[PATCH v3 4/4] mm: zswap: mark the zswap shrinker SHRINKER_NONSLAB

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

With kmem accounting disabled (cgroup.memory=nokmem), memcg reclaim
never invokes the zswap shrinker: a cgroup under memory pressure does
not write back its own zswapped pages to make room; they are only
written back by global reclaim or once the global pool limit kicks in.

The zswap shrinker is registered memcg-aware but without
SHRINKER_NONSLAB, so under nokmem it is treated as a slab shrinker:
before commit 03375203e1da ("mm: do not allocate shrinker info with
cgroup.memory=nokmem") it was skipped by the !memcg_kmem_online()
check in shrink_slab_memcg(); since that commit it is demoted to
non-memcg-aware at registration.  But the zswap shrinker is not a slab
shrinker: it tracks zswap entries and resolves the owning memcg from
the folio's objcg, independent of kmem accounting.

This was noticed by Michal during review of the patch "mm/list_lru:
don't copy stale shrinker id from non-memcg-aware shrinkers" [1].

Mark it SHRINKER_NONSLAB so it keeps its memcg awareness and runs
under memcg reclaim with nokmem.

[1] https://lore.kernel.org/lkml/697713c4-0857-485b-aba7-c74f37a3c8b4@linux.alibaba.com/

Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/zswap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index f3ae3c81e48ea..032136942c172 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1252,8 +1252,8 @@ static struct shrinker *zswap_alloc_shrinker(void)
 {
 	struct shrinker *shrinker;
 
-	shrinker =
-		shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE, "mm-zswap");
+	shrinker = shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE |
+				  SHRINKER_NONSLAB, "mm-zswap");
 	if (!shrinker)
 		return NULL;
 
-- 
2.43.7