Re: [PATCH v2] fs: push nr_cached_objects memcg gating into individual filesystems

[David Sterba]

On Wed, Jul 15, 2026 at 03:35:16AM -0700, Usama Arif wrote:
> Commit 0baad6f9b997 ("fs/super: skip non-memcg-aware nr_cached_objects
> in memcg slab shrink") added a check in fs/super.c that skipped every
> ->nr_cached_objects() hook whenever the shrinker was invoked for a
> non-root memcg, on the assumption that none of them honour sc->memcg.
> 
> That assumption is wrong for XFS, whose inode-reclaim hook is
> intentionally driven from per-memcg contexts to free memcg-charged
> slab. Encoding a blanket "never memcg-aware" policy in fs/super.c
> short-circuits that path.
> 
> Push the check down into the callbacks whose counters really are
> irrelevant to per-memcg reclaim - btrfs_nr_cached_objects() and
> shmem_unused_huge_count() - and drop the fs/super.c gate. Each
> filesystem can now lift the restriction independently if its counter
> later grows memcg awareness, without touching fs/super.c.
> 
> Introduce mem_cgroup_shrink_is_root() in <linux/memcontrol.h> so the
> callbacks don't open-code "sc->memcg is NULL or root".
> 
> Fixes: 0baad6f9b997 ("fs/super: skip non-memcg-aware nr_cached_objects in memcg slab shrink")
> Acked-by: Qi Zheng <qi.zheng@xxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---
> v1 -> v2:
> - Do not gate xfs_fs_nr_cached_objects(); XFS's inode reclaim is
>   intentionally driven from per-memcg contexts to free memcg-charged
>   slab (Dave Chinner).
> - Add mem_cgroup_shrink_is_root() helper in <linux/memcontrol.h> so the
>   filesystem callbacks don't open-code "sc->memcg is NULL or root".
>   (Dave Chinner)
> - Add fixes tag (Dave Chinner)
> ---

For

>  fs/btrfs/super.c           | 10 ++++++++++

Acked-by: David Sterba <dsterba@xxxxxxxx>

Thanks.