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

[Jan Kara]

On Tue 14-07-26 03:14:54, 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 skips the
> ->nr_cached_objects() hook whenever the shrinker is invoked for a
> non-root memcg, because none of the current implementations (btrfs,
> xfs, shmem huge) honour sc->memcg.
> 
> That policy is really a filesystem-owned property: fs/super.c should
> not encode the assumption that these hooks are never memcg-aware,
> since a future implementation might legitimately filter by sc->memcg.
> Move the check into btrfs_nr_cached_objects(), xfs_fs_nr_cached_objects()
> and shmem_unused_huge_count() so each filesystem can lift the
> restriction independently once its underlying counters/scans become
> memcg-aware, without needing a coordinated change to fs/super.c.
> 
> Behaviour is unchanged: calls into these hooks from shrink_slab_memcg()
> still early-return 0 for non-root memcg contexts, keeping the shrinker
> bit clearable in each memcg's bitmap; the global (kswapd or root
> direct reclaim) path still drives them as before.
> 
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Makes sense to me. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  fs/btrfs/super.c   | 11 +++++++++++
>  fs/super.c         | 19 ++-----------------
>  fs/xfs/xfs_super.c | 12 ++++++++++++
>  mm/shmem.c         | 11 +++++++++++
>  4 files changed, 36 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index a7d804219bec..16c24bccc897 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -22,6 +22,7 @@
>  #include <linux/namei.h>
>  #include <linux/miscdevice.h>
>  #include <linux/magic.h>
> +#include <linux/memcontrol.h>
>  #include <linux/slab.h>
>  #include <linux/ratelimit.h>
>  #include <linux/crc32c.h>
> @@ -2434,6 +2435,16 @@ static long btrfs_nr_cached_objects(struct super_block *sb, struct shrink_contro
>  	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
>  	const s64 nr = percpu_counter_read_positive(&fs_info->evictable_extent_maps);
>  
> +	/*
> +	 * The evictable extent map counter 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 and
> +	 * pinning the shrinker bit in every memcg's bitmap.
> +	 */
> +	if (sc->memcg && !mem_cgroup_is_root(sc->memcg))
> +		return 0;
> +
>  	trace_btrfs_extent_map_shrinker_count(fs_info, nr);
>  
>  	return nr;
> diff --git a/fs/super.c b/fs/super.c
> index d2d04a6f4f84..a8fd61136aaf 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -24,7 +24,6 @@
>  #include <linux/export.h>
>  #include <linux/slab.h>
>  #include <linux/blkdev.h>
> -#include <linux/memcontrol.h>
>  #include <linux/mount.h>
>  #include <linux/security.h>
>  #include <linux/writeback.h>		/* for the emergency remount stuff */
> @@ -170,19 +169,6 @@ static void super_wake(struct super_block *sb, unsigned int flag)
>  	wake_up_var(&sb->s_flags);
>  }
>  
> -/*
> - * The s_op->nr_cached_objects hooks (used for example by btrfs and xfs)
> - * operate on filesystem-global state and ignore sc->memcg. Driving them
> - * from per-memcg shrink_slab_memcg() invocations only burns CPU walking
> - * per-cpu counters and queueing duplicate work: the actual reclaim happens on
> - * the global path (kswapd or root direct reclaim) regardless. Restrict them
> - * to that path.
> - */
> -static inline bool super_fs_objects_eligible(struct shrink_control *sc)
> -{
> -	return !sc->memcg || mem_cgroup_is_root(sc->memcg);
> -}
> -
>  /*
>   * One thing we have to be careful of with a per-sb shrinker is that we don't
>   * drop the last active reference to the superblock from within the shrinker.
> @@ -212,7 +198,7 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
>  	if (!super_trylock_shared(sb))
>  		return SHRINK_STOP;
>  
> -	if (sb->s_op->nr_cached_objects && super_fs_objects_eligible(sc))
> +	if (sb->s_op->nr_cached_objects)
>  		fs_objects = sb->s_op->nr_cached_objects(sb, sc);
>  
>  	inodes = list_lru_shrink_count(&sb->s_inode_lru, sc);
> @@ -273,8 +259,7 @@ static unsigned long super_cache_count(struct shrinker *shrink,
>  		return 0;
>  	smp_rmb();
>  
> -	if (sb->s_op && sb->s_op->nr_cached_objects &&
> -	    super_fs_objects_eligible(sc))
> +	if (sb->s_op && sb->s_op->nr_cached_objects)
>  		total_objects = sb->s_op->nr_cached_objects(sb, sc);
>  
>  	total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc);
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index eac7f9503805..f0d8251fad3b 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -54,6 +54,7 @@
>  #include "scrub/rcbag_btree.h"
>  
>  #include <linux/magic.h>
> +#include <linux/memcontrol.h>
>  #include <linux/fs_context.h>
>  #include <linux/fs_parser.h>
>  #include <linux/fserror.h>
> @@ -1242,6 +1243,17 @@ xfs_fs_nr_cached_objects(
>  	/* Paranoia: catch incorrect calls during mount setup or teardown */
>  	if (WARN_ON_ONCE(!sb->s_fs_info))
>  		return 0;
> +
> +	/*
> +	 * The reclaimable inode count 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 and
> +	 * pinning the shrinker bit in every memcg's bitmap.
> +	 */
> +	if (sc->memcg && !mem_cgroup_is_root(sc->memcg))
> +		return 0;
> +
>  	return xfs_reclaim_inodes_count(XFS_M(sb));
>  }
>  
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 5789a0f5a346..3ba812e4f5ba 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -846,6 +846,17 @@ 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 and
> +	 * pinning the shrinker bit in every memcg's bitmap.
> +	 */
> +	if (sc->memcg && !mem_cgroup_is_root(sc->memcg))
> +		return 0;
> +
>  	return READ_ONCE(sbinfo->shrinklist_len);
>  }
>  #else /* !CONFIG_TRANSPARENT_HUGEPAGE */
> -- 
> 2.53.0-Meta
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR