zio_done panic in 10.3

[Youzhong Yang]

Hi Andriy,

This is nice! I am 100% sure it's exactly the same issue I experienced and
then reported to illumos mailing list. In all the crash dumps zio->io_done
= l2arc_read_done, so I thought the crash must be related to L2ARC. Once I
set secondarycache=metadata, the frequency of crash went from one per 2
days down to one per week. I've been puzzled by what could have caused a
zio being destroyed while there's still child zio. Your explanation
definitely makes sense!

By the way, is there a FreeBSD bug report or an illumos bug number tracking
this issue? I would be more than happy to create one if needed, and also
test your potential fix here in our environment.

Thanks,

--Youzhong

On Tue, Nov 21, 2017 at 3:46 PM, Andriy Gapon <avg at freebsd.org> wrote:

>
>
>
> On 21/11/2017 21:30, Shiva Bhanujan wrote:
> > it did get compressed to 0.5G - still too big to send via email.  I did
> send some more debug information by running kgdb on the core file to
> Andriy, and I'm waiting for any analysis that he might provide.
>
> Yes, kgdb-over-email turned out to be a far more efficient compression :-)
> I already have an analysis based on the information provided by Shiva and
> by
> another user who has the same problem and contacted me privately.
> I am discussing possible ways to fix the problem with George Wilson who
> was very
> kind to double-check the analysis, complete it and suggest possible fixes.
>
> A short version is that dbuf_prefetch and dbuf_prefetch_indirect_done
> functions
> chain new zio-s under the same parent zio (a completion of one child zio
> may
> create another child zio).  They do it using arc_read which can create
> either a
> logical zio in most cases or a vdev zio for a read from a cache device
> (2arc).
> zio_done() has a check for the completion of a parent zio's children but
> that
> check is not completely safe and can be broken by the pattern that
> dbuf_prefetch
> can create.  So, under some specific circumstances the parent zio may
> complete
> and get destroyed while there is a child zio.
>
> I believe this problem to be rather rare, but there could be
> configurations and
> workloads where it's triggered more often.
> The problem does not happen if there are no cache devices.
>
> > From: Conrad Meyer [cem at freebsd.org]
> >
> > Sent: Tuesday, November 21, 2017 9:04 AM
> >
> > To: Shiva Bhanujan
> >
> > Cc: Andriy Gapon; freebsd-fs at freebsd.org
> >
> > Subject: Re: zio_done panic in 10.3
> >
> >
> >
> >
> >
> >
> >
> > Have you tried compressing it with e.g. xz or zstd?
> >
> > --
> Andriy Gapon
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