Re: [PATCH v17 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Thu, Feb 25, 2021 at 9:24 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> When we free a HugeTLB page to the buddy allocator, we should allocate
> the vmemmap pages associated with it. But we may cannot allocate vmemmap
> pages when the system is under memory pressure, in this case, we just
> refuse to free the HugeTLB page instead of looping forever trying to
> allocate the pages. This changes some behavior (list below) on some
> corner cases.
>
>  1) Failing to free a huge page triggered by the user (decrease nr_pages).
>
>     Need try again later by the user.
>
>  2) Failing to free a surplus huge page when freed by the application.
>
>     Try again later when freeing a huge page next time.
>
>  3) Failing to dissolve a free huge page on ZONE_MOVABLE via
>     offline_pages().
>
>     This is a bit unfortunate if we have plenty of ZONE_MOVABLE memory
>     but are low on kernel memory. For example, migration of huge pages
>     would still work, however, dissolving the free page does not work.
>     This is a corner cases. When the system is that much under memory
>     pressure, offlining/unplug can be expected to fail. This is
>     unfortunate because it prevents from the memory offlining which
>     shouldn't happen for movable zones. People depending on the memory
>     hotplug and movable zone should carefuly consider whether savings
>     on unmovable memory are worth losing their hotplug functionality
>     in some situations.
>
>  4) Failing to dissolve a huge page on CMA/ZONE_MOVABLE via
>     alloc_contig_range() - once we have that handling in place. Mainly
>     affects CMA and virtio-mem.
>
>     Similar to 3). virito-mem will handle migration errors gracefully.
>     CMA might be able to fallback on other free areas within the CMA
>     region.
>
> Vmemmap pages are allocated from the page freeing context. In order for
> those allocations to be not disruptive (e.g. trigger oom killer)
> __GFP_NORETRY is used. hugetlb_lock is dropped for the allocation
> because a non sleeping allocation would be too fragile and it could fail
> too easily under memory pressure. GFP_ATOMIC or other modes to access
> memory reserves is not used because we want to prevent consuming
> reserves under heavy hugetlb freeing.

Hi,

Since this patch is the only patch that has no reviewed-by tag.
I hope someone (e.g. Mike, Oscar, David or Michal) could review
this. Thanks a lot.

>
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
>  Documentation/admin-guide/mm/hugetlbpage.rst |  8 +++
>  include/linux/mm.h                           |  2 +
>  mm/hugetlb.c                                 | 92 +++++++++++++++++++++-------
>  mm/hugetlb_vmemmap.c                         | 32 ++++++----
>  mm/hugetlb_vmemmap.h                         | 23 +++++++
>  mm/sparse-vmemmap.c                          | 75 ++++++++++++++++++++++-
>  6 files changed, 197 insertions(+), 35 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> index f7b1c7462991..6988895d09a8 100644
> --- a/Documentation/admin-guide/mm/hugetlbpage.rst
> +++ b/Documentation/admin-guide/mm/hugetlbpage.rst
> @@ -60,6 +60,10 @@ HugePages_Surp
>          the pool above the value in ``/proc/sys/vm/nr_hugepages``. The
>          maximum number of surplus huge pages is controlled by
>          ``/proc/sys/vm/nr_overcommit_hugepages``.
> +       Note: When the feature of freeing unused vmemmap pages associated
> +       with each hugetlb page is enabled, the number of surplus huge pages
> +       may be temporarily larger than the maximum number of surplus huge
> +       pages when the system is under memory pressure.
>  Hugepagesize
>         is the default hugepage size (in Kb).
>  Hugetlb
> @@ -80,6 +84,10 @@ returned to the huge page pool when freed by a task.  A user with root
>  privileges can dynamically allocate more or free some persistent huge pages
>  by increasing or decreasing the value of ``nr_hugepages``.
>
> +Note: When the feature of freeing unused vmemmap pages associated with each
> +hugetlb page is enabled, we can fail to free the huge pages triggered by
> +the user when ths system is under memory pressure.  Please try again later.
> +
>  Pages that are used as huge pages are reserved inside the kernel and cannot
>  be used for other purposes.  Huge pages cannot be swapped out under
>  memory pressure.
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4ddfc31f21c6..77693c944a36 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2973,6 +2973,8 @@ static inline void print_vma_addr(char *prefix, unsigned long rip)
>
>  void vmemmap_remap_free(unsigned long start, unsigned long end,
>                         unsigned long reuse);
> +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> +                       unsigned long reuse, gfp_t gfp_mask);
>
>  void *sparse_buffer_alloc(unsigned long size);
>  struct page * __populate_section_memmap(unsigned long pfn,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 43fed6785322..b6e4e3f31ad2 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1304,16 +1304,59 @@ static inline void destroy_compound_gigantic_page(struct page *page,
>                                                 unsigned int order) { }
>  #endif
>
> -static void update_and_free_page(struct hstate *h, struct page *page)
> +static int update_and_free_page(struct hstate *h, struct page *page)
> +       __releases(&hugetlb_lock) __acquires(&hugetlb_lock)
>  {
>         int i;
>         struct page *subpage = page;
> +       int nid = page_to_nid(page);
>
>         if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> -               return;
> +               return 0;
>
>         h->nr_huge_pages--;
> -       h->nr_huge_pages_node[page_to_nid(page)]--;
> +       h->nr_huge_pages_node[nid]--;
> +       VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> +       VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> +       set_page_refcounted(page);
> +       set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> +
> +       /*
> +        * If the vmemmap pages associated with the HugeTLB page can be
> +        * optimized or the page is gigantic, we might block in
> +        * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
> +        * cases, drop the hugetlb_lock.
> +        */
> +       if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> +               spin_unlock(&hugetlb_lock);
> +
> +       if (alloc_huge_page_vmemmap(h, page)) {
> +               spin_lock(&hugetlb_lock);
> +               INIT_LIST_HEAD(&page->lru);
> +               set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> +               h->nr_huge_pages++;
> +               h->nr_huge_pages_node[nid]++;
> +
> +               /*
> +                * If we cannot allocate vmemmap pages, just refuse to free the
> +                * page and put the page back on the hugetlb free list and treat
> +                * as a surplus page.
> +                */
> +               h->surplus_huge_pages++;
> +               h->surplus_huge_pages_node[nid]++;
> +
> +               /*
> +                * The refcount can be perfectly increased by memory-failure or
> +                * soft_offline handlers.
> +                */
> +               if (likely(put_page_testzero(page))) {
> +                       arch_clear_hugepage_flags(page);
> +                       enqueue_huge_page(h, page);
> +               }
> +
> +               return -ENOMEM;
> +       }
> +
>         for (i = 0; i < pages_per_huge_page(h);
>              i++, subpage = mem_map_next(subpage, page, i)) {
>                 subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
> @@ -1321,22 +1364,18 @@ static void update_and_free_page(struct hstate *h, struct page *page)
>                                 1 << PG_active | 1 << PG_private |
>                                 1 << PG_writeback);
>         }
> -       VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> -       VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);
> -       set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> -       set_page_refcounted(page);
> +
>         if (hstate_is_gigantic(h)) {
> -               /*
> -                * Temporarily drop the hugetlb_lock, because
> -                * we might block in free_gigantic_page().
> -                */
> -               spin_unlock(&hugetlb_lock);
>                 destroy_compound_gigantic_page(page, huge_page_order(h));
>                 free_gigantic_page(page, huge_page_order(h));
> -               spin_lock(&hugetlb_lock);
>         } else {
>                 __free_pages(page, huge_page_order(h));
>         }
> +
> +       if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> +               spin_lock(&hugetlb_lock);
> +
> +       return 0;
>  }
>
>  struct hstate *size_to_hstate(unsigned long size)
> @@ -1404,9 +1443,9 @@ static void __free_huge_page(struct page *page)
>         } else if (h->surplus_huge_pages_node[nid]) {
>                 /* remove the page from active list */
>                 list_del(&page->lru);
> -               update_and_free_page(h, page);
>                 h->surplus_huge_pages--;
>                 h->surplus_huge_pages_node[nid]--;
> +               update_and_free_page(h, page);
>         } else {
>                 arch_clear_hugepage_flags(page);
>                 enqueue_huge_page(h, page);
> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
>         /*
>          * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
>          */
> -       if (!in_task()) {
> +       if (!in_atomic()) {
>                 /*
>                  * Only call schedule_work() if hpage_freelist is previously
>                  * empty. Otherwise, schedule_work() had been called but the
> @@ -1699,8 +1738,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
>                                 h->surplus_huge_pages--;
>                                 h->surplus_huge_pages_node[node]--;
>                         }
> -                       update_and_free_page(h, page);
> -                       ret = 1;
> +                       ret = !update_and_free_page(h, page);
>                         break;
>                 }
>         }
> @@ -1713,10 +1751,14 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
>   * nothing for in-use hugepages and non-hugepages.
