On Thu, Feb 25, 2021 at 09:21:25PM +0800, Muchun Song 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. > > 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; This is where I think some optimization is possible, once we are done with vmemmap_end calculation, we can use 6 pages (for 2MiB huge page) as pages for struct page. Is there a reason to not do so? Balbir