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, 21 insertions, 0 deletions

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e1f46a0016fc..5407e4200460 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -520,6 +520,22 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 	return (memcg == root_mem_cgroup);
 }
 
+/**
+ * mem_cgroup_shrink_is_root - is this a global or root-memcg shrink invocation?
+ * @sc: shrink_control describing the current shrinker call
+ *
+ * Returns true when @sc represents a global reclaim shrink (sc->memcg == NULL)
+ * or a root-memcg shrink, i.e. not a per-memcg iteration of
+ * shrink_slab_memcg(). Filesystems whose ->nr_cached_objects()/
+ * ->free_cached_objects() implementations operate on filesystem-global state
+ * and do not honour sc->memcg can use this to early-return 0 in per-memcg
+ * contexts.
+ */
+static inline bool mem_cgroup_shrink_is_root(struct shrink_control *sc)
+{
+	return !sc->memcg || mem_cgroup_is_root(sc->memcg);
+}
+
 static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
 {
 	return objcg->is_root;
@@ -1071,6 +1087,11 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 	return true;
 }
 
+static inline bool mem_cgroup_shrink_is_root(struct shrink_control *sc)
+{
+	return true;
+}
+
 static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg)
 {
 	return true;