[PATCH hotfix] mm/thp: fix deferred split queue not partially_mapped: fix

[Hugh Dickins <hughd@google.com>]

Though even more elusive than before, list_del corruption has still been
seen on THP's deferred split queue.

The idea in commit e66f3185fa04 was right, but its implementation wrong.
The context omitted an important comment just before the critical test:
"split_folio() removes folio from list on success."  In ignoring that
comment, when a THP split succeeded, the code went on to release the
preceding safe folio, preserving instead an irrelevant (formerly head)
folio: which gives no safety because it's not on the list. Fix the logic.

Fixes: e66f3185fa04 ("mm/thp: fix deferred split queue not partially_mapped")
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/huge_memory.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 03fd4bc39ea1..5734d5d5060f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3790,7 +3790,9 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
 		 * in the case it was underused, then consider it used and
 		 * don't add it back to split_queue.
 		 */
-		if (!did_split && !folio_test_partially_mapped(folio)) {
+		if (did_split) {
+			; /* folio already removed from list */
+		} else if (!folio_test_partially_mapped(folio)) {
 			list_del_init(&folio->_deferred_list);
 			removed++;
 		} else {
-- 
2.35.3