[PATCH hotfix] mm/migrate: fix kernel BUG at mm/compaction.c:2761!

[Hugh Dickins]

I hit the VM_BUG_ON(!list_empty(&cc->migratepages)) in compact_zone();
and if DEBUG_VM were off, then pages would be lost on a local list.

Our convention is that if migrate_pages() reports complete success (0),
then the migratepages list will be empty; but if it reports an error or
some pages remaining, then its caller must putback_movable_pages().

There's a new case in which migrate_pages() has been reporting complete
success, but returning with pages left on the migratepages list: when
migrate_pages_batch() successfully split a folio on the deferred list,
but then the "Failure isn't counted" call does not dispose of them all.

Since that block is expecting the large folio to have been counted as 1
failure already, and since the return code is later adjusted to success
whenever the returned list is found empty, the simple way to fix this
safely is to count splitting the deferred folio as "a failure".

Fixes: 7262f208ca68 ("mm/migrate: split source folio if it is on deferred split list")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
A hotfix to 6.10-rc, not needed for stable.

 mm/migrate.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1654,7 +1654,12 @@ static int migrate_pages_batch(struct list_head *from,
 
 			/*
 			 * The rare folio on the deferred split list should
-			 * be split now. It should not count as a failure.
+			 * be split now. It should not count as a failure:
+			 * but increment nr_failed because, without doing so,
+			 * migrate_pages() may report success with (split but
+			 * unmigrated) pages still on its fromlist; whereas it
+			 * always reports success when its fromlist is empty.
+			 *
 			 * Only check it without removing it from the list.
 			 * Since the folio can be on deferred_split_scan()
 			 * local list and removing it can cause the local list
@@ -1669,6 +1674,7 @@ static int migrate_pages_batch(struct list_head *from,
 			if (nr_pages > 2 &&
 			   !list_empty(&folio->_deferred_list)) {
 				if (try_split_folio(folio, split_folios) == 0) {
+					nr_failed++;
 					stats->nr_thp_split += is_thp;
 					stats->nr_split++;
 					continue;
-- 
2.35.3