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@cmpxchg.org: add comment to mem_cgroup_swapout]
[vdavydov@virtuozzo.com: use WARN_ON_ONCE() in mem_cgroup_id_get_online()]
  Link: http://lkml.kernel.org/r/20160803123445.GJ13263@esperanza
Fixes: 73f576c04b941 ("mm: memcontrol: fix cgroup creation failure after many small jobs")
Link: http://lkml.kernel.org/r/5336daa5c9a32e776067773d9da655d2dc126491.1470219853.git.vdavydov@virtuozzo.com
Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: <stable@vger.kernel.org>	[3.19+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>