md/raid5: avoid oops when number of devices is reduced then increased.

The entries in the stripe_cache maintained by raid5 are enlarged
when we increased the number of devices in the array, but not
shrunk when we reduce the number of devices.
So if entries are added after reducing the number of devices, we
much ensure to initialise the whole entry, not just the part that
is currently relevant.  Otherwise if we enlarge the array again,
we will reference uninitialised values.

As grow_buffers/shrink_buffer now want to use a count that is stored
explicity in the raid_conf, they should get it from there rather than
being passed it as a parameter.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -277,12 +277,13 @@ out:
 	return sh;
 }
 
-static void shrink_buffers(struct stripe_head *sh, int num)
+static void shrink_buffers(struct stripe_head *sh)
 {
 	struct page *p;
 	int i;
+	int num = sh->raid_conf->pool_size;
 
-	for (i=0; i<num ; i++) {
+	for (i = 0; i < num ; i++) {
 		p = sh->dev[i].page;
 		if (!p)
 			continue;
@@ -291,11 +292,12 @@ static void shrink_buffers(struct stripe_head *sh, int num)
 	}
 }
 
-static int grow_buffers(struct stripe_head *sh, int num)
+static int grow_buffers(struct stripe_head *sh)
 {
 	int i;
+	int num = sh->raid_conf->pool_size;
 
-	for (i=0; i<num; i++) {
+	for (i = 0; i < num; i++) {
 		struct page *page;
 
 		if (!(page = alloc_page(GFP_KERNEL))) {
@@ -1240,19 +1242,18 @@ static void raid_run_ops(struct stripe_head *sh, unsigned long ops_request)
 static int grow_one_stripe(raid5_conf_t *conf)
 {
 	struct stripe_head *sh;
-	int disks = max(conf->raid_disks, conf->previous_raid_disks);
 	sh = kmem_cache_alloc(conf->slab_cache, GFP_KERNEL);
 	if (!sh)
 		return 0;
-	memset(sh, 0, sizeof(*sh) + (disks-1)*sizeof(struct r5dev));
+	memset(sh, 0, sizeof(*sh) + (conf->pool_size-1)*sizeof(struct r5dev));
 	sh->raid_conf = conf;
 	spin_lock_init(&sh->lock);
 	#ifdef CONFIG_MULTICORE_RAID456
 	init_waitqueue_head(&sh->ops.wait_for_ops);
 	#endif
 
-	if (grow_buffers(sh, disks)) {
-		shrink_buffers(sh, disks);
+	if (grow_buffers(sh)) {
+		shrink_buffers(sh);
 		kmem_cache_free(conf->slab_cache, sh);
 		return 0;
 	}
@@ -1468,7 +1469,7 @@ static int drop_one_stripe(raid5_conf_t *conf)
 	if (!sh)
 		return 0;
 	BUG_ON(atomic_read(&sh->count));
-	shrink_buffers(sh, conf->pool_size);
+	shrink_buffers(sh);
 	kmem_cache_free(conf->slab_cache, sh);
 	atomic_dec(&conf->active_stripes);
 	return 1;