_free_vmemmap_param(char *buf)
>  {
> @@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
>                 return -EINVAL;
>
>         if (!strcmp(buf, "on"))
> -               hugetlb_free_vmemmap_enabled = true;
> +               static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
>         else if (!strcmp(buf, "off"))
> -               hugetlb_free_vmemmap_enabled = false;
> +               static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
>         else
>                 return -EINVAL;
>
> @@ -284,7 +284,7 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
>         BUILD_BUG_ON(__NR_USED_SUBPAGE >=
>                      RESERVE_VMEMMAP_SIZE / sizeof(struct page));
>
> -       if (!hugetlb_free_vmemmap_enabled)
> +       if (!hugetlb_free_vmemmap_enabled())
>                 return;
>
>         vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 0488eed3327c..89c1fde02162 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1341,7 +1341,7 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
>          *       populate a single PMD.
>          */
>         return memmap_on_memory &&
> -              !hugetlb_free_vmemmap_enabled &&
> +              !hugetlb_free_vmemmap_enabled() &&
>                IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
>                size == memory_block_size_bytes() &&
>                IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
> --
> 2.11.0
>

Thanks
barry

Re: [PATCH v3 4/4] selftests: vm: add a hugetlb test case

[Barry Song]

On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> Since the head vmemmap page frame associated with each HugeTLB page is
> reused, we should hide the PG_head flag of tail struct page from the
> user. Add a tese case to check whether it is work properly. The test
> steps are as follows.
>
>   1) alloc 2MB hugeTLB
>   2) get each page frame
>   3) apply those APIs in each page frame
>   4) Those APIs work completely the same as before.
>
> Reading the flags of a page by /proc/kpageflags is done in
> stable_page_flags(), which has invoked PageHead(), PageTail(),
> PageCompound() and compound_head(). If those APIs work properly, the
> head page must have 15 and 17 bits set. And tail pages must have 16
> and 17 bits set but 15 bit unset. Those flags are checked in
> check_page_flags().
>
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  tools/testing/selftests/vm/vmemmap_hugetlb.c | 144 +++++++++++++++++++++++++++
>  1 file changed, 144 insertions(+)
>  create mode 100644 tools/testing/selftests/vm/vmemmap_hugetlb.c
>
> diff --git a/tools/testing/selftests/vm/vmemmap_hugetlb.c b/tools/testing/selftests/vm/vmemmap_hugetlb.c
> new file mode 100644
> index 000000000000..4cc74dd4c333
> --- /dev/null
> +++ b/tools/testing/selftests/vm/vmemmap_hugetlb.c
> @@ -0,0 +1,144 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * A test case of using hugepage memory in a user application using the
> + * mmap system call with MAP_HUGETLB flag.  Before running this program
> + * make sure the administrator has allocated enough default sized huge
> + * pages to cover the 2 MB allocation.
> + *
> + * For ia64 architecture, Linux kernel reserves Region number 4 for hugepages.
> + * That means the addresses starting with 0x800000... will need to be
> + * specified.  Specifying a fixed address is not required on ppc64, i386
> + * or x86_64.
> + */
> +#include <stdlib.h>
> +#include <stdio.h>
> +#include <unistd.h>
> +#include <sys/mman.h>
> +#include <fcntl.h>
> +
> +#define MAP_LENGTH             (2UL * 1024 * 1024)
> +
> +#ifndef MAP_HUGETLB
> +#define MAP_HUGETLB            0x40000 /* arch specific */
> +#endif
> +
> +#define PAGE_SIZE              4096
> +
> +#define PAGE_COMPOUND_HEAD     (1UL << 15)
> +#define PAGE_COMPOUND_TAIL     (1UL << 16)
> +#define PAGE_HUGE              (1UL << 17)
> +
> +#define HEAD_PAGE_FLAGS                (PAGE_COMPOUND_HEAD | PAGE_HUGE)
> +#define TAIL_PAGE_FLAGS                (PAGE_COMPOUND_TAIL | PAGE_HUGE)
> +
> +#define PM_PFRAME_BITS         55
> +#define PM_PFRAME_MASK         ~((1UL << PM_PFRAME_BITS) - 1)
> +

better to move the comment here:

+ * For ia64 architecture, Linux kernel reserves Region number 4 for hugepages.
+ * That means the addresses starting with 0x800000... will need to be
+ * specified.  Specifying a fixed address is not required on ppc64, i386
+ * or x86_64.

