Re: Infinite loop with DAX PMD faults

[Jan Kara <jack@suse.cz>]

On Thu 27-10-16 12:46:32, Dan Williams wrote:
> On Thu, Oct 27, 2016 at 12:07 PM, Jan Kara <jack@suse.cz> wrote:
> > Hello,
> >
> > When testing my DAX patches rebased on top of Ross' DAX PMD series, I've
> > come across the following issue with generic/344 test from xfstests. The
> > test ends in an infinite fault loop when we fault index 0 over and over
> > again never finishing the fault. The problem is that we do a write fault
> > for index 0 when there is PMD for that index. So we enter wp_huge_pmd().
> > For whatever reason that returns VM_FAULT_FALLBACK so we continue to
> > handle_pte_fault(). There we do
> >
> >         if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd))
> >
> > check which is true - the PMD we have is pmd_trans_huge() - so we 'return
> > 0' and that results in retrying the fault and all happens from the
> > beginning again.
> >
> > It isn't quite obvious how to break that cycle to me. The comment before
> > pmd_none_or_trans_huge_or_clear_bad() goes to great lengths explaining
> > possible races when PMD is pmd_trans_huge() so it needs careful evaluation
> > what needs to be done for DAX. Ross, any idea?
> 
> Can you bisect it with CONFIG_BROKEN removed from older kernels?

I can try (but likely won't get to it before Kernel Summit, not sure if
I'll have time for that there).

> I remember tracking down something like this when initially doing the
> pmd support.  It ended up being a missed pmd_devmap() check in the
> fault path, so it may not be the same issue.  It would at least be
> interesting to see if 4.6 fails in a similar manner with this test and
> FS_DAX_PMD enabled.

BTW, the results of checks for the PMD are:

pmd_devmap(*vmf->pmd) == 0
pmd_trans_huge(*vmf->pmd) == 1
pmd_bad(*vmf->pmd) == 1

I'll see if I can get any meaningful test running based on 4.6...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR