[PATCH 1/5] mm: memcg/slab: Fix return child memcg objcg for root memcg

[Muchun Song <songmuchun@xxxxxxxxxxxxx>]

Consider the following memcg hierarchy.

                    root
                   /    \
                  A      B

If we get the objcg of memcg A failed, the get_obj_cgroup_from_current
can return the wrong objcg for the root memcg.

Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1337775b04f3..fcbd79c5023e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2945,7 +2945,7 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
 
 __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
 {
-	struct obj_cgroup *objcg = NULL;
+	struct obj_cgroup *objcg;
 	struct mem_cgroup *memcg;
 
 	if (memcg_kmem_bypass())
@@ -2964,6 +2964,9 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
 	}
 	rcu_read_unlock();
 
+	if (memcg == root_mem_cgroup)
+		return NULL;
+
 	return objcg;
 }
 
-- 
2.20.1