With the change,

Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>

> +/* Only ia64 requires this */
> +#ifdef __ia64__
> +#define MAP_ADDR               (void *)(0x8000000000000000UL)
> +#define MAP_FLAGS              (MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB | MAP_FIXED)
> +#else
> +#define MAP_ADDR               NULL
> +#define MAP_FLAGS              (MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB)
> +#endif
> +
> +static void write_bytes(char *addr, size_t length)
> +{
> +       unsigned long i;
> +
> +       for (i = 0; i < length; i++)
> +               *(addr + i) = (char)i;
> +}
> +
> +static unsigned long virt_to_pfn(void *addr)
> +{
> +       int fd;
> +       unsigned long pagemap;
> +
> +       fd = open("/proc/self/pagemap", O_RDONLY);
> +       if (fd < 0)
> +               return -1UL;
> +
> +       lseek(fd, (unsigned long)addr / PAGE_SIZE * sizeof(pagemap), SEEK_SET);
> +       read(fd, &pagemap, sizeof(pagemap));
> +       close(fd);
> +
> +       return pagemap & ~PM_PFRAME_MASK;
> +}
> +
> +static int check_page_flags(unsigned long pfn)
> +{
> +       int fd, i;
> +       unsigned long pageflags;
> +
> +       fd = open("/proc/kpageflags", O_RDONLY);
> +       if (fd < 0)
> +               return -1;
> +
> +       lseek(fd, pfn * sizeof(pageflags), SEEK_SET);
> +
> +       read(fd, &pageflags, sizeof(pageflags));
> +       if ((pageflags & HEAD_PAGE_FLAGS) != HEAD_PAGE_FLAGS) {
> +               close(fd);
> +               printf("Head page flags (%lx) is invalid\n", pageflags);
> +               return -1;
> +       }
> +
> +       /*
> +        * pages other than the first page must be tail and shouldn't be head;
> +        * this also verifies kernel has correctly set the fake page_head to tail
> +        * while hugetlb_free_vmemmap is enabled.
> +        */
> +       for (i = 1; i < MAP_LENGTH / PAGE_SIZE; i++) {
> +               read(fd, &pageflags, sizeof(pageflags));
> +               if ((pageflags & TAIL_PAGE_FLAGS) != TAIL_PAGE_FLAGS ||
> +                   (pageflags & HEAD_PAGE_FLAGS) == HEAD_PAGE_FLAGS) {
> +                       close(fd);
> +                       printf("Tail page flags (%lx) is invalid\n", pageflags);
> +                       return -1;
> +               }
> +       }
> +
> +       close(fd);
> +
> +       return 0;
> +}
> +
> +int main(int argc, char **argv)
> +{
> +       void *addr;
> +       unsigned long pfn;
> +
> +       addr = mmap(MAP_ADDR, MAP_LENGTH, PROT_READ | PROT_WRITE, MAP_FLAGS, -1, 0);
> +       if (addr == MAP_FAILED) {
> +               perror("mmap");
> +               exit(1);
> +       }
> +
> +       /* Trigger allocation of HugeTLB page. */
> +       write_bytes(addr, MAP_LENGTH);
> +
> +       pfn = virt_to_pfn(addr);
> +       if (pfn == -1UL) {
> +               munmap(addr, MAP_LENGTH);
> +               perror("virt_to_pfn");
> +               exit(1);
> +       }
> +
> +       printf("Returned address is %p whose pfn is %lx\n", addr, pfn);
> +
> +       if (check_page_flags(pfn) < 0) {
> +               munmap(addr, MAP_LENGTH);
> +               perror("check_page_flags");
> +               exit(1);
> +       }
> +
> +       /* munmap() length of MAP_HUGETLB memory must be hugepage aligned */
> +       if (munmap(addr, MAP_LENGTH)) {
> +               perror("munmap");
> +               exit(1);
> +       }
> +
> +       return 0;
> +}
> --
> 2.11.0
>

Thanks
barry

Re: [PATCH v3 1/4] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page

[Muchun Song]

