[PATCH] mm/hugetlb: fix resv_huge_pages double decrement in memfd error path

[Hongfu Li <hongfu.li@linux.dev>]

From: Hongfu Li <lihongfu@kylinos.cn>

alloc_hugetlb_folio_reserve() decrements h->resv_huge_pages when
dequeuing a folio, but unlike the use_global_reservation handling in
hugetlb_alloc_folio(), it does not set HPageRestoreReserve on the folio.

Its sole caller memfd_alloc_folio() pre-allocates a reservation via
hugetlb_reserve_pages() before allocating. When hugetlb_add_to_page_cache()
fails, folio_put() drops the folio without HPageRestoreReserve set, so
free_huge_folio() does not restore the reservation. The subsequent
hugetlb_unreserve_pages() on the err_unresv path decrements the counter
a second time, leaving resv_huge_pages off by one for every failed
allocation.

Set HPageRestoreReserve when consuming the reservation in
alloc_hugetlb_folio_reserve(). On the error path, free_huge_folio() then
restores the reservation before hugetlb_unreserve_pages() releases it.
The success path is unaffected, as hugetlb_add_to_page_cache() clears
the flag once the folio is added to the page cache.

Fixes: 26a8ea80929c ("mm/hugetlb: fix memfd_pin_folios resv_huge_pages leak")
Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
---
 mm/hugetlb.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index d8849196ad9b..fadfe1dbc047 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2178,8 +2178,10 @@ struct folio *alloc_hugetlb_folio_reserve(struct hstate *h, int preferred_nid,
 
 	folio = dequeue_hugetlb_folio_nodemask(h, gfp_mask, preferred_nid,
 					       nmask);
-	if (folio)
+	if (folio) {
+		folio_set_hugetlb_restore_reserve(folio);
 		h->resv_huge_pages--;
+	}
 
 	spin_unlock_irq(&hugetlb_lock);
 	return folio;
-- 
2.54.0