On Fri, 19 Oct 2012, Glauber Costa wrote:

An unlikely branch is used to make sure this case does not affect
performance in the usual slab_free path.

The slab allocator has a time based reaper that would eventually get rid
of the objects, but we can also call it explicitly, since dead caches
are not a likely event.

This is also something that could be done from slab_common since all
allocators have kmem_cache_shrink and kmem_cache_shrink can be used to
drain the caches and free up empty slab pages.