mm/hugetlb: update nr_huge_pages and surplus_huge_pages together

In alloc_surplus_hugetlb_folio(), we increase nr_huge_pages and
surplus_huge_pages separately.  In the middle window, if we set
nr_hugepages to smaller and satisfy count < persistent_huge_pages(h), the
surplus_huge_pages will be increased by adjust_pool_surplus().

After adding delay in the middle window, we can reproduce the problem
easily by following step:

 1. echo 3 > /proc/sys/vm/nr_overcommit_hugepages
 2. mmap two hugepages. When nr_huge_pages=2 and surplus_huge_pages=1,
    goto step 3.
 3. echo 0 > /proc/sys/vm/nr_huge_pages

Finally, nr_huge_pages is less than surplus_huge_pages.

To fix the problem, call only_alloc_fresh_hugetlb_folio() instead and
move down __prep_account_new_huge_page() into the hugetlb_lock.

Link: https://lkml.kernel.org/r/20250305035409.2391344-1-liushixin2@huawei.com
Fixes: 0c397daea1d4 ("mm, hugetlb: further simplify hugetlb allocation API")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Acked-by: Peter Xu <peterx@redhat.com>
Acked-by: Oscar Salvador <osalvador@suse.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 438de55..af9b8c1 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2259,11 +2259,20 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
                goto out_unlock;
        spin_unlock_irq(&hugetlb_lock);
 
-       folio = alloc_fresh_hugetlb_folio(h, gfp_mask, nid, nmask);
+       folio = only_alloc_fresh_hugetlb_folio(h, gfp_mask, nid, nmask, NULL);
        if (!folio)
                return NULL;
 
+       hugetlb_vmemmap_optimize_folio(h, folio);
+
        spin_lock_irq(&hugetlb_lock);
+       /*
+        * nr_huge_pages needs to be adjusted within the same lock cycle
+        * as surplus_pages, otherwise it might confuse
+        * persistent_huge_pages() momentarily.
+        */
+       __prep_account_new_huge_page(h, nid);
+
        /*
         * We could have raced with the pool size change.
         * Double check that and simply deallocate the new page