[PATCH] mm: migrate: requeue destination folio on deferred split queue

[Usama Arif <usama.arif@linux.dev>]

During folio migration, __folio_migrate_mapping() removes the source
folio from the deferred split queue, but the destination folio is never
re-queued.  This causes underutilized THPs to escape the shrinker after
NUMA migration, since they silently drop off the deferred split list.

Fix this by calling deferred_split_folio() on the destination folio
after a successful migration, for large rmappable folios.

Reported-by: Johannes Weiner <hannes@cmpxchg.org>
Fixes: dafff3f4c850 ("mm: split underused THPs")
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 mm/migrate.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index ece77ccb2ec0..98d0a594f7b7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1393,6 +1393,17 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private,
 	if (old_page_state & PAGE_WAS_MAPPED)
 		remove_migration_ptes(src, dst, 0);
 
+	/*
+	 * Requeue the destination folio on the deferred split queue if
+	 * the source was a large folio that was on the queue. Without
+	 * this, NUMA migration causes underutilized THPs to escape
+	 * the shrinker since the source is unqueued in
+	 * __folio_migrate_mapping() and the destination is never
+	 * re-queued.
+	 */
+	if (folio_test_large(dst) && folio_test_large_rmappable(dst))
+		deferred_split_folio(dst, false);
+
 out_unlock_both:
 	folio_unlock(dst);
 	folio_set_owner_migrate_reason(dst, reason);
-- 
2.47.3