Re: linux-next: BUG: Bad page state in process ip6tables-save pfn:1499f4

[Punit Agrawal]

"Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:

> On Tue, Jun 27, 2017 at 05:53:59PM +0100, Punit Agrawal wrote:
>> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> writes:
>> 
>> > On Tue, Jun 27, 2017 at 09:18:15AM +0200, Vlastimil Babka wrote:
>> >> On 06/24/2017 05:08 PM, Andrei Vagin wrote:
>> >> > On Fri, Jun 23, 2017 at 05:17:44PM -0700, Andrei Vagin wrote:
>> >> >> On Thu, Jun 22, 2017 at 11:21:03PM -0700, Andrei Vagin wrote:
>> >> >>> Hello,
>> >> >>>
>> >> >>> We run CRIU tests for linux-next and today they triggered a kernel
>> >> >>> bug. I want to mention that this kernel is built with kasan. This bug
>> >> >>> was triggered in travis-ci. I can't reproduce it on my host. Without
>> >> >>> kasan, kernel crashed but it is impossible to get a kernel log for
>> >> >>> this case.
>> >> >>
>> >> >> We use this tree
>> >> >> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/
>> >> >>
>> >> >> This issue isn't reproduced on the akpm-base branch and
>> >> >> it is reproduced each time on the akpm branch. I didn't
>> >> >> have time today to bisect it, will do on Monday.
>> >> > 
>> >> > c3aab7b2d4e8434d53bc81770442c14ccf0794a8 is the first bad commit
>> >> > 
>> >> > commit c3aab7b2d4e8434d53bc81770442c14ccf0794a8
>> >> > Merge: 849c34f 93a7379
>> >> > Author: Stephen Rothwell
>> >> > Date:   Fri Jun 23 16:40:07 2017 +1000
>> >> > 
>> >> >     Merge branch 'akpm-current/current'
>> >> 
>> >> Hm is it really the merge of mmotm itself and not one of the patches in
>> >> mmotm?
>> >> Anyway smells like THP, adding Kirill.
>> >
>> > Okay, it took a while to figure it out.
>> 
>> I'm sorry you had to go chasing for this one again.
>> 
>> I'd found the same issue while investigating an ltp failure on arm64[0] and
>> sent a fix[1]. The fix is effectively the same as your patch below.
>> 
>> Andrew picked up the patch from v5 posting and I can see it in today's
>> next[2].
>> 
>> 
>> [0] http://lists.infradead.org/pipermail/linux-arm-kernel/2017-June/510318.html
>> [1] https://patchwork.kernel.org/patch/9766193/
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/mm/gup.c?h=next-20170627&id=d31945b5d4ab4490fb5f961dd5b066cc9f560eb3
>
> Ah. Okay, no problem then.
>
> But I think my fix is neater :)

Hehe.. I'm fine with either as they both fix the problem. :)

The reason I kept head and page initialisations separate is to ensure in
the future somebody doesn't conclude the page and head are the same -
which is true in most instances unless you've got contiguous hugepages
where that assumption breaks. But this isn't really full proof anyways.