dm raid1: switch read_record from kmalloc to slab to save memory

With my previous patch to save bi_io_vec, the size of dm_raid1_read_record
is significantly increased (the vector list takes 3072 bytes on 32-bit machines
and 4096 bytes on 64-bit machines).

The structure dm_raid1_read_record used to be allocated with kmalloc,
but kmalloc aligns the size on the next power-of-two so an object
slightly greater than 4096 will allocate 8192 bytes of memory and half of
that memory will be wasted.

This patch turns kmalloc into a slab cache which doesn't have this
padding so it will reduce the memory consumed.

Cc: stable@kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-raid1.c

@@ -145,6 +145,8 @@ struct dm_raid1_read_record {
 	struct dm_bio_details details;
 };
 
+static struct kmem_cache *_dm_raid1_read_record_cache;
+
 /*
  * Every mirror should look like this one.
  */
@@ -764,9 +766,9 @@ static struct mirror_set *alloc_context(unsigned int nr_mirrors,
 	atomic_set(&ms->suspend, 0);
 	atomic_set(&ms->default_mirror, DEFAULT_MIRROR);
 
-	len = sizeof(struct dm_raid1_read_record);
-	ms->read_record_pool = mempool_create_kmalloc_pool(MIN_READ_RECORDS,
-							   len);
+	ms->read_record_pool = mempool_create_slab_pool(MIN_READ_RECORDS,
+						_dm_raid1_read_record_cache);
+
 	if (!ms->read_record_pool) {
 		ti->error = "Error creating mirror read_record_pool";
 		kfree(ms);
@@ -1279,16 +1281,31 @@ static int __init dm_mirror_init(void)
 {
 	int r;
 
+	_dm_raid1_read_record_cache = KMEM_CACHE(dm_raid1_read_record, 0);
+	if (!_dm_raid1_read_record_cache) {
+		DMERR("Can't allocate dm_raid1_read_record cache");
+		r = -ENOMEM;
+		goto bad_cache;
+	}
+
 	r = dm_register_target(&mirror_target);
-	if (r < 0)
+	if (r < 0) {
 		DMERR("Failed to register mirror target");
+		goto bad_target;
+	}
+
+	return 0;
 
+bad_target:
+	kmem_cache_destroy(_dm_raid1_read_record_cache);
+bad_cache:
 	return r;
 }
 
 static void __exit dm_mirror_exit(void)
 {
 	dm_unregister_target(&mirror_target);
+	kmem_cache_destroy(_dm_raid1_read_record_cache);
 }
 
 /* Module hooks */