From: Muchun Song <songmuchun@bytedance.com>

Since the no-hierarchy mode has been deprecated after the commit:

  commit bef8620cd8e0 ("mm: memcg: deprecate the non-hierarchical mode").

As a result, parent_mem_cgroup() will not return NULL except when passing
the root memcg, and the root memcg cannot be offline. Hence, it's safe to
remove the check on the returned value of parent_mem_cgroup(). Remove the
corresponding dead code.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Chen Ridong <chenridong@huawei.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/memcontrol.c | 5 -----
 mm/shrinker.c   | 6 +-----
 2 files changed, 1 insertion(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f9455b4790ba5..c68af9fea0abd 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3352,9 +3352,6 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 		return;
 
 	parent = parent_mem_cgroup(memcg);
-	if (!parent)
-		parent = root_mem_cgroup;
-
 	memcg_reparent_list_lrus(memcg, parent);
 
 	/*
@@ -3645,8 +3642,6 @@ struct mem_cgroup *mem_cgroup_private_id_get_online(struct mem_cgroup *memcg)
 			break;
 		}
 		memcg = parent_mem_cgroup(memcg);
-		if (!memcg)
-			memcg = root_mem_cgroup;
 	}
 	return memcg;
 }
diff --git a/mm/shrinker.c b/mm/shrinker.c
index 4a93fd433689a..e8e092a2f7f41 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -286,14 +286,10 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
 {
 	int nid, index, offset;
 	long nr;
-	struct mem_cgroup *parent;
+	struct mem_cgroup *parent = parent_mem_cgroup(memcg);
 	struct shrinker_info *child_info, *parent_info;
 	struct shrinker_info_unit *child_unit, *parent_unit;
 
-	parent = parent_mem_cgroup(memcg);
-	if (!parent)
-		parent = root_mem_cgroup;
-
 	/* Prevent from concurrent shrinker_info expand */
 	mutex_lock(&shrinker_mutex);
 	for_each_node(nid) {
-- 
2.20.1