[PATCH v3 3/4] mm: thp: restore SHRINKER_NONSLAB on the deferred split shrinker

[Qinyun Tan]

On a system booted with cgroup.memory=nokmem, the deferred split
shrinker is demoted to a non-memcg-aware one.  As a result, partially
unmapped THPs are only split under global reclaim; memcg
(limit-induced) reclaim never splits them, so a cgroup under memory
pressure keeps its underused THPs intact.

The shrinker has carried SHRINKER_NONSLAB since commit 0a432dcbeb32
("mm: shrinker: make shrinker not depend on memcg kmem") precisely
so it would keep working with kmem accounting disabled.  Commit
fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru")
dropped it knowingly: under nokmem the list_lru collapses into a
shared per-node list while __list_lru_add() still sets the shrinker
bit on whichever memcg happens to add the first item, and with the
flag set the shrinker would run twice per reclaim cycle [1].

NONSLAB-backed lrus keep their per-memcg lists under nokmem now, so
the shrinker bit identifies the owning memcg again.  Restore the
flag.

This was noticed by Michal during review of the patch "mm/list_lru:
don't copy stale shrinker id from non-memcg-aware shrinkers" [2].

[1] https://lore.kernel.org/all/ah9PGv12mqai84ES@xxxxxxxxxxx/
[2] https://lore.kernel.org/lkml/697713c4-0857-485b-aba7-c74f37a3c8b4@xxxxxxxxxxxxxxxxx/

Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru")
Suggested-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Qinyun Tan <qinyuntan@xxxxxxxxxxxxxxxxx>
Acked-by: Zi Yan <ziy@xxxxxxxxxx>
Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
Acked-by: Usama Arif <usama.arif@xxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/huge_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c5d11147b69ae..87c9fc48b7cfd 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1064,7 +1064,8 @@ int folio_memcg_alloc_deferred(struct folio *folio)
 static int __init thp_shrinker_init(void)
 {
 	deferred_split_shrinker = shrinker_alloc(SHRINKER_NUMA_AWARE |
-						 SHRINKER_MEMCG_AWARE,
+						 SHRINKER_MEMCG_AWARE |
+						 SHRINKER_NONSLAB,
 						 "thp-deferred_split");
 	if (!deferred_split_shrinker)
 		return -ENOMEM;
-- 
2.43.7