SLUB: Conform to SLAB SLAB_HWCACHE_ALIGN behavior

Currently SLUB is using a strict L1_CACHE_BYTES alignment if SLAB_HWCACHE_ALIGN
is specified. SLAB does not align to a cacheline if the object is smaller
than half of a cacheline. Small objects are aligned by SLAB to a fraction
of a cacheline.

SLUB will just forget about the alignment requirement. It seems that
fractional alignments are no good because they grow the object and
reduce the object density in a cache line. If we are already throwing
the user suggestion of a cache line alignment away then lets do the best
we can.

In any case one needs to explictly specify an alignment during
kmem_cache_create to either slab allocator in order to ensure that the
objects are cacheline aligned.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.21-rc6/mm/slub.c
===================================================================
--- linux-2.6.21-rc6.orig/mm/slub.c	2007-04-20 19:24:23.000000000 -0700
+++ linux-2.6.21-rc6/mm/slub.c	2007-04-20 19:24:29.000000000 -0700
@@ -1490,9 +1490,19 @@ static int calculate_order(int size)
  * various ways of specifying it.
  */
 static unsigned long calculate_alignment(unsigned long flags,
-		unsigned long align)
+		unsigned long align, unsigned long size)
 {
-	if (flags & SLAB_HWCACHE_ALIGN)
+	/*
+	 * If the user wants hardware cache aligned objects then
+	 * follow that suggestion if the object is sufficiently
+	 * large.
+	 *
+	 * The hardware cache alignment cannot override the
+	 * specified alignment though. If that is greater
+	 * then use it.
+	 */
+	if ((flags & SLAB_HWCACHE_ALIGN) &&
+			size > L1_CACHE_BYTES / 2)
 		return max_t(unsigned long, align, L1_CACHE_BYTES);
 
 	if (align < ARCH_SLAB_MINALIGN)
@@ -1681,7 +1691,7 @@ static int calculate_sizes(struct kmem_c
 	 * user specified (this is unecessarily complex due to the attempt
 	 * to be compatible with SLAB. Should be cleaned up some day).
 	 */
-	align = calculate_alignment(flags, align);
+	align = calculate_alignment(flags, align, s->objsize);
 
 	/*
 	 * SLUB stores one object immediately after another beginning from
@@ -2263,7 +2273,7 @@ static struct kmem_cache *find_mergeable
 		return NULL;
 
 	size = ALIGN(size, sizeof(void *));
-	align = calculate_alignment(flags, align);
+	align = calculate_alignment(flags, align, size);
 	size = ALIGN(size, align);
 
 	list_for_each(h, &slab_caches) {