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
>
