Re: [PATCH v3 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup

[Vladimir Davydov]

On Wed, Aug 03, 2016 at 01:35:06PM +0300, Vladimir Davydov wrote:
> An offline memory cgroup might have anonymous memory or shmem left
> charged to it and no swap. Since only swap entries pin the id of an
> offline cgroup, such a cgroup will have no id and so an attempt to
> swapout its anon/shmem will not store memory cgroup info in the swap
> cgroup map. As a result, memcg->swap or memcg->memsw will never get
> uncharged from it and any of its ascendants.
> 
> Fix this by always charging swapout to the first ancestor cgroup that
> hasn't released its id yet.
> 
> [hannes@xxxxxxxxxxx: add comment to mem_cgroup_swapout]
> Fixes: 73f576c04b941 ("mm: memcontrol: fix cgroup creation failure after many small jobs")
> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>	[3.19+]

Andrew, could you please fold this in?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1c0aa59fd333..8c8e68becee9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4044,7 +4044,7 @@ static struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg)
 		 * The root cgroup cannot be destroyed, so it's refcount must
 		 * always be >= 1.
 		 */
-		if (memcg == root_mem_cgroup) {
+		if (WARN_ON_ONCE(memcg == root_mem_cgroup)) {
 			VM_BUG_ON(1);
 			break;
 		}