fs/super: skip non-memcg-aware nr_cached_objects in memcg slab shrink

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 <usama.arif@linux.dev>
Link: https://patch.msgid.link/20260609123047.1948242-1-usama.arif@linux.dev
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/super.c

@@ -24,6 +24,7 @@
 #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 */
@@ -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);