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From: Usama Arif In-Reply-To: <5exadtggg276xp5tayiqbbomnms4hjyaopmgu3chfpl2ug56dq@vyhswy2afzmo> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: CE75C2000A X-Rspam-User: X-Stat-Signature: oz17khbq4p7h8h8px4cegpw5rif3nccb X-Rspamd-Server: rspam08 X-HE-Tag: 1781012345-228107 X-HE-Meta: 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 4Yx8EDOp xUOGkhlevSC8aGKAnyWxsveX3mY9zEO5BbKyarTUWWjbXhnUYN2yt/jO97I+d3Y5PHOqvtgj5o2JVj0LxGSkuYYl2htiqU7oDcqS6ag2yEDLApMbWw+pufSuww87jll9h6VpV8xbWmqKFJ6I2a3D3F44+J3GHkbi1p+QoTGKn1KwB+DJFhmamZA6FjZb/o5jQZA7qqaclZ2vq7f46JLITw2YxATn+fOWxGawBnP0FGuUXSGV3kzJVZ7xCRz/a4ZH6LIyC Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09/06/2026 13:52, Jan Kara wrote: > On Tue 09-06-26 05:30:47, Usama Arif wrote: >> The super_block shrinker is registered with SHRINKER_MEMCG_AWARE because its >> dentry and inode LRUs are memcg-aware (via list_lru). But the optional >> ->nr_cached_objects() hooks that the shrinker also drives are not memcg-aware: >> btrfs extent maps and xfs inode reclaim operate on filesystem-global >> state, and shmem's unused-huge shrinker walks a per-superblock shrinklist. >> None of them filter by sc->memcg. >> >> The mismatch shows up under memcg-heavy slab reclaim. shrink_slab_memcg() >> calls do_shrink_slab() once per (memcg, NUMA node) pair for every memcg >> whose bit is set in the per-superblock shrinker bitmap, which on a busy >> host means hundreds of calls per reclaim pass. Each scan queues the same >> global shrinker work item that's already kicked from the root path. >> >> Because btrfs/xfs global count is typically non-zero on any in-use filesystem, >> the returned total stays positive even if a memcg's own dentry/inode LRUs >> are empty. shrink_slab_memcg() therefore never clears the SB shrinker bit >> in the memcg bitmap, so subsequent reclaim passes from the same memcg >> re-enter super_cache_count() and pay for the global counter walk again. >> >> Restrict ->nr_cached_objects() to the global shrink path (sc->memcg NULL >> or root). The memcg-aware dentry/inode LRUs keep being counted and >> scanned per memcg as before; only the global fs-specific hooks are skipped. >> The root/global shrink path still drives those hooks; only their >> invocation from non-root memcg slab reclaim is removed. >> >> Signed-off-by: Usama Arif > > To me this makes sense. However I'm bit surprised that XFS inode shrinker > (which is what gets counted in nr_cached_objects for XFS) isn't memcg > aware. I guess since these inodes are on their way to a relatively quick > destruction, nobody really bothered. So I'm fine with the change, just > I'd like to make sure XFS folks are aware and don't plan anything in this > area. > Thanks for the review! Yes I am hoping that if there are any objections from xfs or btrfs, it gets raised. Have cc'ed the btrfs and xfs maintainers and reviewers. Thanks! Usama > Honza > >> --- >> fs/super.c | 19 +++++++++++++++++-- >> 1 file changed, 17 insertions(+), 2 deletions(-) >> >> diff --git a/fs/super.c b/fs/super.c >> index 378e81efe643..5216c5dbd4c4 100644 >> --- a/fs/super.c >> +++ b/fs/super.c >> @@ -24,6 +24,7 @@ >> #include >> #include >> #include >> +#include >> #include >> #include >> #include /* for the emergency remount stuff */ >> @@ -169,6 +170,19 @@ 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. >> @@ -198,7 +212,7 @@ static unsigned long super_cache_scan(struct shrinker *shrink, >> if (!super_trylock_shared(sb)) >> return SHRINK_STOP; >> >> - if (sb->s_op->nr_cached_objects) >> + if (sb->s_op->nr_cached_objects && super_fs_objects_eligible(sc)) >> fs_objects = sb->s_op->nr_cached_objects(sb, sc); >> >> inodes = list_lru_shrink_count(&sb->s_inode_lru, sc); >> @@ -259,7 +273,8 @@ static unsigned long super_cache_count(struct shrinker *shrink, >> return 0; >> smp_rmb(); >> >> - if (sb->s_op && sb->s_op->nr_cached_objects) >> + if (sb->s_op && sb->s_op->nr_cached_objects && >> + super_fs_objects_eligible(sc)) >> total_objects = sb->s_op->nr_cached_objects(sb, sc); >> >> total_objects += list_lru_shrink_count(&sb->s_dentry_lru, sc); >> -- >> 2.52.0 >>