mm: memcontrol: remove dead code and comments

Since no-hierarchy mode is deprecated after

  commit bef8620 ("mm: memcg: deprecate the non-hierarchical mode")

so parent_mem_cgroup() cannot return a NULL except root memcg, however, root
memcg cannot be offline, so it is safe to drop the check of returned value
of parent_mem_cgroup().  Remove those dead code.

The comments in memcg_offline_kmem() above memcg_reparent_list_lrus() are
out of date since

  commit 5abc1e3 ("mm: list_lru: allocate list_lru_one only when needed")

There is no ordering requirement between memcg_reparent_list_lrus() and
memcg_reparent_objcgs(), so remove those outdated comments.

Signed-off-by: Muchun Song <[email protected]>
Acked-by: Roman Gushchin <[email protected]>

Link: https://lore.kernel.org/all/[email protected]/

Orabug: 37405594

 Conflicts:
	i. mm/vmscan.c
	(Due to 'commit 96f7b2b mm: vmscan: move shrinker-related code into a
	 separate file', which moved shrinker related functions away from vmscan.c)
	ii. include/linux/memcontrol.h
	(Due to 'commit ca39c5e mm/memcg: update obsolete comment above
	 parent_mem_cgroup()' - which fixed the comments above parent_mem_cgroup())

Signed-off-by: Imran Khan <[email protected]>
Reviewed-by: Kamalesh Babulal <[email protected]>

Author: Muchun Song

mm/memcontrol.c

@@ -3116,17 +3116,7 @@ static void memcg_offline_kmem(struct mem_cgroup *memcg)
 		return;
 
 	parent = parent_mem_cgroup(memcg);
-	if (!parent)
-		parent = root_mem_cgroup;
-
 	memcg_reparent_objcgs(memcg, parent);
-
-	/*
-	 * After we have finished memcg_reparent_objcgs(), all list_lrus
-	 * corresponding to this cgroup are guaranteed to remain empty.
-	 * The ordering is imposed by list_lru_node->lock taken by
-	 * memcg_reparent_list_lrus().
-	 */
 	memcg_reparent_list_lrus(memcg, parent);
 }
 
@@ -4951,8 +4941,6 @@ static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
 			break;
 		}
 		memcg = parent_mem_cgroup(memcg);
-		if (!memcg)
-			memcg = root_mem_cgroup;
 	}
 	return memcg;
 }

mm/shrinker.c

@@ -286,13 +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);