Re: Regression in xfstests on tmpfs-backed NFS exports

[Mark Hemment <markhemm@googlemail.com>]

On Thu, 7 Apr 2022 at 01:19, Hugh Dickins <hughd@google.com> wrote:
>
> On Wed, 6 Apr 2022, Chuck Lever III wrote:
>
> > Good day, Hugh-
>
> Huh! If you were really wishing me a good day, would you tell me this ;-?
>
> >
> > I noticed that several fsx-related tests in the xfstests suite are
> > failing after updating my NFS server to v5.18-rc1. I normally test
> > against xfs, ext4, btrfs, and tmpfs exports. tmpfs is the only export
> > that sees these new failures:
> >
[...]
> > generic/075 fails almost immediately without any NFS-level errors.
> > Likely this is data corruption rather than an overt I/O error.
>
> That's sad.  Thanks for bisecting and reporting.  Sorry for the nuisance.
>
> I suspect this patch is heading for a revert, because I shall not have
> time to debug and investigate.  Cc'ing fsdevel and a few people who have
> an interest in it, to warn of that likely upcoming revert.
>
> But if it's okay with everyone, please may we leave it in for -rc2?
> Given that having it in -rc1 already smoked out another issue (problem
> of SetPageUptodate(ZERO_PAGE(0)) without CONFIG_MMU), I think keeping
> it in a little longer might smoke out even more.
>
> The xfstests info above doesn't actually tell very much, beyond that
> generic/075 generic/091 generic/112 generic/127, each a test with fsx,
> all fall at their first hurdle.  If you have time, please rerun and
> tar up the results/generic directory (maybe filter just those failing)
> and send as attachment.  But don't go to any trouble, it's unlikely
> that I shall even untar it - it would be mainly to go on record if
> anyone has time to look into it later.  And, frankly, it's unlikely
> to tell us anything more enlightening, than that the data seen was
> not as expected: which we do already know.
>
> I've had no problem with xfstests generic 075,091,112,127 testing
> tmpfs here, not before and not in the month or two I've had that
> patch in: so it's something in the way that NFS exercises tmpfs
> that reveals it.  If I had time to duplicate your procedure, I'd be
> asking for detailed instructions: but no, I won't have a chance.
>
> But I can sit here and try to guess.  I notice fs/nfsd checks
> file->f_op->splice_read, and employs fallback if not available:
> if you have time, please try rerunning those xfstests on an -rc1
> kernel, but with mm/shmem.c's .splice_read line commented out.
> My guess is that will then pass the tests, and we shall know more.
>
> What could be going wrong there?  I've thought of two possibilities.
> A minor, hopefully easily fixed, issue would be if fs/nfsd has
> trouble with seeing the same page twice in a row: since tmpfs is
> now using the ZERO_PAGE(0) for all pages of a hole, and I think I
> caught sight of code which looks to see if the latest page is the
> same as the one before.  It's easy to imagine that might go wrong.

When I worked at Veritas, data corruption over NFS was hit when
sending the same page in succession.  This was triggered via VxFS's
shared page cache, after a file had been dedup'ed.
I can't remember all the details (~15yrs ago), but the core issue was
skb_can_coalesce() returning a false-positive for the 'same page' case
(no check for crossing a page boundary).

> A more difficult issue would be, if fsx is racing writes and reads,
> in a way that it can guarantee the correct result, but that correct
> result is no longer delivered: because the writes go into freshly
> allocated tmpfs cache pages, while reads are still delivering
> stale ZERO_PAGEs from the pipe.  I'm hazy on the guarantees there.
>
> But unless someone has time to help out, we're heading for a revert.
>
> Thanks,
> Hugh

Cheers,
Mark