[PATCH v2] mm/hugetlb: fix subpool accounting after cgroup charge failure

[Zhao Li]

alloc_hugetlb_folio() calls hugepage_subpool_get_pages() when map_chg
is set.  For subpools with max_hpages, that increments used_hpages.
If the later hugetlb cgroup charge fails, the unwind must undo that
charge even when gbl_chg > 0.

hugepage_subpool_put_pages() can also restore rsv_hpages if concurrent
frees move used_hpages below min_hpages between the get and put.  When
that happens on the gbl_chg > 0 path, restore the matching global
reservation as well.

Skip the gbl_chg > 0 put when max_hpages is unset.  For a min_size-only
subpool, get_pages() did not change subpool state and put_pages() would
create a false reservation.

Signed-off-by: Zhao Li <enderaoelyther@xxxxxxxxx>
---
Changes in v2:
- Handle rsv_hpages restoration when racing frees cross min_hpages.
- Skip gbl_chg > 0 put_pages() when max_hpages is unset.

 mm/hugetlb.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f24bf49be047e..4065d66fdcb5c 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3026,12 +3026,23 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 						    h_cg);
 out_subpool_put:
 	/*
-	 * put page to subpool iff the quota of subpool's rsv_hpages is used
-	 * during hugepage_subpool_get_pages.
+	 * map_chg means hugepage_subpool_get_pages() succeeded above.
+	 * If max_hpages accounting was touched, undo it.  If racing frees
+	 * moved the subpool below min_hpages, the put path may restore a
+	 * subpool reservation.  Restore the matching global reservation too.
 	 */
-	if (map_chg && !gbl_chg) {
-		gbl_reserve = hugepage_subpool_put_pages(spool, 1);
-		hugetlb_acct_memory(h, -gbl_reserve);
+	if (map_chg) {
+		if (!gbl_chg) {
+			gbl_reserve = hugepage_subpool_put_pages(spool, 1);
+			hugetlb_acct_memory(h, -gbl_reserve);
+		} else if (spool && spool->max_hpages != -1) {
+			gbl_reserve = hugepage_subpool_put_pages(spool, 1);
+			if (!gbl_reserve) {
+				spin_lock_irq(&hugetlb_lock);
+				h->resv_huge_pages++;
+				spin_unlock_irq(&hugetlb_lock);
+			}
+		}
 	}


--
2.50.1 (Apple Git-155)