Re: [PATCH 2/3] nbd: fsync and kill block device on shutdown

Paul Clements
2013-02-12T22:15:18Z

On Tue, Feb 12, 2013 at 11:06 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> There are two problems with shutdown in the NBD driver. The first is
> that receiving the NBD_DISCONNECT ioctl does not sync the filesystem;
> this is useful because BLKFLSBUF is restricted to processes that have
> CAP_SYS_ADMIN, and the NBD client may not possess it (fsync of the
> block device does not sync the filesystem, either).

> The second is that once we clear the socket we have no guarantee that
> later reads will come from the same backing storage. Thus the page cache
> must be cleaned, lest reads that hit on the page cache will return stale
> data from the previously-accessible disk.

Paolo,

Thanks for this. A problem indeed...

Acked-by: Paul.Clements@steeleye.com

> Cc: <stable@vger.kernel.org>
> Cc: <nbd-general@lists.sf.net>
> Cc: Paul Clements <Paul.Clements@steeleye.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> drivers/block/nbd.c | 10 ++++++++++
> 1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 5603765..a9c5c7a 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -608,12 +608,20 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> struct request sreq;
>
> dev_info(disk_to_dev(nbd->disk), "NBD_DISCONNECT\n");
> + if (!nbd->sock)
> + return -EINVAL;
>
> + mutex_unlock(&nbd->tx_lock);
> + fsync_bdev(bdev);
> + mutex_lock(&nbd->tx_lock);
> blk_rq_init(NULL, &sreq);
> sreq.cmd_type = REQ_TYPE_SPECIAL;
> nbd_cmd(&sreq) = NBD_CMD_DISC;
> +
> + /* Check again after getting mutex back. */
> if (!nbd->sock)
> return -EINVAL;
> +
> nbd_send_req(nbd, &sreq);
> return 0;
> }
> @@ -627,6 +635,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> nbd_clear_que(nbd);
> BUG_ON(!list_empty(&nbd->queue_head));
> BUG_ON(!list_empty(&nbd->waiting_queue));
> + kill_bdev(bdev);
> if (file)
> fput(file);
> return 0;
> @@ -719,6 +728,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
> nbd->file = NULL;
> nbd_clear_que(nbd);
> dev_warn(disk_to_dev(nbd->disk), "queue cleared\n");
> + kill_bdev(bdev);
> queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
> if (file)
> fput(file);
> --
> 1.7.1
>
>