>   * This function returns values like below:
>   *
> - *  -EBUSY: failed to dissolved free hugepages or the hugepage is in-use
> - *          (allocated or reserved.)
> - *       0: successfully dissolved free hugepages or the page is not a
> - *          hugepage (considered as already dissolved)
> + *  -ENOMEM: failed to allocate vmemmap pages to free the freed hugepages
> + *           when the system is under memory pressure and the feature of
> + *           freeing unused vmemmap pages associated with each hugetlb page
> + *           is enabled.
> + *  -EBUSY:  failed to dissolved free hugepages or the hugepage is in-use
> + *           (allocated or reserved.)
> + *       0:  successfully dissolved free hugepages or the page is not a
> + *           hugepage (considered as already dissolved)
>   */
>  int dissolve_free_huge_page(struct page *page)
>  {
> @@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
>                 h->free_huge_pages--;
>                 h->free_huge_pages_node[nid]--;
>                 h->max_huge_pages--;
> -               update_and_free_page(h, head);
> -               rc = 0;
> +               rc = update_and_free_page(h, head);
> +               if (rc) {
> +                       h->surplus_huge_pages--;
> +                       h->surplus_huge_pages_node[nid]--;
> +                       h->max_huge_pages++;
> +               }
>         }
>  out:
>         spin_unlock(&hugetlb_lock);
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 0209b736e0b4..f7ab3d99250a 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -181,21 +181,31 @@
>  #define RESERVE_VMEMMAP_NR             2U
>  #define RESERVE_VMEMMAP_SIZE           (RESERVE_VMEMMAP_NR << PAGE_SHIFT)
>
> -/*
> - * How many vmemmap pages associated with a HugeTLB page that can be freed
> - * to the buddy allocator.
> - *
> - * Todo: Returns zero for now, which means the feature is disabled. We will
> - * enable it once all the infrastructure is there.
> - */
> -static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
>  {
> -       return 0;
> +       return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
>  }
>
> -static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
>  {
> -       return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> +       unsigned long vmemmap_addr = (unsigned long)head;
> +       unsigned long vmemmap_end, vmemmap_reuse;
> +
> +       if (!free_vmemmap_pages_per_hpage(h))
> +               return 0;
> +
> +       vmemmap_addr += RESERVE_VMEMMAP_SIZE;
> +       vmemmap_end = vmemmap_addr + free_vmemmap_pages_size_per_hpage(h);
> +       vmemmap_reuse = vmemmap_addr - PAGE_SIZE;
> +       /*
> +        * The pages which the vmemmap virtual address range [@vmemmap_addr,
> +        * @vmemmap_end) are mapped to are freed to the buddy allocator, and
> +        * the range is mapped to the page which @vmemmap_reuse is mapped to.
> +        * When a HugeTLB page is freed to the buddy allocator, previously
> +        * discarded vmemmap pages must be allocated and remapping.
> +        */
> +       return vmemmap_remap_alloc(vmemmap_addr, vmemmap_end, vmemmap_reuse,
> +                                  GFP_KERNEL | __GFP_NORETRY | __GFP_THISNODE);
>  }
>
>  void free_huge_page_vmemmap(struct hstate *h, struct page *head)
> diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> index 6923f03534d5..a37771b0b82a 100644
> --- a/mm/hugetlb_vmemmap.h
> +++ b/mm/hugetlb_vmemmap.h
> @@ -11,10 +11,33 @@
>  #include <linux/hugetlb.h>
>
>  #ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head);
>  void free_huge_page_vmemmap(struct hstate *h, struct page *head);
> +
> +/*
> + * How many vmemmap pages associated with a HugeTLB page that can be freed
> + * to the buddy allocator.
> + *
> + * Todo: Returns zero for now, which means the feature is disabled. We will
> + * enable it once all the infrastructure is there.
