mm/hugetlb: fix subpool minimum reservation rollback

When a reservation request is partially covered by a subpool minimum and
the remaining global reservation fails, the error path first calls
hugepage_subpool_put_pages() for the subpool-backed portion.  It removes
the failed global portion from used_hpages only afterwards.

hugepage_subpool_put_pages() uses used_hpages to decide whether rsv_hpages
should be restored.  Since used_hpages still includes the global portion,
it can remain at or above min_hpages and prevent that restoration.  It
then reports the subpool reservation as releasable, causing
hugetlb_acct_memory() to incorrectly decrement h->resv_huge_pages.

This was reproduced with four 2 MB huge pages and a hugetlbfs mount with
size=10M,min_size=8M.  After a successful three-page reservation, a
two-page reservation which needed one subpool page and one global page
failed with -ENOMEM.  HugePages_Rsvd incorrectly dropped from four to
three even though the subpool minimum was still four pages.

Roll back the failed global portion from used_hpages first, so that
hugepage_subpool_put_pages() evaluates the minimum reservation against the
current usage and returns the correct global adjustment.

Link: https://lore.kernel.org/20260907132055.26696-1-zhoujinmeng@bytedance.com
Fixes: a833a693a490 ("mm: hugetlb: fix incorrect fallback for subpool")
Signed-off-by: Jinmeng Zhou <zhoujinmeng@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Ma Wupeng <mawupeng1@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>

1 files changed, 9 insertions, 9 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a78a9fa26958..290150eb12a6 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6862,15 +6862,6 @@ long hugetlb_reserve_pages(struct inode *inode,
 
 out_put_pages:
 	spool_resv = chg - gbl_reserve;
-	if (spool_resv) {
-		/* put sub pool's reservation back, chg - gbl_reserve */
-		gbl_resv = hugepage_subpool_put_pages(spool, spool_resv);
-		/*
-		 * subpool's reserved pages can not be put back due to race,
-		 * return to hstate.
-		 */
-		hugetlb_acct_memory(h, -gbl_resv);
-	}
 	/* Restore used_hpages for pages that failed global reservation */
 	if (gbl_reserve && spool) {
 		unsigned long flags;
@@ -6880,6 +6871,15 @@ out_put_pages:
 			spool->used_hpages -= gbl_reserve;
 		unlock_or_release_subpool(spool, flags);
 	}
+	if (spool_resv) {
+		/* put sub pool's reservation back, chg - gbl_reserve */
+		gbl_resv = hugepage_subpool_put_pages(spool, spool_resv);
+		/*
+		 * subpool's reserved pages can not be put back due to race,
+		 * return to hstate.
+		 */
+		hugetlb_acct_memory(h, -gbl_resv);
+	}
 out_uncharge_cgroup:
 	hugetlb_cgroup_uncharge_cgroup_rsvd(hstate_index(h),
 					    chg * pages_per_huge_page(h), h_cg);