Re: [PATCH] mm/list_lru: don't copy stale shrinker id from non-memcg-aware shrinkers

["Michal Koutný" <mkoutny@suse.com>]

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Hello Qinyun.

On Tue, Sep 01, 2026 at 07:51:04PM +0800, Qinyun Tan <qinyuntan@linux.alibaba.com> wrote:
> With cgroup.memory=nokmem, shrinker_memcg_alloc() fails with -ENOSYS
> for shrinkers without SHRINKER_NONSLAB, and shrinker_alloc() falls
> back to a non-memcg-aware shrinker.  On this fallback path,
> shrinker->id is never assigned and keeps 0 from kzalloc(), which is a
> valid id belonging to whichever memcg-aware shrinker registers first.
> 
> __list_lru_init() copies shrinker->id unconditionally, so every
> list_lru backed by such a fallback shrinker (thp-deferred_split,
> zswap-shrinker, workingset shadow nodes, superblock lrus, ...) ends
> up with lru->shrinker_id == 0 instead of -1.

<del>thp-deferred_split shrinek has SHRINKER_NONSLAB so, the id should be
assigned (at least I see it in thp_shrinker_init()).</del> I was looking
at 6.12 kernel.

> Under nokmem the list_lru collapses to the shared per-node lists, but
> __list_lru_add() still calls set_shrinker_bit() against the memcg of
> the added object.  Most list_lru users are unaffected because their
> objects resolve to a NULL memcg without kmem accounting, but the THP
> deferred split queue holds user folios, which are charged regardless
> of nokmem.  Since no memcg-aware shrinker can register under nokmem,

Not sure I understand here, the SHRINKER_NONSLAB are excluded and should
still register for per-memcg info.

> shrinker_nr_max stays 0 and every memcg's shrinker_info has
> map_nr_max == 0, so the first folio added by khugepaged triggers on
> every boot:
> 
>   WARNING: mm/shrinker.c:212 at set_shrinker_bit+0x99/0xa0
> 
> On systems where a SHRINKER_NONSLAB shrinker (btrfs, xfs) did register
> and expand the maps, there is no warning; instead bit 0 is set
> spuriously for an unrelated shrinker.

Could it be that you see this issue because of a shrinker which isn't
marked as SHRINKER_NONSLAB?

I'd even go as far as pointing at
fafaeceb89a5e ("mm: switch deferred split shrinker to list_lru")
which removed the flag from the the thp-deferred_split shrinker.

IOW, the proper fix should be addition of SHRINKER_NONSLAB so that
per-memcg maps are properly allocated.

Thanks,
Michal

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