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FAILED: patch "[PATCH] fs: push nr_cached_objects memcg gating into individual" failed to apply to 7.2-stable tree

PATCH [email protected] 1 participant 1 message Sep 23

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Stable branch Memory management Btrfs file system memcg +1 more subsystems
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The patch below does not apply to the 7.2-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-7.2.y
git checkout FETCH_HEAD
git cherry-pick -x 0ef8faff490be6aa1a1e5dfcb0c8492689e91c0f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026092315-overtone-rename-4864@gregkh' --subject-prefix 'PATCH 7.2.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 0ef8faff490be6aa1a1e5dfcb0c8492689e91c0f Mon Sep 17 00:00:00 2001
From: Usama Arif <[email protected]>
Date: Wed, 15 Jul 2026 03:35:16 -0700
Subject: [PATCH] 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 <[email protected]>
Reviewed-by Jan Kara <[email protected]>
Reviewed-by Shakeel Butt <[email protected]>
Signed-off-by Usama Arif <[email protected]>
Link https://patch.msgid.link/[email protected] open here
Acked-by David Sterba <[email protected]>
Reviewed-by Baolin Wang <[email protected]>
Signed-off-by Christian Brauner (Amutable) <[email protected]>
fs/btrfs/super.c +10 -0
index a7d804219bec..cc4537435399 100644
@@ -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;
include/linux/memcontrol.h +21 -0
index e1f46a0016fc..5407e4200460 100644
@@ -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;
mm/shmem.c +10 -0
index b51f83c970bb..9001aaf3b7b9 100644
@@ -846,6 +846,16 @@ 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.
+	 */
+	if (!mem_cgroup_shrink_is_root(sc))
+		return 0;
+
 	return READ_ONCE(sbinfo->shrinklist_len);
 }
 #else /* !CONFIG_TRANSPARENT_HUGEPAGE */