[PATCH v4 3/5] mm: list_lru: keep per-memcg lists with nokmem for NONSLAB-backed lrus

[Qinyun Tan <qinyuntan@linux.alibaba.com>]

With cgroup.memory=nokmem, __list_lru_init() collapses every list_lru
into per-node lists.  That is fine for slab objects, which are not
charged per memcg without kmem accounting.  But a lru backed by a
SHRINKER_NONSLAB shrinker, e.g. the THP deferred split queue or the
zswap entries, holds user memory, which is charged regardless of
nokmem.  Collapsing such an lru loses the per-memcg view of its
objects.  Reclaim can no longer target just the cgroup under pressure.

Keep such lrus memcg aware under nokmem when their backing shrinker is
registered SHRINKER_NONSLAB.

This leaves the savings of nokmem intact: slab-backed lrus (e.g. the
superblock dentry/inode lrus) still fall back to the per-node lists,
and the per-memcg lists are allocated only when a memcg actually holds
such objects.

Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru")
Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/list_lru.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index 8e9577c5cb19..7c8d0b959bed 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -690,7 +690,9 @@ int __list_lru_init(struct list_lru *lru, bool memcg_aware, struct shrinker *shr
 	else
 		lru->shrinker_id = -1;
 
-	if (mem_cgroup_disabled() || mem_cgroup_kmem_disabled())
+	if (mem_cgroup_disabled() ||
+	    (mem_cgroup_kmem_disabled() &&
+	     (!shrinker || !(shrinker->flags & SHRINKER_NONSLAB))))
 		memcg_aware = false;
 #endif
 
-- 
2.43.7