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SubjectRe: [External] Re: [PATCH v16 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
On Tue, Feb 23, 2021 at 8:01 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 2/19/21 2:49 AM, 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.
>
> Thank you for listing changes in behavior and possible side effects of
> not being able to allocate vmemmmap and free huge page to buddy!
>
> I will not repeat Michal's comment about the check for an atomic context
> in free_huge_page path.
>
> >
> > 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.
> >
> > 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.
> >
> > We do not want to use GFP_ATOMIC to allocate vmemmap pages. Because it
> > grants access to memory reserves and we do not think it is reasonable
> > to use memory reserves. We use GFP_KERNEL in alloc_huge_page_vmemmap().
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> > Documentation/admin-guide/mm/hugetlbpage.rst | 8 +++
> > include/linux/mm.h | 2 +
> > mm/hugetlb.c | 81 ++++++++++++++++++++--------
> > mm/hugetlb_vmemmap.c | 22 ++++++++
> > mm/hugetlb_vmemmap.h | 6 +++
> > mm/sparse-vmemmap.c | 75 +++++++++++++++++++++++++-
> > 6 files changed, 171 insertions(+), 23 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/mm/hugetlbpage.rst b/Documentation/admin-guide/mm/hugetlbpage.rst
> > index f7b1c7462991..fb8f649e5635 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 the surplus huge
>
> Small wording change:
>
> with each hugetlb page is enabled, the number of surplus huge

Thanks. I will update this.

>
> > + 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 failed to free the huge pages triggered by
>
> Small wording change:
>
> hugetlb page is enabled, we can fail to free the huge pages triggered by

Thanks. I will update this.

>
> > +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 d7dddf334779..33c5911afe18 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2981,6 +2981,8 @@ 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 4cfca27c6d32..bcf856974c48 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1305,37 +1305,68 @@ 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;
> > + 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_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> > + set_page_refcounted(page);
>
> I think you added the set_page_refcounted() because the huge page will
> appear as just a compound page without a reference after dropping the
> hugetlb lock?

Right.

> It might be better to set the reference before modifying
> the destructor. Otherwise, page scanning code could find the non-hugetlb
> compound page with no reference. I could not find any code where this
> would be a problem, but I think it would be safer to set the reference
> first.

Make sense to me. It is better to set the refcount first.

>
> > + spin_unlock(&hugetlb_lock);
>
> I really like the way this code is structured. It is much simpler than
> previous versions with retries or workqueue. There is nothing wrong with
> always dropping the lock here. However, I wonder if we should think about
> optimizing for the case where this feature is not enabled and we are not
> freeing a 1G huge page. I suspect this will be the most common case for
> some time, and there is no need to drop the lock in this case.
>
> Please do not change the code based on my comment. I just wanted to bring
> this up for thought.

At least make sense to me. It may take a long time to free a 1G
huge page. Dropping the lock may be a good choice. But I also
want to listen to Oscar and Michal's opinion on this.

>
> Is it as simple as checking?
> if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> spin_unlock(&hugetlb_lock);

>
> /* before return */
> if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> spin_lock(&hugetlb_lock);
>
> > +
> > + if (alloc_huge_page_vmemmap(h, page)) {
> > + int zeroed;
> > +
> > + 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]++;
> > +
> > + /*
> > + * This page is now managed by the hugetlb allocator and has
> > + * no users -- drop the last reference.
> > + */
> > + zeroed = put_page_testzero(page);
> > + VM_BUG_ON_PAGE(!zeroed, page);
> > + arch_clear_hugepage_flags(page);
> > + enqueue_huge_page(h, page);
> > +
> > + return -ENOMEM;
> > + }
> > +
> > for (i = 0; i < pages_per_huge_page(h); i++) {
> > page[i].flags &= ~(1 << PG_locked | 1 << PG_error |
> > 1 << PG_referenced | 1 << PG_dirty |
> > 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));
> > }
> > +
> > + spin_lock(&hugetlb_lock);
> > +
> > + return 0;
> > }
> >
> > struct hstate *size_to_hstate(unsigned long size)
> > @@ -1403,9 +1434,9 @@ 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);
> > @@ -1693,6 +1724,7 @@ static int free_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed,
> > struct page *page =
> > list_entry(h->hugepage_freelists[node].next,
> > struct page, lru);
> > + ClearHPageFreed(page);
>
> Quick question. Is this change directly related to the vmemmap changes,
> or is it a cleanup that you noticed?

Just a cleanup. Maybe there should be a separate patch for this.

>
> > list_del(&page->lru);
> > h->free_huge_pages--;
> > h->free_huge_pages_node[node]--;
> > @@ -1700,8 +1732,7 @@ 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;
> > }
> > }
> > @@ -1714,10 +1745,14 @@ 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)
> > {
> > @@ -1768,12 +1803,14 @@ int dissolve_free_huge_page(struct page *page)
> > SetPageHWPoison(page);
> > ClearPageHWPoison(head);
> > }
> > + ClearHPageFreed(page);
> > list_del(&head->lru);
> > 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->max_huge_pages++;
>
> Since update_and_free_page failed, the number of surplus pages was
> incremented. Surplus pages are the number of pages greater than
> max_huge_pages. Since we are incrementing max_huge_pages, we should
> decrement (undo) the addition to surplus_huge_pages and
> surplus_huge_pages_node[nid]. So, I think we want
> h->surplus_huge_pages--;
> h->surplus_huge_pages_node[nid]--;
> here as well.

You are right. Thanks for reminding me of this.

>
> > }
> > out:
> > spin_unlock(&hugetlb_lock);
>
> In previous version of this patch series, we discussed and refined the
> vmemmap manipulation routines below. They still look good to me.
>
> In general, I like the approach taken in this patch. Hopefully, others
> will comment and we can move the series forward.
> --
> Mike Kravetz
>
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 0209b736e0b4..29a3380f3b20 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -198,6 +198,28 @@ static inline unsigned long free_vmemmap_pages_size_per_hpage(struct hstate *h)
> > return (unsigned long)free_vmemmap_pages_per_hpage(h) << PAGE_SHIFT;
> > }
> >
> > +int alloc_huge_page_vmemmap(struct hstate *h, struct page *head)
> > +{
> > + 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)
> > {
> > unsigned long vmemmap_addr = (unsigned long)head;
> > diff --git a/mm/hugetlb_vmemmap.h b/mm/hugetlb_vmemmap.h
> > index 6923f03534d5..e5547d53b9f5 100644
> > --- a/mm/hugetlb_vmemmap.h
> > +++ b/mm/hugetlb_vmemmap.h
> > @@ -11,8 +11,14 @@
> > #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);
> > #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)
> > {
> > }
> > 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.
> >

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