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authorOscar Salvador <osalvador@suse.de>2021-05-05 04:35:26 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2021-05-05 21:27:22 +0300
commit369fa227c21949b22fd7374506c4992a0d7bb580 (patch)
tree5735544698728e2ae27bbd076d9516e99721567d /mm/hugetlb.c
parentd3d99fcc4e28f1a613744608c289d4f18b60b12f (diff)
downloadlinux-369fa227c21949b22fd7374506c4992a0d7bb580.tar.xz
mm: make alloc_contig_range handle free hugetlb pages
alloc_contig_range will fail if it ever sees a HugeTLB page within the range we are trying to allocate, even when that page is free and can be easily reallocated. This has proved to be problematic for some users of alloc_contic_range, e.g: CMA and virtio-mem, where those would fail the call even when those pages lay in ZONE_MOVABLE and are free. We can do better by trying to replace such page. Free hugepages are tricky to handle so as to no userspace application notices disruption, we need to replace the current free hugepage with a new one. In order to do that, a new function called alloc_and_dissolve_huge_page is introduced. This function will first try to get a new fresh hugepage, and if it succeeds, it will replace the old one in the free hugepage pool. The free page replacement is done under hugetlb_lock, so no external users of hugetlb will notice the change. To allocate the new huge page, we use alloc_buddy_huge_page(), so we do not have to deal with any counters, and prep_new_huge_page() is not called. This is valulable because in case we need to free the new page, we only need to call __free_pages(). Once we know that the page to be replaced is a genuine 0-refcounted huge page, we remove the old page from the freelist by remove_hugetlb_page(). Then, we can call __prep_new_huge_page() and __prep_account_new_huge_page() for the new huge page to properly initialize it and increment the hstate->nr_huge_pages counter (previously decremented by remove_hugetlb_page()). Once done, the page is enqueued by enqueue_huge_page() and it is ready to be used. There is one tricky case when page's refcount is 0 because it is in the process of being released. A missing PageHugeFreed bit will tell us that freeing is in flight so we retry after dropping the hugetlb_lock. The race window should be small and the next retry should make a forward progress. E.g: CPU0 CPU1 free_huge_page() isolate_or_dissolve_huge_page PageHuge() == T alloc_and_dissolve_huge_page alloc_buddy_huge_page() spin_lock_irq(hugetlb_lock) // PageHuge() && !PageHugeFreed && // !PageCount() spin_unlock_irq(hugetlb_lock) spin_lock_irq(hugetlb_lock) 1) update_and_free_page PageHuge() == F __free_pages() 2) enqueue_huge_page SetPageHugeFreed() spin_unlock_irq(&hugetlb_lock) spin_lock_irq(hugetlb_lock) 1) PageHuge() == F (freed by case#1 from CPU0) 2) PageHuge() == T PageHugeFreed() == T - proceed with replacing the page In the case above we retry as the window race is quite small and we have high chances to succeed next time. With regard to the allocation, we restrict it to the node the page belongs to with __GFP_THISNODE, meaning we do not fallback on other node's zones. Note that gigantic hugetlb pages are fenced off since there is a cyclic dependency between them and alloc_contig_range. Link: https://lkml.kernel.org/r/20210419075413.1064-6-osalvador@suse.de Signed-off-by: Oscar Salvador <osalvador@suse.de> Acked-by: Michal Hocko <mhocko@suse.com> Acked-by: David Hildenbrand <david@redhat.com> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com> Cc: Muchun Song <songmuchun@bytedance.com> Cc: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/hugetlb.c')
-rw-r--r--mm/hugetlb.c116
1 files changed, 116 insertions, 0 deletions
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 63760be2688e..92f3cd08946f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2267,6 +2267,122 @@ static void restore_reserve_on_error(struct hstate *h,
}
}
+/*
+ * alloc_and_dissolve_huge_page - Allocate a new page and dissolve the old one
+ * @h: struct hstate old page belongs to
+ * @old_page: Old page to dissolve
+ * Returns 0 on success, otherwise negated error.
