memcg: fix low limit calculation

A memcg is considered low limited even when the current usage is equal to
the low limit.  This leads to interesting side effects e.g.
groups/hierarchies with no memory accounted are considered protected and
so the reclaim will emit MEMCG_LOW event when encountering them.

Another and much bigger issue was reported by Joonsoo Kim.  He has hit a
NULL ptr dereference with the legacy cgroup API which even doesn't have
low limit exposed.  The limit is 0 by default but the initial check fails
for memcg with 0 consumption and parent_mem_cgroup() would return NULL if
use_hierarchy is 0 and so page_counter_read would try to dereference NULL.

I suppose that the current implementation is just an overlook because the
documentation in Documentation/cgroups/unified-hierarchy.txt says:

  "The memory.low boundary on the other hand is a top-down allocated
  reserve.  A cgroup enjoys reclaim protection when it and all its
  ancestors are below their low boundaries"

Fix the usage and the low limit comparision in mem_cgroup_low accordingly.

Fixes: 241994ed (mm: memcontrol: default hierarchy interface for memory)
Reported-by: NJoonsoo Kim <js1304@gmail.com>
Signed-off-by: NMichal Hocko <mhocko@suse.cz>
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -5426,7 +5426,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
 	if (memcg == root_mem_cgroup)
 		return false;
 
-	if (page_counter_read(&memcg->memory) > memcg->low)
+	if (page_counter_read(&memcg->memory) >= memcg->low)
 		return false;
 
 	while (memcg != root) {
@@ -5435,7 +5435,7 @@ bool mem_cgroup_low(struct mem_cgroup *root, struct mem_cgroup *memcg)
 		if (memcg == root_mem_cgroup)
 			break;
 
-		if (page_counter_read(&memcg->memory) > memcg->low)
+		if (page_counter_read(&memcg->memory) >= memcg->low)
 			return false;
 	}
 	return true;