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

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

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@linux.dev>
---
 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