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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Sep 02, 2026 at 03:37:59PM +0800, 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>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>