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

[Barry Song <21cnbao@gmail.com>]

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