Re: [PATCH] mm/shrinker: fix bogus set_shrinker_bit() with cgroup.memory=nokmem

[Usama Arif <usama.arif@linux.dev>]

On 02/09/2026 08:37, Jiayuan Chen wrote:
> With cgroup.memory=nokmem, shrinker_memcg_alloc() bails out early and
> never allocates an id, so shrinker->id keeps the 0 it got from the
> kzalloc() in shrinker_alloc(). __list_lru_init() then copies that 0
> into lru->shrinker_id, where it looks like a valid bit index.
> 
> Nothing calls expand_shrinker_info() on nokmem either, so
> shrinker_nr_max stays 0 and every memcg ends up with an empty map
> (map_nr_max == 0).
> 
> deferred_split_folio() hands a real memcg to __list_lru_add()
> regardless of whether the lru is memcg aware, so the first THP queued
> in a cgroup does set_shrinker_bit(memcg, nid, 0) and trips the bounds
> check:
> 
> WARNING: mm/shrinker.c:212 at set_shrinker_bit+0x7d/0x90, CPU#126
> Call Trace:
>  <TASK>
>  deferred_split_folio+0x18c/0x220
>  map_anon_folio_pmd_nopf+0xdd/0x130
>  map_anon_folio_pmd_pf+0x14/0xb0
>  do_huge_pmd_anonymous_page+0x1a1/0x620
>  __handle_mm_fault+0xea9/0x10d0
>  handle_mm_fault+0xe5/0x320
>  do_user_addr_fault+0x1cc/0x870
>  exc_page_fault+0x81/0x1b0
>  asm_exc_page_fault+0x27/0x30
>  </TASK>
> 
> Harmless, the WARN_ON_ONCE() is what keeps the out of bounds unit[]
> read from happening, but the id should not look valid in the first
> place. Clear it before returning.
> 
> Two other spots could paper over this: drop the id in
> __list_lru_init() when nokmem turns memcg_aware off, or make
> deferred_split_folio() pass NULL like list_lru_add_obj() does. Both
> leave shrinker->id lying around for the next caller, so fix it where
> the id is handed out.
> 
> Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  mm/shrinker.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/shrinker.c b/mm/shrinker.c
> index a70aab124a0e..7ec2a9704f6f 100644
> --- a/mm/shrinker.c
> +++ b/mm/shrinker.c
> @@ -227,6 +227,8 @@ static int shrinker_memcg_alloc(struct shrinker *shrinker)
>  {
>  	int id;
>  
> +	shrinker->id = -1;
> +
>  	if (mem_cgroup_disabled())
>  		return -ENOSYS;
>  	if (mem_cgroup_kmem_disabled() && !(shrinker->flags & SHRINKER_NONSLAB))


Thanks for the fix!

I think the initialization would look better in shrinker_alloc()?