[PATCH v6 5/6] mm: vmscan: Avoid split during shrink_folio_list()

[Ryan Roberts]

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.

THP_SWPOUT and THP_SWPOUT_FALLBACK counters should continue to count
events only for PMD-mappable folios to avoid user confusion. THP_SWPOUT
already has the appropriate guard. Add a guard for THP_SWPOUT_FALLBACK.
It may be appropriate to add per-size counters in future.

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Barry Song <v-songbaohua@xxxxxxxx>
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
 mm/vmscan.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 00adaf1cb2c3..ffc4553c8615 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1223,11 +1223,12 @@ 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 right
+					 * away. We can free the unmapped pages
+					 * without IO.
 					 */
-					if (!folio_entire_mapcount(folio) &&
+					if (data_race(!list_empty(
+						&folio->_deferred_list)) &&
 					    split_folio_to_list(folio,
 								folio_list))
 						goto activate_locked;
@@ -1240,8 +1241,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 								folio_list))
 						goto activate_locked;
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
-					count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
-					count_vm_event(THP_SWPOUT_FALLBACK);
+					if (nr_pages >= HPAGE_PMD_NR) {
+						count_memcg_folio_events(folio,
+							THP_SWPOUT_FALLBACK, 1);
+						count_vm_event(
+							THP_SWPOUT_FALLBACK);
+					}
 #endif
 					if (!add_to_swap(folio))
 						goto activate_locked_split;
-- 
2.25.1