memcg: destroy kmem caches when last slab is freed

memcg: destroy kmem caches when last slab is freed

When the memcg_cache_params->refcnt goes to 0, schedule the worker that
will unregister the cache.  To prevent this from happening when the owner
memcg is alive, keep the refcnt incremented during memcg lifetime.

Note, this doesn't guarantee that the cache that belongs to a dead memcg
will go away as soon as the last object is freed, because SL[AU]B
implementation can cache empty slabs for performance reasons.  Hence the
cache may be hanging around indefinitely after memcg offline.  This is to
be resolved by the next patches.

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