On Fri, Sep 24, 2021 at 6:21 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > Currently, we only free 6 vmemmap pages associated with a 2MB HugeTLB
> > page. However, we can remap all tail vmemmap pages to the page frame
> > mapped to with the head vmemmap page. Finally, we can free 7 vmemmap
> > pages for a 2MB HugeTLB page. It is a fine gain (e.g. we can save
> > extra 2GB memory when there is 1TB HugeTLB pages in the system
> > compared with the current implementation).
> >
> > But the head vmemmap page is not freed to the buddy allocator and all
> > tail vmemmap pages are mapped to the head vmemmap page frame. So we
> > can see more than one struct page struct with PG_head (e.g. 8 per 2 MB
> > HugeTLB page) associated with each HugeTLB page. We should adjust
> > compound_head() to make it returns the real head struct page when the
> > parameter is the tail struct page but with PG_head flag.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt |  2 +-
> >  include/linux/page-flags.h                      | 77 +++++++++++++++++++++++--
> >  mm/hugetlb_vmemmap.c                            | 60 ++++++++++---------
> >  mm/sparse-vmemmap.c                             | 21 +++++++
> >  4 files changed, 128 insertions(+), 32 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 91ba391f9b32..5aaf2f271980 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -1617,7 +1617,7 @@
> >                         [KNL] Reguires CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> >                         enabled.
> >                         Allows heavy hugetlb users to free up some more
> > -                       memory (6 * PAGE_SIZE for each 2MB hugetlb page).
> > +                       memory (7 * PAGE_SIZE for each 2MB hugetlb page).
> >                         Format: { on | off (default) }
> >
> >                         on:  enable the feature
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index a68af80649a4..b47a7f51d2c3 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -184,13 +184,68 @@ enum pageflags {
> >
> >  #ifndef __GENERATING_BOUNDS_H
> >
> > +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > +extern bool hugetlb_free_vmemmap_enabled;
> > +
> > +/*
> > + * If the feature of freeing some vmemmap pages associated with each HugeTLB
> > + * page is enabled, the head vmemmap page frame is reused and all of the tail
> > + * vmemmap addresses map to the head vmemmap page frame (furture details can
> > + * refer to the figure at the head of the mm/hugetlb_vmemmap.c).  In other
> > + * words, there are more than one page struct with PG_head associated with each
> > + * HugeTLB page.  We __know__ that there is only one head page struct, the tail
> > + * page structs with PG_head are fake head page structs.  We need an approach
> > + * to distinguish between those two different types of page structs so that
> > + * compound_head() can return the real head page struct when the parameter is
> > + * the tail page struct but with PG_head.
> > + *
> > + * The page_fixed_fake_head() returns the real head page struct if the @page is
> > + * fake page head, otherwise, returns @page which can either be a true page
> > + * head or tail.
> > + */
> > +static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> > +{
> > +       if (!hugetlb_free_vmemmap_enabled)
> > +               return page;
> > +       /*
> > +        * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> > +        * struct page. The alignment check aims to avoid access the fields (
> > +        * e.g. compound_head) of the @page[1]. It can avoid touch a (possibly)
> > +        * cold cacheline in some cases.
> > +        */
> > +       if (IS_ALIGNED((unsigned long)page, PAGE_SIZE) &&
> > +           test_bit(PG_head, &page->flags)) {
> > +               /*
> > +                * We can safely access the field of the @page[1] with PG_head
> > +                * because the @page is a compound page composed with at least
> > +                * two contiguous pages.
> > +                */
> > +               unsigned long head = READ_ONCE(page[1].compound_head);
> > +
> > +               if (likely(head & 1))
> > +                       return (const struct page *)(head - 1);
> > +       }
> > +       return page;
> > +}
> > +#else
> > +static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> > +{
> > +       return page;
> > +}
> > +#endif
> > +
> > +static __always_inline int page_is_fake_head(struct page *page)
> > +{
> > +       return page_fixed_fake_head(page) != page;
> > +}
> > +
> >  static inline unsigned long _compound_head(const struct page *page)
> >  {
> >         unsigned long head = READ_ONCE(page->compound_head);
> >
> >         if (unlikely(head & 1))
> >                 return head - 1;
> > -       return (unsigned long)page;
> > +       return (unsigned long)page_fixed_fake_head(page);
> >  }
> >
> >  #define compound_head(page)    ((typeof(page))_compound_head(page))
> > @@ -225,12 +280,13 @@ static inline unsigned long _compound_head(const struct page *page)
> >
> >  static __always_inline int PageTail(struct page *page)
> >  {
> > -       return READ_ONCE(page->compound_head) & 1;
> > +       return READ_ONCE(page->compound_head) & 1 || page_is_fake_head(page);
> >  }
> >
> >  static __always_inline int PageCompound(struct page *page)
> >  {
> > -       return test_bit(PG_head, &page->flags) || PageTail(page);
> > +       return test_bit(PG_head, &page->flags) ||
> > +              READ_ONCE(page->compound_head) & 1;
> >  }
> >
> >  #define        PAGE_POISON_PATTERN     -1l
> > @@ -675,7 +731,20 @@ static inline bool test_set_page_writeback(struct page *page)
> >         return set_page_writeback(page);
> >  }
> >
> > -__PAGEFLAG(Head, head, PF_ANY) CLEARPAGEFLAG(Head, head, PF_ANY)
> > +static __always_inline bool folio_test_head(struct folio *folio)
> > +{
> > +       return test_bit(PG_head, folio_flags(folio, FOLIO_PF_ANY));
> > +}
> > +
> > +static __always_inline int PageHead(struct page *page)
> > +{
> > +       PF_POISONED_CHECK(page);
> > +       return test_bit(PG_head, &page->flags) && !page_is_fake_head(page);
> > +}
> > +
> > +__SETPAGEFLAG(Head, head, PF_ANY)
> > +__CLEARPAGEFLAG(Head, head, PF_ANY)
> > +CLEARPAGEFLAG(Head, head, PF_ANY)
> >
> >  /* Whether there are one or multiple pages in a folio */
> >  static inline bool folio_test_single(struct folio *folio)
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index c540c21e26f5..527bcaa44a48 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -124,9 +124,9 @@
> >   * 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
> > + * Therefore, we can remap pages 1 to 7 to page 0. Only 1 pages of page structs
>
> 1 page.

