slab: Avoid fragmentation

During free_block we unconditionally put a block at the end of the
partial list. This was done to insure that there is maximum time
for the other entries in the slab to be freed. If we would leave
the slab at the beginning of the partial list if there are only
a  few free entries then the slab has more of a chance to get
back onto the fully used list. The partial slabs already on the
partial list have therefore more chances of getting all their
objects freed.

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

Index: linux-2.6.18-rc3/mm/slab.c
===================================================================
--- linux-2.6.18-rc3.orig/mm/slab.c	2006-08-03 22:44:16.879298921 -0700
+++ linux-2.6.18-rc3/mm/slab.c	2006-08-03 22:51:02.708671592 -0700
@@ -3134,11 +3134,21 @@ static void free_block(struct kmem_cache
 				list_add(&slabp->list, &l3->slabs_free);
 			}
 		} else {
-			/* Unconditionally move a slab to the end of the
-			 * partial list on free - maximum time for the
-			 * other objects to be freed, too.
+			/*
+			 * Move slab to the end of the list if the number
+			 * of allocated objects is low to increase the
+			 * likelyhood of freeing the rest.
+			 *
+			 * If the number of allocated objects is high then
+			 * we want this at the start of the list in order
+			 * to fill the slab up again.
+			 *
+			 * This help to reduce fragmentation.
 			 */
-			list_add_tail(&slabp->list, &l3->slabs_partial);
+			if (cachep->num - slabp->inuse <= cachep->num / 4)
+				list_add(&slabp->list, &l3->slabs_partial);
+			else
+				list_add_tail(&slabp->list, &l3->slabs_partial);
 		}
 	}
 }