slub: reduce failure of this_cpu_cmpxchg in put_cpu_partial() after unfreezing

In current implementation, after unfreezing, we doesn't touch oldpage,
so it remain 'NOT NULL'. When we call this_cpu_cmpxchg()
with this old oldpage, this_cpu_cmpxchg() is mostly be failed.

We can change value of oldpage to NULL after unfreezing,
because unfreeze_partial() ensure that all the cpu partial slabs is removed
from cpu partial list. In this time, we could expect that
this_cpu_cmpxchg is mostly succeed.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -1962,6 +1962,7 @@ int put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 				local_irq_save(flags);
 				unfreeze_partials(s);
 				local_irq_restore(flags);
+				oldpage = NULL;
 				pobjects = 0;
 				pages = 0;
 				stat(s, CPU_PARTIAL_DRAIN);