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

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

[Mauricio Faria de Oliveira]

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.

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

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Co-developed-by: Eric Desrochers <eric.desrochers@canonical.com>
Signed-off-by: Eric Desrochers <eric.desrochers@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 e5ff328f0917..49517482e061 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 (!(lo->lo_flags & LO_FLAGS_READ_ONLY) && filp->f_op->fsync)
+		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

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

[Gabriel Krisman Bertazi]

Mauricio Faria de Oliveira <mfo@canonical.com> writes:

> 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.
>
> 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
>
> Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
> Co-developed-by: Eric Desrochers <eric.desrochers@canonical.com>
> Signed-off-by: Eric Desrochers <eric.desrochers@canonical.com>

This sign-off chain is not sorted correctly.  See Documentation/process/submitting-patches.rst.

Other than that, I think the fix makes sense.

Tested-by: Gabriel Krisman Bertazi <krisman@collabora.com>

> ---
>  drivers/block/loop.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index e5ff328f0917..49517482e061 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 (!(lo->lo_flags & LO_FLAGS_READ_ONLY) && filp->f_op->fsync)
> +		blk_queue_write_cache(lo->lo_queue, false, false);
> +
>  	/* freeze request queue during the transition */
>  	blk_mq_freeze_queue(lo->lo_queue);

-- 
Gabriel Krisman Bertazi

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

[Mauricio Faria de Oliveira]

On Sat, Jan 2, 2021 at 2:14 AM Gabriel Krisman Bertazi
<krisman@collabora.com> wrote:
>
> Mauricio Faria de Oliveira <mfo@canonical.com> writes:
[snip]
> > Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
> > Co-developed-by: Eric Desrochers <eric.desrochers@canonical.com>
> > Signed-off-by: Eric Desrochers <eric.desrochers@canonical.com>
>
> This sign-off chain is not sorted correctly.  See Documentation/process/submitting-patches.rst.
>
> Other than that, I think the fix makes sense.
>
> Tested-by: Gabriel Krisman Bertazi <krisman@collabora.com>
>

Hey Krisman,

Thanks for testing and catching that; sent V2.

[snip]
> --
> Gabriel Krisman Bertazi


cheers,
-- 
Mauricio Faria de Oliveira