Great. Thanks

>
> >   * will be used for each HugeTLB page. This will allow us to free the remaining
> > - * 6 pages to the buddy allocator.
> > + * 7 pages to the buddy allocator.
> >   *
> >   * Here is how things look after remapping.
> >   *
> > @@ -134,30 +134,30 @@
> >   * +-----------+ ---virt_to_page---> +-----------+   mapping to   +-----------+
> >   * |           |                     |     0     | -------------> |     0     |
> >   * |           |                     +-----------+                +-----------+
> > - * |           |                     |     1     | -------------> |     1     |
> > - * |           |                     +-----------+                +-----------+
> > - * |           |                     |     2     | ----------------^ ^ ^ ^ ^ ^
> > - * |           |                     +-----------+                   | | | | |
> > - * |           |                     |     3     | ------------------+ | | | |
> > - * |           |                     +-----------+                     | | | |
> > - * |           |                     |     4     | --------------------+ | | |
> > - * |    PMD    |                     +-----------+                       | | |
> > - * |   level   |                     |     5     | ----------------------+ | |
> > - * |  mapping  |                     +-----------+                         | |
> > - * |           |                     |     6     | ------------------------+ |
> > - * |           |                     +-----------+                           |
> > - * |           |                     |     7     | --------------------------+
> > + * |           |                     |     1     | ---------------^ ^ ^ ^ ^ ^ ^
> > + * |           |                     +-----------+                  | | | | | |
> > + * |           |                     |     2     | -----------------+ | | | | |
> > + * |           |                     +-----------+                    | | | | |
> > + * |           |                     |     3     | -------------------+ | | | |
> > + * |           |                     +-----------+                      | | | |
> > + * |           |                     |     4     | ---------------------+ | | |
> > + * |    PMD    |                     +-----------+                        | | |
> > + * |   level   |                     |     5     | -----------------------+ | |
> > + * |  mapping  |                     +-----------+                          | |
> > + * |           |                     |     6     | -------------------------+ |
> > + * |           |                     +-----------+                            |
> > + * |           |                     |     7     | ---------------------------+
> >   * |           |                     +-----------+
> >   * |           |
> >   * |           |
> >   * |           |
> >   * +-----------+
> >   *
> > - * When a HugeTLB is freed to the buddy system, we should allocate 6 pages for
> > + * When a HugeTLB is freed to the buddy system, we should allocate 7 pages for
> >   * vmemmap pages and restore the previous mapping relationship.
> >   *
> >   * For the HugeTLB page of the pud level mapping. It is similar to the former.
> > - * We also can use this approach to free (PAGE_SIZE - 2) vmemmap pages.
> > + * We also can use this approach to free (PAGE_SIZE - 1) vmemmap pages.
> >   *
> >   * Apart from the HugeTLB page of the pmd/pud level mapping, some architectures
> >   * (e.g. aarch64) provides a contiguous bit in the translation table entries
> > @@ -166,7 +166,13 @@
> >   *
> >   * The contiguous bit is used to increase the mapping size at the pmd and pte
> >   * (last) level. So this type of HugeTLB page can be optimized only when its
> > - * size of the struct page structs is greater than 2 pages.
> > + * size of the struct page structs is greater than 1 pages.
>
> 1 page.

