From: Qi Zheng <zhengqi.arch@bytedance.com>

Refactor the memcg_reparent_objcgs() to facilitate subsequent reparenting
LRU folios here.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 mm/memcontrol.c | 37 +++++++++++++++++++++++++++----------
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 930dacd6ce31a..3daa99a0c65fe 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -206,24 +206,41 @@ static struct obj_cgroup *obj_cgroup_alloc(void)
 	return objcg;
 }
 
-static void memcg_reparent_objcgs(struct mem_cgroup *memcg)
+static inline void __memcg_reparent_objcgs(struct mem_cgroup *src,
+				    struct mem_cgroup *dst)
 {
 	struct obj_cgroup *objcg, *iter;
-	struct mem_cgroup *parent = parent_mem_cgroup(memcg);
-
-	objcg = rcu_replace_pointer(memcg->objcg, NULL, true);
-
-	spin_lock_irq(&objcg_lock);
 
+	objcg = rcu_replace_pointer(src->objcg, NULL, true);
 	/* 1) Ready to reparent active objcg. */
-	list_add(&objcg->list, &memcg->objcg_list);
+	list_add(&objcg->list, &src->objcg_list);
 	/* 2) Reparent active objcg and already reparented objcgs to parent. */
-	list_for_each_entry(iter, &memcg->objcg_list, list)
-		WRITE_ONCE(iter->memcg, parent);
+	list_for_each_entry(iter, &src->objcg_list, list)
+		WRITE_ONCE(iter->memcg, dst);
 	/* 3) Move already reparented objcgs to the parent's list */
-	list_splice(&memcg->objcg_list, &parent->objcg_list);
+	list_splice(&src->objcg_list, &dst->objcg_list);
+}
+
+static inline void reparent_locks(struct mem_cgroup *src, struct mem_cgroup *dst)
+{
+	spin_lock_irq(&objcg_lock);
+}
 
+static inline void reparent_unlocks(struct mem_cgroup *src, struct mem_cgroup *dst)
+{
 	spin_unlock_irq(&objcg_lock);
+}
+
+static void memcg_reparent_objcgs(struct mem_cgroup *src)
+{
+	struct obj_cgroup *objcg = rcu_dereference_protected(src->objcg, true);
+	struct mem_cgroup *dst = parent_mem_cgroup(src);
+
+	reparent_locks(src, dst);
+
+	__memcg_reparent_objcgs(src, dst);
+
+	reparent_unlocks(src, dst);
 
 	percpu_ref_kill(&objcg->refcnt);
 }
-- 
2.20.1

On Wed, Dec 17, 2025 at 03:27:50PM +0800, Qi Zheng wrote:
> +static void memcg_reparent_objcgs(struct mem_cgroup *src)
> +{
> +	struct obj_cgroup *objcg = rcu_dereference_protected(src->objcg, true);
> +	struct mem_cgroup *dst = parent_mem_cgroup(src);
> +
> +	reparent_locks(src, dst);
> +
> +	__memcg_reparent_objcgs(src, dst);

Please have __memcg_reparent_objcgs() return the dead objcg for the
percpu_ref_kill(), instead of doing the deref twice.

And please use @child, @parent (or @memcg, @parent) throughout instead
of @src and @dst.

> +
> +	reparent_unlocks(src, dst);
>  
>  	percpu_ref_kill(&objcg->refcnt);

With that,

Acked-by: Johannes Weiner <hannes@cmpxchg.org>