[PATCH] mm/migrate: Putback split folios when numa hint migration fails

Peter Xu
2024-07-08T21:55:42Z

This issue is not from any report yet, but by code observation only.

This is yet another fix besides Hugh's patch [1] but on relevant code path,
where eager split of folio can happen if the folio is already on deferred
list during a folio migration.

Here the issue is NUMA path (migrate_misplaced_folio()) may start to
encounter such folio split now even with MR_NUMA_MISPLACED hint applied.
Then when migrate_pages() didn't migrate all the folios, it's possible the
split small folios be put onto the list instead of the original folio.
Then putting back only the head page won't be enough.

Fix it by putting back all the folios on the list.

[1] https://lore.kernel.org/all/46c948b4-4dd8-6e03-4c7b-ce4e81cfa536@google.com/

Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Fixes: 7262f208ca68 ("mm/migrate: split source folio if it is on deferred split list")
Signed-off-by: Peter Xu <peterx@redhat.com>
---

Don't need to copy stable if this can still hit 6.10.. Only smoke tested.
---
mm/migrate.c | 10 ++--------
1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index e10d2445fbd8..20da2595527a 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2615,14 +2615,8 @@ int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma,
nr_remaining = migrate_pages(&migratepages, alloc_misplaced_dst_folio,
NULL, node, MIGRATE_ASYNC,
MR_NUMA_MISPLACED, &nr_succeeded);
- if (nr_remaining) {
- if (!list_empty(&migratepages)) {
- list_del(&folio->lru);
- node_stat_mod_folio(folio, NR_ISOLATED_ANON +
- folio_is_file_lru(folio), -nr_pages);
- folio_putback_lru(folio);
- }
- }
+ if (nr_remaining && !list_empty(&migratepages))
+ putback_movable_pages(&migratepages);
if (nr_succeeded) {
count_vm_numa_events(NUMA_PAGE_MIGRATE, nr_succeeded);
if (!node_is_toptier(folio_nid(folio)) && node_is_toptier(node))
--
2.45.0