[PATCH] 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.

Fixes: 0c397daea1d4 ("mm, hugetlb: further simplify hugetlb allocation API")
Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
---
 mm/hugetlb.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9faa1034704ff..a900562ea7679 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2248,14 +2248,17 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
 	if (hstate_is_gigantic(h))
 		return NULL;
 
+	mutex_lock(&h->resize_lock);
 	spin_lock_irq(&hugetlb_lock);
 	if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages)
 		goto out_unlock;
 	spin_unlock_irq(&hugetlb_lock);
 
 	folio = alloc_fresh_hugetlb_folio(h, gfp_mask, nid, nmask);
-	if (!folio)
+	if (!folio) {
+		mutex_unlock(&h->resize_lock);
 		return NULL;
+	}
 
 	spin_lock_irq(&hugetlb_lock);
 	/*
@@ -2268,6 +2271,7 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
 	if (h->surplus_huge_pages >= h->nr_overcommit_huge_pages) {
 		folio_set_hugetlb_temporary(folio);
 		spin_unlock_irq(&hugetlb_lock);
+		mutex_unlock(&h->resize_lock);
 		free_huge_folio(folio);
 		return NULL;
 	}
@@ -2277,6 +2281,7 @@ static struct folio *alloc_surplus_hugetlb_folio(struct hstate *h,
 
 out_unlock:
 	spin_unlock_irq(&hugetlb_lock);
+	mutex_unlock(&h->resize_lock);
 
 	return folio;
 }
-- 
2.34.1