Re: panic on 4.20 server exporting xfs filesystem

J. Bruce Fields
2015-03-05T04:08:55Z

On Thu, Mar 05, 2015 at 09:56:23AM +1100, Dave Chinner wrote:
> On Wed, Mar 04, 2015 at 04:49:09PM -0600, Eric Sandeen wrote:
> > On 3/4/15 4:45 PM, Dave Chinner wrote:
> > > On Wed, Mar 04, 2015 at 05:27:09PM -0500, J. Bruce Fields wrote:
> > >> On Thu, Mar 05, 2015 at 09:09:00AM +1100, Dave Chinner wrote:
> > >>> On Wed, Mar 04, 2015 at 10:54:21AM -0500, J. Bruce Fields wrote:
> > >>>> On Tue, Mar 03, 2015 at 09:08:26PM -0500, J. Bruce Fields wrote:
> > >>>>> On Wed, Mar 04, 2015 at 09:44:56AM +1100, Dave Chinner wrote:
> > >>>>>> On Tue, Mar 03, 2015 at 05:10:33PM -0500, J. Bruce Fields wrote:
> > >>>>>>> I'm getting mysterious crashes on a server exporting an xfs filesystem.
> > >>>>>>>
> > >>>>>>> Strangely, I've reproduced this on
> > >>>>>>>
> > >>>>>>> 93aaa830fc17 "Merge tag 'xfs-pnfs-for-linus-3.20-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs
> > >>>>>>>
> > >>>>>>> but haven't yet managed to reproduce on either of its parents
> > >>>>>>> (24a52e412ef2 or 781355c6e5ae). That might just be chance, I'll try
> > >>>>>>> again.
> > >>>>>>
> > >>>>>> I think you'll find that the bug is only triggered after that XFS
> > >>>>>> merge because it's what enabled block layout support in the server,
> > >>>>>> i.e. nfsd4_setup_layout_type() is now setting the export type to
> > >>>>>> LAYOUT_BLOCK_VOLUME because XFS has added the necessary functions to
> > >>>>>> it's export ops.
> > >>>>>
> > >>>>> Doh--after all the discussion I didn't actually pay attention to what
> > >>>>> happened in the end. OK, I see, you're right, it's all more-or-less
> > >>>>> dead code till that merge.
> > >>>>>
> > >>>>> Christoph's code was passing all my tests before that, so maybe we
> > >>>>> broke something in the merge process.
> > >>>>>
> > >>>>> Alternatively, it could be because I've added more tests--I'll rerun my
> > >>>>> current tests on his original branch....
> > >>>>
> > >>>> The below is on Christoph's pnfsd-for-3.20-4 (at cd4b02e). Doesn't look
> > >>>> very informative. I'm running xfstests over NFSv4.1 with client and
> > >>>> server running the same kernel, the filesystem in question is xfs, but
> > >>>> isn't otherwise available to the client (so the client shouldn't be
> > >>>> doing pnfs).
> > >>>>
> > >>>> --b.
> > >>>>
> > >>>> BUG: unable to handle kernel paging request at 00000000757d4900
> > >>>> IP: [<ffffffff810b59af>] cpuacct_charge+0x5f/0xa0
> > >>>> PGD 0
> > >>>> Thread overran stack, or stack corrupted
> > >>>
> > >>> Hmmmm. That is not at all informative, especially as it's only
> > >>> dumped the interrupt stack and not the stack or the task that it
> > >>> has detected as overrun or corrupted.
> > >>>
> > >>> Can you turn on all the stack overrun debug options? Maybe even
> > >>> turn on the stack tracer to get an idea of whether we are recursing
> > >>> deeply somewhere we shouldn't be?
> > >>
> > >> Digging around under "Kernel hacking".... I already have
> > >> DEBUG_STACK_USAGE, DEBUG_STACKOVERFLOW, and STACK_TRACER, and I can try
> > >> turning on the latter. (Will I be able to get information out of it
> > >> before the panic?)
> > >
> > > just keep taking samples of the worst case stack usage as the test
> > > runs. If there's anything unusual before the failure then it will
> > > show up, otherwise I'm not sure how to track this down...
> >
> > I think it should print "maximum stack depth" messages whenever a stack
> > reaches a new max excursion...
>
> That gets printed only when the process exits, IIRC.

Ah-hah:

static void
nfsd4_cb_layout_fail(struct nfs4_layout_stateid *ls)
{
...
nfsd4_cb_layout_fail(ls);

That'd do it!

Haven't tried to figure out why exactly that's getting called, and why
only rarely. Some intermittent problem with the callback path, I guess.

Anyway, I think that solves most of the mystery....

--b.