slab: set free_limit for dead caches to 0

slab: set free_limit for dead caches to 0

We mustn't keep empty slabs on dead memcg caches, because otherwise they
will never be destroyed.

Currently, we check if the cache is dead in free_block and drop empty slab
if so irrespective of the node's free_limit.  This can be pretty
expensive.  So let's avoid this additional check by zeroing nodes'
free_limit for dead caches on kmem_cache_shrink.  This way no additional
overhead is added to free hot path.

Note, since ->free_limit can be updated on cpu/memory hotplug, we must
handle it properly there.

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