[PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

Ryan Roberts
2024-03-11T15:01:22Z

Now that swap supports storing all mTHP sizes, avoid splitting large
folios before swap-out. This benefits performance of the swap-out path
by eliding split_folio_to_list(), which is expensive, and also sets us
up for swapping in large folios in a future series.

If the folio is partially mapped, we continue to split it since we want
to avoid the extra IO overhead and storage of writing out pages
uneccessarily.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
mm/vmscan.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index cf7d4cf47f1a..0ebec99e04c6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
if (!can_split_folio(folio, NULL))
goto activate_locked;
/*
- * Split folios without a PMD map right
- * away. Chances are some or all of the
- * tail pages can be freed without IO.
+ * Split partially mapped folios map
+ * right away. Chances are some or all
+ * of the tail pages can be freed
+ * without IO.
*/
- if (!folio_entire_mapcount(folio) &&
+ if (!list_empty(&folio->_deferred_list) &&
split_folio_to_list(folio,
folio_list))
goto activate_locked;
--
2.25.1