nbd: don't update block size after device is started

Mounted NBD device can be resized, one use case is rbd-nbd.

Fix the issue by setting up default block size, then not touch it
in nbd_size_update() any more. This kind of usage is aligned with loop
which has same use case too.

Cc: stable@vger.kernel.org
Fixes: c8a83a6b ("nbd: Use set_blocksize() to set device blocksize")
Reported-by: lining <lining2020x@163.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Jan Kara <jack@suse.cz>
Tested-by: lining <lining2020x@163.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -296,7 +296,7 @@ static void nbd_size_clear(struct nbd_device *nbd)
 	}
 }
 
-static void nbd_size_update(struct nbd_device *nbd)
+static void nbd_size_update(struct nbd_device *nbd, bool start)
 {
 	struct nbd_config *config = nbd->config;
 	struct block_device *bdev = bdget_disk(nbd->disk, 0);
@@ -313,7 +313,8 @@ static void nbd_size_update(struct nbd_device *nbd)
 	if (bdev) {
 		if (bdev->bd_disk) {
 			bd_set_nr_sectors(bdev, nr_sectors);
-			set_blocksize(bdev, config->blksize);
+			if (start)
+				set_blocksize(bdev, config->blksize);
 		} else
 			set_bit(GD_NEED_PART_SCAN, &nbd->disk->state);
 		bdput(bdev);
@@ -328,7 +329,7 @@ static void nbd_size_set(struct nbd_device *nbd, loff_t blocksize,
 	config->blksize = blocksize;
 	config->bytesize = blocksize * nr_blocks;
 	if (nbd->task_recv != NULL)
-		nbd_size_update(nbd);
+		nbd_size_update(nbd, false);
 }
 
 static void nbd_complete_rq(struct request *req)
@@ -1308,7 +1309,7 @@ static int nbd_start_device(struct nbd_device *nbd)
 		args->index = i;
 		queue_work(nbd->recv_workq, &args->work);
 	}
-	nbd_size_update(nbd);
+	nbd_size_update(nbd, true);
 	return error;
 }