Cmnt: [SRU][F][PATCH 1/1] loop: fix I/O error on fsync() in detached loop devices

[Thadeu Lima de Souza Cascardo]

Not a formal NACK, but rather a comment when applying and when sending other
fixes:

The author information is incorrect here. Upstream commit says it is Mauricio,
but the pseudo-header From: is missing from your message. How did you prepare
and submit this patch to the list?

Cascardo.

On Wed, Mar 29, 2023 at 05:20:28PM -0300, Jorge Merlino wrote:
> BugLink: https://bugs.launchpad.net/bugs/1856871
> 
> 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 at canonical.com>
> Signed-off-by: Eric Desrochers <eric.desrochers at canonical.com>
> Signed-off-by: Mauricio Faria de Oliveira <mfo at canonical.com>
> Tested-by: Gabriel Krisman Bertazi <krisman at collabora.com>
> Reviewed-by: Ming Lei <ming.lei at redhat.com>
> Signed-off-by: Jens Axboe <axboe at kernel.dk>
> (cherry picked from commit 4ceddce55eb35d15b0f87f5dcf6f0058fd15d3a4)
> Signed-off-by: Jorge Merlino <jorge.merlino at canonical.com>
> ---
>  drivers/block/loop.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 5ab7985fff8a..74ceba11ca0d 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1186,6 +1186,9 @@ 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);
>  
> -- 
> 2.25.1
> 
> 
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