Re: [problem] xfstests generic/311 unreliable...

[Dave Chinner <david@fromorbit.com>]

Argh, add the cc to Josef...

On Tue, May 07, 2013 at 05:11:02PM +1000, Dave Chinner wrote:
> Hi Josef,
> 
> I was just looking at a generic/311, and I think there's something
> fundamentally wrong with the way it is checking the scratch device.
> 
> You reported it was failing for internal test 19 on XFS, but I'm
> seeing is fail after the first test or 2, randomly. It has never
> made it past test 3. So I had a little bit of a closer look at it's
> structure. Essentially it is doing this (and the contents seen by
> each step:
> 
> scratch dev + mkfs
> 	+-------------------------------+
> overlay dm-flakey
> 	D-------------------------------D
> mount/write/kill/unmount dm-flakey
> 	Dx-x-x-x-x-x-x------------------D
> 
> All good up to here. Now, you can _check_scratch_fs which sees:
> 
> scratch dev + check
> 	+-------------------------------+
> 
> i.e. it's not seeing all the changes written to dm-flakey and so
> xfs-check it seeing corruption.
> 
> After I realised this was stacking block devices and checking the
> underlying block device, the cause was pretty obvious: scratch-dev
> and dm-flakey have different address spaces, so changes written
> throughone address space will not be seen through the other address
> space if there is stale cached data in the original address space.
> 
> And that's exactly what is happening. This patch:
> 
> --- a/tests/generic/311
> +++ b/tests/generic/311
> @@ -79,6 +79,7 @@ _mount_flakey()
>  _unmount_flakey()
>  {
>         $UMOUNT_PROG $SCRATCH_MNT
> +       echo 3 > /proc/sys/vm/drop_caches
>  }
>  
>  _load_flakey_table()
> 
> Makes the problem go away for xfs_check. But really, I don't like
> the assumption that the test is built on - that writes through one
> block device are visible through another. It's just asking for weird
> problems.
> 
> Is there some way that you can restructure this test so it doesn't
> have this problem (e.g. do everything on dm-flakey)?
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
> 
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> 

-- 
Dave Chinner
david@fromorbit.com

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