memcg: fix endless loop caused by mem_cgroup_iter

[platform/adaptation/renesas_rcar/renesas_kernel.git] / mm / memcontrol.c

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c8336e8..da07784 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1158,7 +1158,15 @@ mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
        if (iter->last_dead_count == *sequence) {
                smp_rmb();
                position = iter->last_visited;
-               if (position && !css_tryget(&position->css))
+
+               /*
+                * We cannot take a reference to root because we might race
+                * with root removal and returning NULL would end up in
+                * an endless loop on the iterator user level when root
+                * would be returned all the time.
+                */
+               if (position && position != root &&
+                               !css_tryget(&position->css))
                        position = NULL;
        }
        return position;
@@ -1167,9 +1175,11 @@ mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
 static void mem_cgroup_iter_update(struct mem_cgroup_reclaim_iter *iter,
                                   struct mem_cgroup *last_visited,
                                   struct mem_cgroup *new_position,
+                                  struct mem_cgroup *root,
                                   int sequence)
 {
-       if (last_visited)
+       /* root reference counting symmetric to mem_cgroup_iter_load */
+       if (last_visited && last_visited != root)
                css_put(&last_visited->css);
        /*
         * We store the sequence count from the time @last_visited was
@@ -1244,7 +1254,8 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
                memcg = __mem_cgroup_iter_next(root, last_visited);
 
                if (reclaim) {
-                       mem_cgroup_iter_update(iter, last_visited, memcg, seq);
+                       mem_cgroup_iter_update(iter, last_visited, memcg, root,
+                                       seq);
 
                        if (!memcg)
                                iter->generation++;