memcg: do not expose uninitialized mem_cgroup_per_node to world

If somebody is touching data too early, it might be easier to diagnose a
problem when dereferencing NULL at mem->info.nodeinfo[node] than trying to
understand why mem_cgroup_per_zone is [un|partly]initialized.
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Acked-by: NMichal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -4742,7 +4742,6 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *memcg, int node)
 	if (!pn)
 		return 1;
 
-	memcg->info.nodeinfo[node] = pn;
 	for (zone = 0; zone < MAX_NR_ZONES; zone++) {
 		mz = &pn->zoneinfo[zone];
 		for_each_lru(l)
@@ -4751,6 +4750,7 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *memcg, int node)
 		mz->on_tree = false;
 		mz->mem = memcg;
 	}
+	memcg->info.nodeinfo[node] = pn;
 	return 0;
 }