mm, memcg: clean up reclaim iter array

The mem_cgroup_reclaim_cookie is only used in memcg softlimit reclaim now,
and the priority of the reclaim is always 0.  We don't need to define the
iter in struct mem_cgroup_per_node as an array any more.  That could make
the code more clear and save some space.

Link: http://lkml.kernel.org/r/1569897728-1686-1-git-send-email-laoar.shao@gmail.com
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -58,7 +58,6 @@ enum mem_cgroup_protection {
 
 struct mem_cgroup_reclaim_cookie {
 	pg_data_t *pgdat;
-	int priority;
 	unsigned int generation;
 };
 
@@ -126,7 +125,7 @@ struct mem_cgroup_per_node {
 
 	unsigned long		lru_zone_size[MAX_NR_ZONES][NR_LRU_LISTS];
 
-	struct mem_cgroup_reclaim_iter	iter[DEF_PRIORITY + 1];
+	struct mem_cgroup_reclaim_iter	iter;
 
 	struct memcg_shrinker_map __rcu	*shrinker_map;
 

mm/memcontrol.c

@@ -1052,7 +1052,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		struct mem_cgroup_per_node *mz;
 
 		mz = mem_cgroup_nodeinfo(root, reclaim->pgdat->node_id);
-		iter = &mz->iter[reclaim->priority];
+		iter = &mz->iter;
 
 		if (prev && reclaim->generation != iter->generation)
 			goto out_unlock;
@@ -1152,15 +1152,11 @@ static void __invalidate_reclaim_iterators(struct mem_cgroup *from,
 	struct mem_cgroup_reclaim_iter *iter;
 	struct mem_cgroup_per_node *mz;
 	int nid;
-	int i;
 
 	for_each_node(nid) {
 		mz = mem_cgroup_nodeinfo(from, nid);
-		for (i = 0; i <= DEF_PRIORITY; i++) {
-			iter = &mz->iter[i];
-			cmpxchg(&iter->position,
-				dead_memcg, NULL);
-		}
+		iter = &mz->iter;
+		cmpxchg(&iter->position, dead_memcg, NULL);
 	}
 }
 
@@ -1705,7 +1701,6 @@ static int mem_cgroup_soft_reclaim(struct mem_cgroup *root_memcg,
 	unsigned long nr_scanned;
 	struct mem_cgroup_reclaim_cookie reclaim = {
 		.pgdat = pgdat,
-		.priority = 0,
 	};
 
 	excess = soft_limit_excess(root_memcg);