arm64: segfaults on next-20170602 with LTP tests

Will Deacon [off-list ref] writes:

On Fri, Jun 02, 2017 at 05:42:04PM +0300, Yury Norov wrote:

quoted

On Fri, Jun 02, 2017 at 03:37:51PM +0300, Yury Norov wrote:

quoted

On Fri, Jun 02, 2017 at 01:19:18PM +0100, Will Deacon wrote:

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Hi Yury,

[adding Steve and Punit]

On Fri, Jun 02, 2017 at 02:11:51PM +0300, Yury Norov wrote:

quoted

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.

Apologies for the breakage. It looks like anonymous hugepages are not
happy with the change. Let me dig into this.

Will