Re: [BUG, TOT] xfs w/ dax failure in __follow_pte_pmd()

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Jan 2, 2019 at 4:04 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Wed, Jan 02, 2019 at 02:50:05PM -0800, Matthew Wilcox wrote:
> > On Wed, Jan 02, 2019 at 01:25:31PM -0800, Matthew Wilcox wrote:
> > > On Thu, Jan 03, 2019 at 08:13:32AM +1100, Dave Chinner wrote:
> > > > Hi folks,
> > > >
> > > > An overnight test run on a current TOT kernel failed generic/413
> > > > with the following dmesg output:
> > > >
> > > > [ 9487.276402] RIP: 0010:__follow_pte_pmd+0x22d/0x340
> > > > [ 9487.305065] Call Trace:
> > > > [ 9487.307310]  dax_entry_mkclean+0xbb/0x1f0
> > >
> > > We've only got one commit touching dax_entry_mkclean and it's Jerome's.
> > > Looking through ac46d4f3c43241ffa23d5bf36153a0830c0e02cc, I'd say
> > > it's missing a call to mmu_notifier_range_init().
> >
> > Could I persuade you to give this a try?
>
> Yup, that fixes it.
>
> And looking at the code, the dax mmu notifier code clearly wasn't
> tested. i.e. dax_entry_mkclean() is the *only* code that exercises
> the conditional range parameter code paths inside
> __follow_pte_pmd().  This means it wasn't tested before it was
> proposed for inclusion and since inclusion no-one using -akpm,
> linux-next or the current mainline TOT has done any filesystem DAX
> testing until I tripped over it.
>
> IOws, this is the second "this was never tested before it was merged
> into mainline" XFS regression that I've found in the last 3 weeks.
> Both commits have been merged through the -akpm tree, and that
> implies we currently have no significant filesystem QA coverage on
> changes being merged through this route. This seems like an area
> that needs significant improvement to me....

Yes, this is also part of a series I explicitly NAK'd [1] because
there are no upstream users for it. I didn't bother to test it because
I thought the NAK was sufficient.

Andrew, any reason to not revert the set? They provide no upstream
value and actively break DAX.

[1]: https://www.spinics.net/lists/linux-fsdevel/msg137309.html