SLUB: allocate smallest object size if the user asks for 0 bytes

Makes SLUB behave like SLAB in this area to avoid issues....

Throw a stack dump to alert people.

At some point the behavior should be switched back.  NULL is no memory as
far as I can tell and if the use asked for 0 bytes then he need to get no
memory.
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/slub_def.h

@@ -80,8 +80,12 @@ extern struct kmem_cache kmalloc_caches[KMALLOC_SHIFT_HIGH + 1];
  */
 static inline int kmalloc_index(int size)
 {
-	if (size == 0)
-		return 0;
+	/*
+	 * We should return 0 if size == 0 but we use the smallest object
+	 * here for SLAB legacy reasons.
+	 */
+	WARN_ON_ONCE(size == 0);
+
 	if (size > 64 && size <= 96)
 		return 1;
 	if (size > 128 && size <= 192)

mm/slub.c

@@ -1979,7 +1979,7 @@ static struct kmem_cache *get_slab(size_t size, gfp_t flags)
 {
 	int index = kmalloc_index(size);
 
-	if (!size)
+	if (!index)
 		return NULL;
 
 	/* Allocation too large? */