Great. Thanks

>
> > + *
> > + * Notice: The head vmemmap page is not freed to the buddy allocator and all
> > + * tail vmemmap pages are mapped to the head vmemmap page frame. So we can see
> > + * more than one struct page struct with PG_head (e.g. 8 per 2 MB HugeTLB page)
> > + * associated with each HugeTLB page. The compound_head() can handle this
> > + * correctly (more details refer to the comment above compound_head()).
> >   */
> >  #define pr_fmt(fmt)    "HugeTLB: " fmt
> >
> > @@ -175,14 +181,16 @@
> >  /*
> >   * There are a lot of struct page structures associated with each HugeTLB page.
> >   * For tail pages, the value of compound_head is the same. So we can reuse first
> > - * page of tail page structures. We map the virtual addresses of the remaining
> > - * pages of tail page structures to the first tail page struct, and then free
> > - * these page frames. Therefore, we need to reserve two pages as vmemmap areas.
> > + * page of head page structures. We map the virtual addresses of all the pages
> > + * of tail page structures to the head page struct, and then free these page
> > + * frames. Therefore, we need to reserve one pages as vmemmap areas.
> >   */
> > -#define RESERVE_VMEMMAP_NR             2U
> > +#define RESERVE_VMEMMAP_NR             1U
> >  #define RESERVE_VMEMMAP_SIZE           (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
> >
> > -bool hugetlb_free_vmemmap_enabled = IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> > +bool hugetlb_free_vmemmap_enabled __read_mostly =
> > +       IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> > +EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
> >
> >  static int __init early_hugetlb_free_vmemmap_param(char *buf)
> >  {
> > @@ -236,7 +244,6 @@ int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> >          */
> >         ret = vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> >                                   GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
> > -
> >         if (!ret)
> >                 ClearHPageVmemmapOptimized(head);
> >
> > @@ -282,9 +289,8 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> >
> >         vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
> >         /*
> > -        * The head page and the first tail page are not to be freed to buddy
> > -        * allocator, the other pages will map to the first tail page, so they
> > -        * can be freed.
> > +        * The head page is not to be freed to buddy allocator, the other tail
> > +        * pages will map to the head page, so they can be freed.
> >          *
> >          * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
> >          * on some architectures (e.g. aarch64). See Documentation/arm64/
> > diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> > index bdce883f9286..62e3d20648ce 100644
> > --- a/mm/sparse-vmemmap.c
> > +++ b/mm/sparse-vmemmap.c
> > @@ -53,6 +53,17 @@ struct vmemmap_remap_walk {
> >         struct list_head *vmemmap_pages;
> >  };
> >
> > +/*
> > + * How many struct page structs need to be reset. When we reuse the head
> > + * struct page, the special metadata (e.g. page->flags or page->mapping)
> > + * cannot copy to the tail struct page structs. The invalid value will be
> > + * checked in the free_tail_pages_check(). In order to avoid the message
> > + * of "corrupted mapping in tail page". We need to reset at least 3 (one
> > + * head struct page struct and two tail struct page structs) struct page
> > + * structs.
> > + */
> > +#define NR_RESET_STRUCT_PAGE           3
> > +
> >  static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start,
> >                                   struct vmemmap_remap_walk *walk)
> >  {
> > @@ -245,6 +256,15 @@ static void vmemmap_remap_pte(pte_t *pte, unsigned long addr,
> >         set_pte_at(&init_mm, addr, pte, entry);
> >  }
> >
> > +static inline void reset_struct_pages(struct page *start)
> > +{
> > +       int i;
> > +       struct page *from = start + NR_RESET_STRUCT_PAGE;
> > +
> > +       for (i = 0; i < NR_RESET_STRUCT_PAGE; i++)
> > +               memcpy(start + i, from, sizeof(*from));
> > +}
> > +
> >  static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> >                                 struct vmemmap_remap_walk *walk)
> >  {
> > @@ -258,6 +278,7 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> >         list_del(&page->lru);
> >         to = page_to_virt(page);
> >         copy_page(to, (void *)walk->reuse_addr);
> > +       reset_struct_pages(to);
>
> Is this done unconditionally? Are we going here while
> hugetlb_free_vmemmap_enabled=false?

