[PATCH v3] mm/hugetlb: update nr_huge_pages and surplus_huge_pages together

[Liu Shixin]

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.

Fixes: 0c397daea1d4 ("mm, hugetlb: further simplify hugetlb allocation API")
Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
v2->v3: Modify the comment suggested by Oscar.
 mm/hugetlb.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9faa1034704ff..0e08d2fff2360 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2253,11 +2253,20 @@ 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
-- 
2.34.1