Now, the only reason to keep kmemcg_id till css free is list_lru, which
uses it to distribute elements between per-memcg lists.  However, it can
be easily sorted out - we only need to change kmemcg_id of an offline
cgroup to its parent's id, making further list_lru_add()'s add elements to
the parent's list, and then move all elements from the offline cgroup's
list to the one of its parent.  It will work, because a racing
list_lru_del() does not need to know the list it is deleting the element
from.  It can decrement the wrong nr_items counter though, but the ongoing
reparenting will fix it.  After list_lru reparenting is done we are free
to release kmemcg_id saving a valuable slot in a per-memcg array for new
cgroups.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>