arm64: segfaults on next-20170602 with LTP tests

[will.deacon@arm.com (Will Deacon)]

On Fri, Jun 02, 2017 at 05:42:04PM +0300, Yury Norov wrote:
> On Fri, Jun 02, 2017 at 03:37:51PM +0300, Yury Norov wrote:
> > On Fri, Jun 02, 2017 at 01:19:18PM +0100, Will Deacon wrote:
> > > Hi Yury,
> > > 
> > > [adding Steve and Punit]
> > > 
> > > On Fri, Jun 02, 2017 at 02:11:51PM +0300, Yury Norov wrote:
> > > > I see that latest and yesterday's linux-next segfaults with tests pth_str01,
> > > > pth_str03, rwtest04. Rwtest04 hangs sometimes. Crashes are not always
> > > > reproducible. About week ago everything was fine. Kernel log and config file
> > > > are attached. The testing is performed on qemu.
> > > 
> > > It's weird that these haven't cropped up in our nightly tests, especially
> > > given that defconfig is very similar to the one you're using. That said,
> > > I see huge pmds cropping up in the traces below and there have been some
> > > recent changes from Punit and Steve in that area, in particular things
> > > like 55f379263bcc ("mm, gup: ensure real head page is ref-counted when using
> > > hugepages").
> > > 
> > > Are you in a position to bisect this, or is it too fiddly to reproduce?
> 
> I have bisected the bug to exactly this patch. If I revert it, the
> pth_str01/03 are passed.

Thanks for doing that: I had my suspicion ;) I can also reproduce the
failure locally on my Juno.

Punit -- please can you investigate this? Otherwise I think we have to
revert this for now and bring it back after some better testing.

Will