+ */
+static int alloc_and_dissolve_huge_page(struct hstate *h, struct page *old_page)
+{
+ gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
+ int nid = page_to_nid(old_page);
+ struct page *new_page;
+ int ret = 0;
+
+ /*
+ * Before dissolving the page, we need to allocate a new one for the
+ * pool to remain stable. Using alloc_buddy_huge_page() allows us to
+ * not having to deal with prep_new_huge_page() and avoids dealing of any
+ * counters. This simplifies and let us do the whole thing under the
+ * lock.
+ */
+ new_page = alloc_buddy_huge_page(h, gfp_mask, nid, NULL, NULL);
+ if (!new_page)
+ return -ENOMEM;
+
+retry:
+ spin_lock_irq(&hugetlb_lock);
+ if (!PageHuge(old_page)) {
+ /*
+ * Freed from under us. Drop new_page too.
+ */
+ goto free_new;
+ } else if (page_count(old_page)) {
+ /*
+ * Someone has grabbed the page, fail for now.
+ */
+ ret = -EBUSY;
+ goto free_new;
+ } else if (!HPageFreed(old_page)) {
+ /*
+ * Page's refcount is 0 but it has not been enqueued in the
+ * freelist yet. Race window is small, so we can succeed here if
+ * we retry.
+ */
+ spin_unlock_irq(&hugetlb_lock);
+ cond_resched();
+ goto retry;
+ } else {
+ /*
+ * Ok, old_page is still a genuine free hugepage. Remove it from
+ * the freelist and decrease the counters. These will be
+ * incremented again when calling __prep_account_new_huge_page()
+ * and enqueue_huge_page() for new_page. The counters will remain
+ * stable since this happens under the lock.
+ */
+ remove_hugetlb_page(h, old_page, false);
+
+ /*
+ * new_page needs to be initialized with the standard hugetlb
+ * state. This is normally done by prep_new_huge_page() but
+ * that takes hugetlb_lock which is already held so we need to
+ * open code it here.
+ * Reference count trick is needed because allocator gives us
+ * referenced page but the pool requires pages with 0 refcount.
+ */
+ __prep_new_huge_page(new_page);
+ __prep_account_new_huge_page(h, nid);
+ page_ref_dec(new_page);
+ enqueue_huge_page(h, new_page);
+
+ /*
+ * Pages have been replaced, we can safely free the old one.
+ */
+ spin_unlock_irq(&hugetlb_lock);
+ update_and_free_page(h, old_page);
+ }
+
+ return ret;
+
+free_new:
+ spin_unlock_irq(&hugetlb_lock);
+ __free_pages(new_page, huge_page_order(h));
+
+ return ret;
+}
+
+int isolate_or_dissolve_huge_page(struct page *page)
+{
+ struct hstate *h;
+ struct page *head;
+
+ /*
+ * The page might have been dissolved from under our feet, so make sure
+ * to carefully check the state under the lock.
+ * Return success when racing as if we dissolved the page ourselves.
+ */
+ spin_lock_irq(&hugetlb_lock);
+ if (PageHuge(page)) {
+ head = compound_head(page);
+ h = page_hstate(head);
+ } else {
+ spin_unlock_irq(&hugetlb_lock);
+ return 0;
+ }
+ spin_unlock_irq(&hugetlb_lock);
+
+ /*
+ * Fence off gigantic pages as there is a cyclic dependency between
+ * alloc_contig_range and them. Return -ENOMEM as this has the effect
+ * of bailing out right away without further retrying.
+ */
+ if (hstate_is_gigantic(h))
+ return -ENOMEM;
+
+ return alloc_and_dissolve_huge_page(h, head);
+}
+
struct page *alloc_huge_page(struct vm_area_struct *vma,
unsigned long addr, int avoid_reserve)
{