mm/memcontrol.c: introduce helper mem_cgroup_zoneinfo_zone()

Introduce helper mem_cgroup_zoneinfo_zone().  This makes mem_cgroup_iter()
code more compact.

Signed-off-by: Jianyu Zhan <nasa4836@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Jianyu Zhan <nasa4836@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/memcontrol.c

@@ -683,6 +683,15 @@ mem_cgroup_zoneinfo(struct mem_cgroup *memcg, int nid, int zid)
 	return &memcg->nodeinfo[nid]->zoneinfo[zid];
 }
 
+static struct mem_cgroup_per_zone *
+mem_cgroup_zoneinfo_zone(struct mem_cgroup *memcg, struct zone *zone)
+{
+       int nid = zone_to_nid(zone);
+       int zid = zone_idx(zone);
+
+       return mem_cgroup_zoneinfo(memcg, nid, zid);
+}
+
 struct cgroup_subsys_state *mem_cgroup_css(struct mem_cgroup *memcg)
 {
 	return &memcg->css;
@@ -1234,11 +1243,9 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		int uninitialized_var(seq);
 
 		if (reclaim) {
-			int nid = zone_to_nid(reclaim->zone);
-			int zid = zone_idx(reclaim->zone);
 			struct mem_cgroup_per_zone *mz;
 
-			mz = mem_cgroup_zoneinfo(root, nid, zid);
+			mz = mem_cgroup_zoneinfo_zone(root, reclaim->zone);
 			iter = &mz->reclaim_iter[reclaim->priority];
 			if (prev && reclaim->generation != iter->generation) {
 				iter->last_visited = NULL;
@@ -1345,7 +1352,7 @@ struct lruvec *mem_cgroup_zone_lruvec(struct zone *zone,
 		goto out;
 	}
 
-	mz = mem_cgroup_zoneinfo(memcg, zone_to_nid(zone), zone_idx(zone));
+	mz = mem_cgroup_zoneinfo_zone(memcg, zone);
 	lruvec = &mz->lruvec;
 out:
 	/*