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Re: [Qemu-devel] post-copy is broken?

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] post-copy is broken?
Date: Thu, 14 Apr 2016 11:13:03 +0100
User-agent: Mutt/1.5.24 (2015-08-30)

* Andrea Arcangeli (address@hidden) wrote:
> On Wed, Apr 13, 2016 at 01:50:53PM +0100, Dr. David Alan Gilbert wrote:
> > * Dr. David Alan Gilbert (address@hidden) wrote:
> > 
> > > +            if ( ((b + 1) % 255) == last_byte && !hit_edge) {
> > 
> > Ahem, that should be 256.
> > 
> > I'm going to bisect the kernel and see where we get to.
> > Andrea's userfaultfd self-test passes on 2.5, so it's something more
> > subtle.
> > 
> David already tracked down 1df59b8497f47495e873c23abd6d3d290c730505
> good and 984065055e6e39f8dd812529e11922374bd39352 bad.
> git diff 
> 1df59b8497f47495e873c23abd6d3d290c730505..984065055e6e39f8dd812529e11922374bd39352
>  fs/userfaultfd.c mm/userfaultfd.c
> Nothing that could break it in the diff of the relevant two files.
> The only other userfault related change in this commit range that
> comes to mind is in fixup_user_fault, but if that was buggy you don't
> userfault into futexes with postcopy so you couldn't notice, so the
> only other user of that is s390.
> The next suspect is the massive THP refcounting change that went
> upstream recently:


> As further debug hint, can you try to disable THP and see if that
> makes the problem go away?

Yeh, looks like it is THP.
My bisect is currently at 17ec4cd985780a7e30aa45bb8f272237c12502a4
and with that from a fresh boot it fails, if I disable THP it works
and if I reenable THP back to madvise it fails.

I spotted that my previous bisect point it failed before I'd done
the next kernel build but failed after I'd done the build (but before
I rebooted!) - so I guess after the build it couldn't find any THPs to do.


> Thanks,
> Andrea
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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