On Tue, May 07, 2013 at 01:37:17AM -0600, Dave Chinner wrote:
> 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)?
Yup I can do that, honestly the only reason I was doing it this way was because
my original script which this test is based on did this all to a raw disk with
a real reboot in there. I'll fix it up and send a patch. Thanks,
Josef
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