fs: push nr_cached_objects memcg gating into individual filesystems

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@linux.dev>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
Link: https://patch.msgid.link/20260715103516.2410175-1-usama.arif@linux.dev
Acked-by: David Sterba <dsterba@suse.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index a7d804219bec..cc4537435399 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 +2435,15 @@ 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.
+	 */
+	if (!mem_cgroup_shrink_is_root(sc))
+		return 0;
+
 	trace_btrfs_extent_map_shrinker_count(fs_info, nr);
 
 	return nr;