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

[Baolin Wang]

On 7/15/26 6:35 PM, 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>
> ---

[snip]

> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e1f46a0016fc..5407e4200460 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -520,6 +520,22 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
>   	return (memcg == root_mem_cgroup);
>   }
>   
> +/**
> + * mem_cgroup_shrink_is_root - is this a global or root-memcg shrink invocation?
> + * @sc: shrink_control describing the current shrinker call
> + *
> + * Returns true when @sc represents a global reclaim shrink (sc->memcg == NULL)
> + * or a root-memcg shrink, i.e. not a per-memcg iteration of
> + * shrink_slab_memcg(). Filesystems whose ->nr_cached_objects()/
> + * ->free_cached_objects() implementations operate on filesystem-global state
> + * and do not honour sc->memcg can use this to early-return 0 in per-memcg
> + * contexts.
> + */
> +static inline bool mem_cgroup_shrink_is_root(struct shrink_control *sc)
> +{
> +	return !sc->memcg || mem_cgroup_is_root(sc->memcg);
> +}
> +
>   static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
>   {
>   	return objcg->is_root;
> @@ -1071,6 +1087,11 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
>   	return true;
>   }
>   
> +static inline bool mem_cgroup_shrink_is_root(struct shrink_control *sc)
> +{
> +	return true;
> +}
> +
>   static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
>   {
>   	return true;
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 5789a0f5a346..dc8cd4f563f4 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -846,6 +846,16 @@ static long shmem_unused_huge_count(struct super_block *sb,
>   		struct shrink_control *sc)
>   {
>   	struct shmem_sb_info *sbinfo = SHMEM_SB(sb);
> +
> +	/*
> +	 * The per-superblock shrinklist is filesystem-global and does not
> +	 * honour sc->memcg, so it is only meaningful on the global (kswapd or
> +	 * root direct reclaim) shrink path. Skip the per-memcg iterations of
> +	 * shrink_slab_memcg() to avoid queueing duplicate global work.
> +	 */
> +	if (!mem_cgroup_shrink_is_root(sc))
> +		return 0;
> +
>   	return READ_ONCE(sbinfo->shrinklist_len);
>   }
>   #else /* !CONFIG_TRANSPARENT_HUGEPAGE */

For shmem part, LGTM.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>