Re: [PATCH v2 3/4] mm: thp: restore SHRINKER_NONSLAB on the deferred split shrinker

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

Hi Johannes,

On 9/10/26 2:28 AM, Johannes Weiner wrote:
> On Mon, Sep 07, 2026 at 07:01:10PM +0800, 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.
> 
> Here is the reason why it was dropped:
> https://lore.kernel.org/all/ah9PGv12mqai84ES@cmpxchg.org/
> 
> After your patch to make list_lru expand for NONSLAB shrinkers, it can
> be restored.
> 

Thanks for the pointer. I'll update the commit message in v3 and
reference that thread.

>> 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>
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>> Acked-by: Usama Arif <usama.arif@linux.dev>
>> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>


Thanks again,
Qinyun Tan