memcg: keep root group unchanged if creation fails

If the request is to create non-root group and we fail to meet it, we
should leave the root unchanged.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4907,9 +4907,9 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 		int cpu;
 		enable_swap_cgroup();
 		parent = NULL;
-		root_mem_cgroup = memcg;
 		if (mem_cgroup_soft_limit_tree_init())
 			goto free_out;
+		root_mem_cgroup = memcg;
 		for_each_possible_cpu(cpu) {
 			struct memcg_stock_pcp *stock =
 						&per_cpu(memcg_stock, cpu);
@@ -4948,7 +4948,6 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 	return &memcg->css;
 free_out:
 	__mem_cgroup_free(memcg);
-	root_mem_cgroup = NULL;
 	return ERR_PTR(error);
 }