[PATCH 12/16] mm/migration: fix potential page refcounts leak in migrate_pages

[Miaohe Lin]

In -ENOMEM case, there might be some subpages of fail-to-migrate THPs
left in thp_split_pages list. We should move them back to migration
list so that they could be put back to the right list by the caller
otherwise the page refcnt will be leaked here. Also adjust nr_failed
and nr_thp_failed accordingly to make vm events account more accurate.

Fixes: b5bade978e9b ("mm: migrate: fix the return value of migrate_pages()")
Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
---
 mm/migrate.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index e0db06927f02..6c2dfed2ddb8 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1422,6 +1422,15 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 				}
 
 				nr_failed_pages += nr_subpages;
+				/*
+				 * There might be some subpages of fail-to-migrate THPs
+				 * left in thp_split_pages list. Move them back to migration
+				 * list so that they could be put back to the right list by
+				 * the caller otherwise the page refcnt will be leaked.
+				 */
+				list_splice_init(&thp_split_pages, from);
+				nr_failed += retry;
+				nr_thp_failed += thp_retry;
 				goto out;
 			case -EAGAIN:
 				if (is_thp)
-- 
2.23.0