On Fri, Mar 5, 2021 at 4:55 PM Balbir Singh <bsingharora@xxxxxxxxx> wrote:
>
> 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?
If you mean that we reuse part of a huge page as vmemmap while
freeing. You can look at the discussion here.
https://patchwork.kernel.org/project/linux-mm/patch/20210117151053.24600-6-songmuchun@xxxxxxxxxxxxx/
Thanks.
>
> Balbir
