revert "memcg, vmscan: do not attempt soft limit reclaim if it would not scan anything"

Revert commit e839b6a1 ("memcg, vmscan: do not attempt soft limit
reclaim if it would not scan anything")

I merged this prematurely - Michal and Johannes still disagree about the
overall design direction and the future remains unclear.

Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1872,11 +1872,7 @@ enum mem_cgroup_filter_t
 mem_cgroup_soft_reclaim_eligible(struct mem_cgroup *memcg,
 		struct mem_cgroup *root)
 {
-	struct mem_cgroup *parent;
-
-	if (!memcg)
-		memcg = root_mem_cgroup;
-	parent = memcg;
+	struct mem_cgroup *parent = memcg;
 
 	if (res_counter_soft_limit_excess(&memcg->res))
 		return VISIT;

mm/vmscan.c

@@ -142,9 +142,7 @@ static bool global_reclaim(struct scan_control *sc)
 
 static bool mem_cgroup_should_soft_reclaim(struct scan_control *sc)
 {
-	struct mem_cgroup *root = sc->target_mem_cgroup;
-	return !mem_cgroup_disabled() &&
-		mem_cgroup_soft_reclaim_eligible(root, root) != SKIP_TREE;
+	return !mem_cgroup_disabled();
 }
 #else
 static bool global_reclaim(struct scan_control *sc)