> + */
> +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +{
> +       return 0;
> +}
>  #else
> +static inline int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> +{
> +       return 0;
> +}
> +
>  static inline void free_huge_page_vmemmap(struct hstate *h, struct page *head)
>  {
>  }
> +
> +static inline unsigned int free_vmemmap_pages_per_hpage(struct hstate *h)
> +{
> +       return 0;
> +}
>  #endif /* CONFIG_HUGETLB_PAGE_FREE_VMEMMAP */
>  #endif /* _LINUX_HUGETLB_VMEMMAP_H */
> diff --git a/mm/sparse-vmemmap.c b/mm/sparse-vmemmap.c
> index d3076a7a3783..60fc6cd6cd23 100644
> --- a/mm/sparse-vmemmap.c
> +++ b/mm/sparse-vmemmap.c
> @@ -40,7 +40,8 @@
>   * @remap_pte:         called for each lowest-level entry (PTE).
>   * @reuse_page:                the page which is reused for the tail vmemmap pages.
>   * @reuse_addr:                the virtual address of the @reuse_page page.
> - * @vmemmap_pages:     the list head of the vmemmap pages that can be freed.
> + * @vmemmap_pages:     the list head of the vmemmap pages that can be freed
> + *                     or is mapped from.
>   */
>  struct vmemmap_remap_walk {
>         void (*remap_pte)(pte_t *pte, unsigned long addr,
> @@ -237,6 +238,78 @@ void vmemmap_remap_free(unsigned long start, unsigned long end,
>         free_vmemmap_page_list(&vmemmap_pages);
>  }
>
> +static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
> +                               struct vmemmap_remap_walk *walk)
> +{
> +       pgprot_t pgprot = PAGE_KERNEL;
> +       struct page *page;
> +       void *to;
> +
> +       BUG_ON(pte_page(*pte) != walk->reuse_page);
> +
> +       page = list_first_entry(walk->vmemmap_pages, struct page, lru);
> +       list_del(&page->lru);
> +       to = page_to_virt(page);
> +       copy_page(to, (void *)walk->reuse_addr);
> +
> +       set_pte_at(&init_mm, addr, pte, mk_pte(page, pgprot));
> +}
> +
> +static int alloc_vmemmap_page_list(unsigned long start, unsigned long end,
> +                                  gfp_t gfp_mask, struct list_head *list)
> +{
> +       unsigned long nr_pages = (end - start) >> PAGE_SHIFT;
> +       int nid = page_to_nid((struct page *)start);
> +       struct page *page, *next;
> +
> +       while (nr_pages--) {
> +               page = alloc_pages_node(nid, gfp_mask, 0);
> +               if (!page)
> +                       goto out;
> +               list_add_tail(&page->lru, list);
> +       }
> +
> +       return 0;
> +out:
> +       list_for_each_entry_safe(page, next, list, lru)
> +               __free_pages(page, 0);
> +       return -ENOMEM;
> +}
> +
> +/**
> + * vmemmap_remap_alloc - remap the vmemmap virtual address range [@start, end)
> + *                      to the page which is from the @vmemmap_pages
> + *                      respectively.
> + * @start:     start address of the vmemmap virtual address range that we want
> + *             to remap.
> + * @end:       end address of the vmemmap virtual address range that we want to
> + *             remap.
> + * @reuse:     reuse address.
> + * @gpf_mask:  GFP flag for allocating vmemmap pages.
> + */
> +int vmemmap_remap_alloc(unsigned long start, unsigned long end,
> +                       unsigned long reuse, gfp_t gfp_mask)
> +{
> +       LIST_HEAD(vmemmap_pages);
> +       struct vmemmap_remap_walk walk = {
> +               .remap_pte      = vmemmap_restore_pte,
> +               .reuse_addr     = reuse,
> +               .vmemmap_pages  = &vmemmap_pages,
> +       };
> +
> +       /* See the comment in the vmemmap_remap_free(). */
> +       BUG_ON(start - reuse != PAGE_SIZE);
> +
> +       might_sleep_if(gfpflags_allow_blocking(gfp_mask));
> +
> +       if (alloc_vmemmap_page_list(start, end, gfp_mask, &vmemmap_pages))
> +               return -ENOMEM;
> +
> +       vmemmap_remap_range(reuse, end, &walk);
> +
> +       return 0;
> +}
> +
>  /*
>   * Allocate a block of memory to be used to back the virtual memory map
>   * or to back the page tables that are used to create the mapping.
> --
> 2.11.0
>