The whole thing is only used in the case of
hugetlb_free_vmemmap_enabled=false.

Maybe we could make vmemmap_remap_free()
and vmemmap_remap_alloc() compiled under
CONFIG_HUGETLB_PAGE_FREE_VMEMMAP.
But this should be in another separate patch.

Thanks.

>
> >
> >         set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> >  }
> > --
> > 2.11.0
> >
>
> Thanks
> barry

Re: [PATCH v3 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key

[Muchun Song]

On Fri, Sep 24, 2021 at 6:24 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > The page_head_if_fake() is used throughout memory management and the
> > conditional check requires checking a global variable, although the
> > overhead of this check may be small, it increases when the memory
> > cache comes under pressure. Also, the global variable will not be
> > modified after system boot, so it is very appropriate to use static
> > key machanism.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  include/linux/hugetlb.h    | 10 ++++++++--
> >  include/linux/page-flags.h |  6 ++++--
> >  mm/hugetlb_vmemmap.c       | 12 ++++++------
> >  mm/memory_hotplug.c        |  2 +-
> >  4 files changed, 19 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 1faebe1cd0ed..4cc647a5dbf8 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -1066,9 +1066,15 @@ static inline void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr
> >  #endif /* CONFIG_HUGETLB_PAGE */
> >
> >  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > -extern bool hugetlb_free_vmemmap_enabled;
> > +static inline bool hugetlb_free_vmemmap_enabled(void)
> > +{
> > +       return static_key_enabled(&hugetlb_free_vmemmap_enabled_key);
>
> could it be
>        if (static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
>                                 &hugetlb_free_vmemmap_enabled_key))
>
> then we are able to remove the duplication in page_fixed_fake_head()?

Definitely. I'll update in the next version.

Thanks.

>
> > +}
> >  #else
> > -#define hugetlb_free_vmemmap_enabled   false
> > +static inline bool hugetlb_free_vmemmap_enabled(void)
> > +{
> > +       return false;
> > +}
> >  #endif
> >
> >  static inline spinlock_t *huge_pte_lock(struct hstate *h,
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index b47a7f51d2c3..54e119e44496 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -185,7 +185,8 @@ enum pageflags {
> >  #ifndef __GENERATING_BOUNDS_H
> >
> >  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> > -extern bool hugetlb_free_vmemmap_enabled;
> > +DECLARE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> > +                        hugetlb_free_vmemmap_enabled_key);
> >
> >  /*
> >   * If the feature of freeing some vmemmap pages associated with each HugeTLB
> > @@ -205,7 +206,8 @@ extern bool hugetlb_free_vmemmap_enabled;
> >   */
> >  static __always_inline const struct page *page_fixed_fake_head(const struct page *page)
> >  {
> > -       if (!hugetlb_free_vmemmap_enabled)
> > +       if (!static_branch_maybe(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> > +                                &hugetlb_free_vmemmap_enabled_key))
> >                 return page;
> >         /*
> >          * Only addresses aligned with PAGE_SIZE of struct page may be fake head
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 527bcaa44a48..47517e878ed5 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -188,9 +188,9 @@
> >  #define RESERVE_VMEMMAP_NR             1U
> >  #define RESERVE_VMEMMAP_SIZE           (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
> >
> > -bool hugetlb_free_vmemmap_enabled __read_mostly =
> > -       IS_ENABLED(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON);
> > -EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled);
> > +DEFINE_STATIC_KEY_MAYBE(CONFIG_HUGETLB_PAGE_FREE_VMEMMAP_DEFAULT_ON,
> > +                       hugetlb_free_vmemmap_enabled_key);
> > +EXPORT_SYMBOL(hugetlb_free_vmemmap_enabled_key);
> >
> >  static int __init early_hugetlb_free_vmemmap_param(char *buf)
> >  {
> > @@ -204,9 +204,9 @@ static int __init early_hugetlb_free_vmemmap_param(char *buf)
> >                 return -EINVAL;
> >
> >         if (!strcmp(buf, "on"))
> > -               hugetlb_free_vmemmap_enabled = true;
> > +               static_branch_enable(&hugetlb_free_vmemmap_enabled_key);
> >         else if (!strcmp(buf, "off"))
> > -               hugetlb_free_vmemmap_enabled = false;
> > +               static_branch_disable(&hugetlb_free_vmemmap_enabled_key);
> >         else
> >                 return -EINVAL;
> >
> > @@ -284,7 +284,7 @@ void __init hugetlb_vmemmap_init(struct hstate *h)
> >         BUILD_BUG_ON(__NR_USED_SUBPAGE >=
> >                      RESERVE_VMEMMAP_SIZE / sizeof(struct page));
> >
> > -       if (!hugetlb_free_vmemmap_enabled)
> > +       if (!hugetlb_free_vmemmap_enabled())
> >                 return;
> >
> >         vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> > index 0488eed3327c..89c1fde02162 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -1341,7 +1341,7 @@ bool mhp_supports_memmap_on_memory(unsigned long size)
> >          *       populate a single PMD.
> >          */
> >         return memmap_on_memory &&
> > -              !hugetlb_free_vmemmap_enabled &&
> > +              !hugetlb_free_vmemmap_enabled() &&
> >                IS_ENABLED(CONFIG_MHP_MEMMAP_ON_MEMORY) &&
> >                size == memory_block_size_bytes() &&
> >                IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
> > --
> > 2.11.0
> >
>
> Thanks
> barry

