slub: Do not dereference NULL pointer in node_match

The variables accessed in slab_alloc are volatile and therefore
the page pointer passed to node_match can be NULL. The processing
of data in slab_alloc is tentative until either the cmpxhchg
succeeds or the __slab_alloc slowpath is invoked. Both are
able to perform the same allocation from the freelist.

Check for the NULL pointer in node_match.

A false positive will lead to a retry of the loop in __slab_alloc.
Signed-off-by: NChristoph Lameter <cl@linux.com>
Signed-off-by: NPekka Enberg <penberg@kernel.org>

mm/slub.c

@@ -2042,7 +2042,7 @@ static void flush_all(struct kmem_cache *s)
 static inline int node_match(struct page *page, int node)
 {
 #ifdef CONFIG_NUMA
-	if (node != NUMA_NO_NODE && page_to_nid(page) != node)
+	if (!page || (node != NUMA_NO_NODE && page_to_nid(page) != node))
 		return 0;
 #endif
 	return 1;