[PATCH 0/2] mm: restore SHRINKER_NONSLAB on THP and zswap shrinkers

[PATCH 0/2] mm: restore SHRINKER_NONSLAB on THP and zswap shrinkers

[Qinyun Tan]

While reviewing the patch "mm/list_lru: don't copy stale shrinker id
from non-memcg-aware shrinkers" (now in mm-unstable), Michal noticed
that the two non-slab memcg-aware shrinkers, the THP deferred split
shrinker and the zswap shrinker, are registered without
SHRINKER_NONSLAB [1].

Without that flag, booting with cgroup.memory=nokmem silently demotes
both shrinkers to non-memcg-aware ones: memcg (limit-induced) reclaim
neither splits partially unmapped THPs nor writes back zswapped pages
of the cgroup under pressure.

Patch 1 restores the flag on the deferred split shrinker; it was lost
when commit fafaeceb89a5 ("mm: switch deferred split shrinker to
list_lru") re-registered the shrinker.

Patch 2 adds the flag to the zswap shrinker, which has never carried
it since its introduction in commit b5ba474f3f51 ("zswap: shrink zswap
pool based on memory pressure").

The series is based on mm-new, on top of the list_lru patch above,
but the changes are independent of it.

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

Qinyun Tan (2):
  mm: thp: restore SHRINKER_NONSLAB on the deferred split shrinker
  mm: zswap: mark the zswap shrinker SHRINKER_NONSLAB

 mm/huge_memory.c | 3 ++-
 mm/zswap.c       | 4 ++--
 2 files changed, 4 insertions(+), 3 deletions(-)

-- 
2.43.7

[PATCH 1/2] 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 quietly 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.

This is a regression from commit fafaeceb89a5 ("mm: switch deferred
split shrinker to list_lru"), which re-registered the shrinker without
SHRINKER_NONSLAB.  The shrinker had carried this flag since commit
0a432dcbeb32 ("mm: shrinker: make shrinker not depend on memcg kmem")
precisely so it would keep working with kmem accounting disabled.
Without the flag, shrinker_memcg_alloc() fails with -ENOSYS under
nokmem and the shrinker loses its memcg awareness.

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].

Restore the flag.

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

Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru")
Suggested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
---
 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 644d6905b49cc..16f9aa5b2d077 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1022,7 +1022,8 @@ 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

Re: [PATCH 1/2] mm: thp: restore SHRINKER_NONSLAB on the deferred split shrinker

[Zi Yan]

On Thu Sep 3, 2026 at 11:35 PM EDT, Qinyun Tan wrote:
> On a system booted with cgroup.memory=nokmem, the deferred split
> shrinker is quietly 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.
>
> This is a regression from commit fafaeceb89a5 ("mm: switch deferred
> split shrinker to list_lru"), which re-registered the shrinker without
> SHRINKER_NONSLAB.  The shrinker had carried this flag since commit
> 0a432dcbeb32 ("mm: shrinker: make shrinker not depend on memcg kmem")
> precisely so it would keep working with kmem accounting disabled.
> Without the flag, shrinker_memcg_alloc() fails with -ENOSYS under
> nokmem and the shrinker loses its memcg awareness.
>
> 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].
>
> Restore the flag.
>
> [1] https://lore.kernel.org/lkml/697713c4-0857-485b-aba7-c74f37a3c8b4@linux.alibaba.com/
>
> Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru")
> Suggested-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Qinyun Tan <qinyuntan@linux.alibaba.com>
> ---
>  mm/huge_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>

Makes sense.

Acked-by: Zi Yan <ziy@nvidia.com>



-- 
Best Regards,
Yan, Zi

[PATCH 2/2] mm: zswap: mark the zswap shrinker SHRINKER_NONSLAB

[Qinyun Tan]

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>
---
 mm/zswap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 761cd699e0a3e..8bcc2997d7719 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1263,8 +1263,8 @@ 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