[PATCH] memcg: fix __mem_cgroup_same_or_subtree

[Michal Hocko]

__mem_cgroup_same_or_subtree relies on css_is_ancestor if hierarchy is
enabled for ages. This, however, is not correct because use_hierarchy
doesn't need to be true all the way up the cgroup hierarchy. Consider
the following example:
root (use_hierarchy=0)
 \
  A (use_hierarchy=0)
   \
    B (use_hierarchy=1)
     \
      C (use_hierarchy=1)

__mem_cgroup_same_or_subtree(A, C) would return true even though C is
not from the same hierarchy subtree. The bug shouldn't be critical but
at least dump_tasks might print unrelated tasks (via
task_in_mem_cgroup).

Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/memcontrol.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f608546..177bec2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1470,9 +1470,12 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
 {
 	if (root_memcg == memcg)
 		return true;
-	if (!root_memcg->use_hierarchy || !memcg)
+	if (!memcg)
 		return false;
-	return css_is_ancestor(&memcg->css, &root_memcg->css);
+	while ((memcg = parent_mem_cgroup(memcg)))
+		if (memcg == root_memcg)
+			return true;
+	return false;
 }
 
 static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
-- 
1.7.10.4

-- 
Michal Hocko
SUSE Labs
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