Re: [PATCH v3 4/4] selftests: vm: add a hugetlb test case

[Muchun Song]

On Fri, Sep 24, 2021 at 6:29 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@bytedance.com> wrote:
> >
> > Since the head vmemmap page frame associated with each HugeTLB page is
> > reused, we should hide the PG_head flag of tail struct page from the
> > user. Add a tese case to check whether it is work properly. The test
> > steps are as follows.
> >
> >   1) alloc 2MB hugeTLB
> >   2) get each page frame
> >   3) apply those APIs in each page frame
> >   4) Those APIs work completely the same as before.
> >
> > Reading the flags of a page by /proc/kpageflags is done in
> > stable_page_flags(), which has invoked PageHead(), PageTail(),
> > PageCompound() and compound_head(). If those APIs work properly, the
> > head page must have 15 and 17 bits set. And tail pages must have 16
> > and 17 bits set but 15 bit unset. Those flags are checked in
> > check_page_flags().
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  tools/testing/selftests/vm/vmemmap_hugetlb.c | 144 +++++++++++++++++++++++++++
> >  1 file changed, 144 insertions(+)
> >  create mode 100644 tools/testing/selftests/vm/vmemmap_hugetlb.c
> >
> > diff --git a/tools/testing/selftests/vm/vmemmap_hugetlb.c b/tools/testing/selftests/vm/vmemmap_hugetlb.c
> > new file mode 100644
> > index 000000000000..4cc74dd4c333
> > --- /dev/null
> > +++ b/tools/testing/selftests/vm/vmemmap_hugetlb.c
> > @@ -0,0 +1,144 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * A test case of using hugepage memory in a user application using the
> > + * mmap system call with MAP_HUGETLB flag.  Before running this program
> > + * make sure the administrator has allocated enough default sized huge
> > + * pages to cover the 2 MB allocation.
> > + *
> > + * For ia64 architecture, Linux kernel reserves Region number 4 for hugepages.
> > + * That means the addresses starting with 0x800000... will need to be
> > + * specified.  Specifying a fixed address is not required on ppc64, i386
> > + * or x86_64.
> > + */
> > +#include <stdlib.h>
> > +#include <stdio.h>
> > +#include <unistd.h>
> > +#include <sys/mman.h>
> > +#include <fcntl.h>
> > +
> > +#define MAP_LENGTH             (2UL * 1024 * 1024)
> > +
> > +#ifndef MAP_HUGETLB
> > +#define MAP_HUGETLB            0x40000 /* arch specific */
> > +#endif
> > +
> > +#define PAGE_SIZE              4096
> > +
> > +#define PAGE_COMPOUND_HEAD     (1UL << 15)
> > +#define PAGE_COMPOUND_TAIL     (1UL << 16)
> > +#define PAGE_HUGE              (1UL << 17)
> > +
> > +#define HEAD_PAGE_FLAGS                (PAGE_COMPOUND_HEAD | PAGE_HUGE)
> > +#define TAIL_PAGE_FLAGS                (PAGE_COMPOUND_TAIL | PAGE_HUGE)
> > +
> > +#define PM_PFRAME_BITS         55
> > +#define PM_PFRAME_MASK         ~((1UL << PM_PFRAME_BITS) - 1)
> > +
>
> better to move the comment here:
>
> + * For ia64 architecture, Linux kernel reserves Region number 4 for hugepages.
> + * That means the addresses starting with 0x800000... will need to be
> + * specified.  Specifying a fixed address is not required on ppc64, i386
> + * or x86_64.
>
> With the change,
>
> Reviewed-by: Barry Song <song.bao.hua@hisilicon.com>

Thanks. Will do.

