Re: [PATCH v3] loop: fix I/O error on fsync() in detached loop devices

[Ming Lei <ming.lei@redhat.com>]

On Thu, Jan 07, 2021 at 03:17:34PM -0300, Mauricio Faria de Oliveira wrote:
> There's an I/O error on fsync() in a detached loop device
> if it has been previously attached.
> 
> The issue is write cache is enabled in the attach path in
> loop_configure() but it isn't disabled in the detach path;
> thus it remains enabled in the block device regardless of
> whether it is attached or not.
> 
> Now fsync() can get an I/O request that will just be failed
> later in loop_queue_rq() as device's state is not 'Lo_bound'.
> 
> So, disable write cache in the detach path.
> 
> Do so based on the queue flag, not the loop device flag for
> read-only (used to enable) as the queue flag can be changed
> via sysfs even on read-only loop devices (e.g., losetup -r.)
> 
> Test-case:
> 
>     # DEV=/dev/loop7
> 
>     # IMG=/tmp/image
>     # truncate --size 1M $IMG
> 
>     # losetup $DEV $IMG
>     # losetup -d $DEV
> 
> Before:
> 
>     # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
>     fsync(3)                                = -1 EIO (Input/output error)
>     Warning: Error fsyncing/closing /dev/loop7: Input/output error
>     [  982.529929] blk_update_request: I/O error, dev loop7, sector 0 op 0x1:(WRITE) flags 0x800 phys_seg 0 prio class 0
> 
> After:
> 
>     # strace -e fsync parted -s $DEV print 2>&1 | grep fsync
>     fsync(3)                                = 0
> 
> Co-developed-by: Eric Desrochers <eric.desrochers@canonical.com>
> Signed-off-by: Eric Desrochers <eric.desrochers@canonical.com>
> Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
> Tested-by: Gabriel Krisman Bertazi <krisman@collabora.com>
> ---
>  drivers/block/loop.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> v3:
> - Disable write cache based on QUEUE_FLAG_WC, not just
>   !LO_FLAGS_READ_ONLY, as the former can be enabled in
>   sysfs even for LO_FLAGS_READ_ONLY, thus not disabled
>   later. Mention that in commit message. (thanks, Ming)
> v2:
> - Fix ordering of Co-developed-by:/Signed-off-by: tags.
>   (thanks, Krisman)
> - Add Tested-by: tag. (likewise.)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index e5ff328f0917..e94a11dbb5bd 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1212,6 +1212,9 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
>  		goto out_unlock;
>  	}
>  
> +	if (test_bit(QUEUE_FLAG_WC, &lo->lo_queue->queue_flags))
> +		blk_queue_write_cache(lo->lo_queue, false, false);
> +
>  	/* freeze request queue during the transition */
>  	blk_mq_freeze_queue(lo->lo_queue);

Reviewed-by: Ming Lei <ming.lei@redhat.com>

-- 
Ming