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

[Usama Arif <usama.arif@linux.dev>]

On 14/07/2026 23:22, Dave Chinner wrote:
> On Tue, Jul 14, 2026 at 03:14:54AM -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 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.
> 
> I don't see that commit in an upstream tree. I'm guessing I wasn't
> cc'd on it, either, because I don't recall seeing that as a patch,
> either.

Ah sorry about this, I just ran get_maintainers.pl on the patch, plus added
shmem, btrfs and xfs maintainers on it. get_maintainers.pl didn't report
shrinker maintainers (likely because the change was in fs/) and it didn't
come to my mind as well :(

The patch is here: https://lore.kernel.org/all/20260609123047.1948242-1-usama.arif@linux.dev/> 
> However, if anything is gating the fs_objects callouts to the fs
> when memcg is being srhunk, then it is fundamentally broken and you
> are correct to fix it.
> 
>>
>> 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.
> 
> However, your assumptions here are incorrect.
> 
> That is, the XFS fs_objects implementation is intended to be called
> even when memcg shrinking is occurring. I architected it that way
> all those years ago when I introduced the fs_objects shrinker
> callout. i.e. XFS is not aging reclaimable inodes via this call - it
> is purely an expedite freeing of recently reclaimed VFS inodes when
> there is memory pressure of any kind.
> 
> Reclaimable XFS inodes are still charged to memcgs, but we don't
> track them per-memcg because it is extremely inefficient when their
> lifetime after being released by the VFS is usually less than 5
> seconds. Indeed, this low level cache is not for working set
> retention; the VFS inode cache does that. The XFS inodes need to
> through a GC state before they can be freed, this takes a little bit
> of time, so we don't hold up the VFS inode cache shrinker for that.
> 
> The VFS inode cache shrinker is memcg aware, so it only pushes
> inodes that are being reclaimed by memcg reclaim into the the
> reclaimable state at the XFS level. Hence we don't need to track
> reclaimable inodes by memcg - we know that memcg owned inodes that
> are to be freed have already been pushed into reclaim by the VFS
> shrinker doing memcg reclaim. Hence all we need to do is sweep them
> and free them.
> 
> This is what the XFS fs objects shrinker does.  It needs to run in
> conjunction with -any- shrinker context to immediately free the
> clean inodes that the VFS shrinker released that pass, and to
> release any of the recently released VFS inodes that needed GC and
> are now clean and can be freed.
> 
> IOWs, the XFS fs_objects callout is not "memcg-aware" in the classic
> sense of "it tracks objects by their owner memcg". However, the
> -combined superblock shrinker algorithm- is memcg aware and that
> results in XFS only seeing objects from the memcg being reclaimed
> transitioning to RECLAIMABLE state which it then immediately sweeps
> away.
> 
> i.e. the architecture of the VFS and XFS inode cache reclaim is
> entirely memcg aware, and that is why I implemented the fs_objects
> callout for XFS all those years ago. It works efficiently, it works
> correctly with memcg based reclaim, and XFS doesn't need to care
> about memcgs in it's low level reclaim code. Win, win, win.

Ack on above, and Thanks for explaining this!> 
>> Behaviour is unchanged: calls into these hooks from shrink_slab_memcg()
> 
> Behaviour was broken by the above commit, it needs to change.

Ack.> 
>> 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@linux.dev>
>>
>> @@ -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);
>> -}
> 
> This wrapper should still exist (with a better name) so filesystems
> don't need to open code memcg cruft.

Ack,

I am going to send a v2, which will include Fixes tag and follow above.> 
> -Dave.