slab: NUMA kmem_cache diet

Some NUMA machines have a big MAX_NUMNODES (possibly 1024), but fewer
possible nodes.  This patch dynamically sizes the 'struct kmem_cache' to
allocate only needed space.

I moved nodelists[] field at the end of struct kmem_cache, and use the
following computation in kmem_cache_init()

cache_cache.buffer_size = offsetof(struct kmem_cache, nodelists) +
                                 nr_node_ids * sizeof(struct kmem_list3 *);

On my two nodes x86_64 machine, kmem_cache.obj_size is now 192 instead of 704
(This is because on x86_64, MAX_NUMNODES is 64)

On bigger NUMA setups, this might reduce the gfporder of "cache_cache"
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Andy Whitcroft <apw@shadowen.org>
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

@@ -389,7 +389,6 @@ struct kmem_cache {
 	unsigned int buffer_size;
 	u32 reciprocal_buffer_size;
 /* 3) touched by every alloc & free from the backend */
-	struct kmem_list3 *nodelists[MAX_NUMNODES];
 
 	unsigned int flags;		/* constant flags */
 	unsigned int num;		/* # of objs per slab */
@@ -444,6 +443,17 @@ struct kmem_cache {
 	int obj_offset;
 	int obj_size;
 #endif
+	/*
+	 * We put nodelists[] at the end of kmem_cache, because we want to size
+	 * this array to nr_node_ids slots instead of MAX_NUMNODES
+	 * (see kmem_cache_init())
+	 * We still use [MAX_NUMNODES] and not [1] or [0] because cache_cache
+	 * is statically defined, so we reserve the max number of nodes.
+	 */
+	struct kmem_list3 *nodelists[MAX_NUMNODES];
+	/*
+	 * Do not add fields after nodelists[]
+	 */
 };
 
 #define CFLGS_OFF_SLAB		(0x80000000UL)
@@ -678,9 +688,6 @@ static struct kmem_cache cache_cache = {
 	.shared = 1,
 	.buffer_size = sizeof(struct kmem_cache),
 	.name = "kmem_cache",
-#if DEBUG
-	.obj_size = sizeof(struct kmem_cache),
-#endif
 };
 
 #define BAD_ALIEN_MAGIC 0x01020304ul
@@ -1440,6 +1447,15 @@ void __init kmem_cache_init(void)
 	cache_cache.array[smp_processor_id()] = &initarray_cache.cache;
 	cache_cache.nodelists[node] = &initkmem_list3[CACHE_CACHE];
 
+	/*
+	 * struct kmem_cache size depends on nr_node_ids, which
+	 * can be less than MAX_NUMNODES.
+	 */
+	cache_cache.buffer_size = offsetof(struct kmem_cache, nodelists) +
+				 nr_node_ids * sizeof(struct kmem_list3 *);
+#if DEBUG
+	cache_cache.obj_size = cache_cache.buffer_size;
+#endif
 	cache_cache.buffer_size = ALIGN(cache_cache.buffer_size,
 					cache_line_size());
 	cache_cache.reciprocal_buffer_size =