>
> > +/* Only ia64 requires this */
> > +#ifdef __ia64__
> > +#define MAP_ADDR               (void *)(0x8000000000000000UL)
> > +#define MAP_FLAGS              (MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB | MAP_FIXED)
> > +#else
> > +#define MAP_ADDR               NULL
> > +#define MAP_FLAGS              (MAP_PRIVATE | MAP_ANONYMOUS | MAP_HUGETLB)
> > +#endif
> > +
> > +static void write_bytes(char *addr, size_t length)
> > +{
> > +       unsigned long i;
> > +
> > +       for (i = 0; i < length; i++)
> > +               *(addr + i) = (char)i;
> > +}
> > +
> > +static unsigned long virt_to_pfn(void *addr)
> > +{
> > +       int fd;
> > +       unsigned long pagemap;
> > +
> > +       fd = open("/proc/self/pagemap", O_RDONLY);
> > +       if (fd < 0)
> > +               return -1UL;
> > +
> > +       lseek(fd, (unsigned long)addr / PAGE_SIZE * sizeof(pagemap), SEEK_SET);
> > +       read(fd, &pagemap, sizeof(pagemap));
> > +       close(fd);
> > +
> > +       return pagemap & ~PM_PFRAME_MASK;
> > +}
> > +
> > +static int check_page_flags(unsigned long pfn)
> > +{
> > +       int fd, i;
> > +       unsigned long pageflags;
> > +
> > +       fd = open("/proc/kpageflags", O_RDONLY);
> > +       if (fd < 0)
> > +               return -1;
> > +
> > +       lseek(fd, pfn * sizeof(pageflags), SEEK_SET);
> > +
> > +       read(fd, &pageflags, sizeof(pageflags));
> > +       if ((pageflags & HEAD_PAGE_FLAGS) != HEAD_PAGE_FLAGS) {
> > +               close(fd);
> > +               printf("Head page flags (%lx) is invalid\n", pageflags);
> > +               return -1;
> > +       }
> > +
> > +       /*
> > +        * pages other than the first page must be tail and shouldn't be head;
> > +        * this also verifies kernel has correctly set the fake page_head to tail
> > +        * while hugetlb_free_vmemmap is enabled.
> > +        */
> > +       for (i = 1; i < MAP_LENGTH / PAGE_SIZE; i++) {
> > +               read(fd, &pageflags, sizeof(pageflags));
> > +               if ((pageflags & TAIL_PAGE_FLAGS) != TAIL_PAGE_FLAGS ||
> > +                   (pageflags & HEAD_PAGE_FLAGS) == HEAD_PAGE_FLAGS) {
> > +                       close(fd);
> > +                       printf("Tail page flags (%lx) is invalid\n", pageflags);
> > +                       return -1;
> > +               }
> > +       }
> > +
> > +       close(fd);
> > +
> > +       return 0;
> > +}
> > +
> > +int main(int argc, char **argv)
> > +{
> > +       void *addr;
> > +       unsigned long pfn;
> > +
> > +       addr = mmap(MAP_ADDR, MAP_LENGTH, PROT_READ | PROT_WRITE, MAP_FLAGS, -1, 0);
> > +       if (addr == MAP_FAILED) {
> > +               perror("mmap");
> > +               exit(1);
> > +       }
> > +
> > +       /* Trigger allocation of HugeTLB page. */
> > +       write_bytes(addr, MAP_LENGTH);
> > +
> > +       pfn = virt_to_pfn(addr);
> > +       if (pfn == -1UL) {
> > +               munmap(addr, MAP_LENGTH);
> > +               perror("virt_to_pfn");
> > +               exit(1);
> > +       }
> > +
> > +       printf("Returned address is %p whose pfn is %lx\n", addr, pfn);
> > +
> > +       if (check_page_flags(pfn) < 0) {
> > +               munmap(addr, MAP_LENGTH);
> > +               perror("check_page_flags");
> > +               exit(1);
> > +       }
> > +
> > +       /* munmap() length of MAP_HUGETLB memory must be hugepage aligned */
> > +       if (munmap(addr, MAP_LENGTH)) {
> > +               perror("munmap");
> > +               exit(1);
> > +       }
> > +
> > +       return 0;
> > +}
> > --
> > 2.11.0
> >
>
> Thanks
> barry