dm thin: don't allow changing data device during thin-pool reload

The existing code allows changing the data device when the thin-pool
target is reloaded.

This capability is not required and only complicates device lifetime
guarantees. This can cause crashes like the one reported here:
	https://bugzilla.redhat.com/show_bug.cgi?id=1788596
where the kernel tries to issue a flush bio located in a structure that
was already freed.

Take the first step to simplifying the thin-pool's data device lifetime
by disallowing changing it. Like the thin-pool's metadata device, the
data device is now set in pool_create() and it cannot be changed for a
given thin-pool.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin.c

@@ -231,6 +231,7 @@ struct pool {
 	struct dm_target *ti;	/* Only set if a pool target is bound */
 
 	struct mapped_device *pool_md;
+	struct block_device *data_dev;
 	struct block_device *md_dev;
 	struct dm_pool_metadata *pmd;
 
@@ -2933,6 +2934,7 @@ static struct kmem_cache *_new_mapping_cache;
 
 static struct pool *pool_create(struct mapped_device *pool_md,
 				struct block_device *metadata_dev,
+				struct block_device *data_dev,
 				unsigned long block_size,
 				int read_only, char **error)
 {
@@ -3040,6 +3042,7 @@ static struct pool *pool_create(struct mapped_device *pool_md,
 	pool->last_commit_jiffies = jiffies;
 	pool->pool_md = pool_md;
 	pool->md_dev = metadata_dev;
+	pool->data_dev = data_dev;
 	__pool_table_insert(pool);
 
 	return pool;
@@ -3081,6 +3084,7 @@ static void __pool_dec(struct pool *pool)
 
 static struct pool *__pool_find(struct mapped_device *pool_md,
 				struct block_device *metadata_dev,
+				struct block_device *data_dev,
 				unsigned long block_size, int read_only,
 				char **error, int *created)
 {
@@ -3091,19 +3095,23 @@ static struct pool *__pool_find(struct mapped_device *pool_md,
 			*error = "metadata device already in use by a pool";
 			return ERR_PTR(-EBUSY);
 		}
+		if (pool->data_dev != data_dev) {
+			*error = "data device already in use by a pool";
+			return ERR_PTR(-EBUSY);
+		}
 		__pool_inc(pool);
 
 	} else {
 		pool = __pool_table_lookup(pool_md);
 		if (pool) {
-			if (pool->md_dev != metadata_dev) {
+			if (pool->md_dev != metadata_dev || pool->data_dev != data_dev) {
 				*error = "different pool cannot replace a pool";
 				return ERR_PTR(-EINVAL);
 			}
 			__pool_inc(pool);
 
 		} else {
-			pool = pool_create(pool_md, metadata_dev, block_size, read_only, error);
+			pool = pool_create(pool_md, metadata_dev, data_dev, block_size, read_only, error);
 			*created = 1;
 		}
 	}
@@ -3356,7 +3364,7 @@ static int pool_ctr(struct dm_target *ti, unsigned argc, char **argv)
 		goto out;
 	}
 
-	pool = __pool_find(dm_table_get_md(ti->table), metadata_dev->bdev,
+	pool = __pool_find(dm_table_get_md(ti->table), metadata_dev->bdev, data_dev->bdev,
 			   block_size, pf.mode == PM_READ_ONLY, &ti->error, &pool_created);
 	if (IS_ERR(pool)) {
 		r = PTR_ERR(pool);
@@ -4098,7 +4106,7 @@ static struct target_type pool_target = {
 	.name = "thin-pool",
 	.features = DM_TARGET_SINGLETON | DM_TARGET_ALWAYS_WRITEABLE |
 		    DM_TARGET_IMMUTABLE,
-	.version = {1, 21, 0},
+	.version = {1, 22, 0},
 	.module = THIS_MODULE,
 	.ctr = pool_ctr,
 	.dtr = pool_dtr,
@@ -4475,7 +4483,7 @@ static void thin_io_hints(struct dm_target *ti, struct queue_limits *limits)
 
 static struct target_type thin_target = {
 	.name = "thin",
-	.version = {1, 21, 0},
+	.version = {1, 22, 0},
 	.module	= THIS_MODULE,
 	.ctr = thin_ctr,
 	.dtr = thin_dtr,