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

From: Muchun Song
Date: Thu Sep 23 2021 - 22:58:22 EST

Next message: Yang Shi: "Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache"
Previous message: Guo Zhi: "Re: [PATCH] staging: vchip_arm: Fix misuse of %x"
In reply to: Barry Song: "Re: [PATCH v3 1/4] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page"
Next in thread: Muchun Song: "[PATCH v3 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

On Fri, Sep 24, 2021 at 6:21 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Wed, Sep 22, 2021 at 10:27 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > ---
> > 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

Next message: Yang Shi: "Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache"
Previous message: Guo Zhi: "Re: [PATCH] staging: vchip_arm: Fix misuse of %x"
In reply to: Barry Song: "Re: [PATCH v3 1/4] mm: hugetlb: free the 2nd vmemmap page associated with each HugeTLB page"
Next in thread: Muchun Song: "[PATCH v3 2/4] mm: hugetlb: replace hugetlb_free_vmemmap_enabled with a static_key"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]