SLAB: don't allocate empty shared caches

We can avoid allocating empty shared caches and avoid unecessary check of
cache->limit.  We save some memory.  We avoid bringing into CPU cache
unecessary cache lines.

All accesses to l3->shared are already checking NULL pointers so this patch is
safe.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

diff --git a/mm/slab.c b/mm/slab.c
index ae440b5..00f98b9 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1223,19 +1223,20 @@
 		 */
 		list_for_each_entry(cachep, &cache_chain, next) {
 			struct array_cache *nc;
-			struct array_cache *shared;
+			struct array_cache *shared = NULL;
 			struct array_cache **alien = NULL;
 
 			nc = alloc_arraycache(node, cachep->limit,
 						cachep->batchcount);
 			if (!nc)
 				goto bad;
-			shared = alloc_arraycache(node,
+			if (cachep->shared) {
+				shared = alloc_arraycache(node,
 					cachep->shared * cachep->batchcount,
 					0xbaadf00d);
-			if (!shared)
-				goto bad;
-
+				if (!shared)
+					goto bad;
+			}
 			if (use_alien_caches) {
                                 alien = alloc_alien_cache(node, cachep->limit);
                                 if (!alien)
@@ -1317,8 +1318,8 @@
 
 			shared = l3->shared;
 			if (shared) {
-				free_block(cachep, l3->shared->entry,
-					   l3->shared->avail, node);
+				free_block(cachep, shared->entry,
+					   shared->avail, node);
 				l3->shared = NULL;
 			}
 
@@ -3870,12 +3871,15 @@
                                 goto fail;
                 }
 
-		new_shared = alloc_arraycache(node,
+		new_shared = NULL;
+		if (cachep->shared) {
+			new_shared = alloc_arraycache(node,
 				cachep->shared*cachep->batchcount,
 					0xbaadf00d);
-		if (!new_shared) {
-			free_alien_cache(new_alien);
-			goto fail;
+			if (!new_shared) {
+				free_alien_cache(new_alien);
+				goto fail;
+			}
 		}
 
 		l3 = cachep->nodelists[node];