linux-kernel - Re: [delayed_fput] BUG: unable to handle kernel paging request at ffff8800122a0ad0

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Message-ID: <CAADnVQL+O0ZFmF99+Ba4Ua2bwLnaO2KDJM1o86aUvG24xS08gg@mail.gmail.com>
Date:	Wed, 30 Jul 2014 19:53:15 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jet Chen <jet.chen@...el.com>, Su Tao <tao.su@...el.com>,
	Yuanhan Liu <yuanhan.liu@...el.com>,
	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [delayed_fput] BUG: unable to handle kernel paging request at ffff8800122a0ad0

On Wed, Jul 30, 2014 at 7:40 PM, Fengguang Wu <fengguang.wu@...el.com> wrote:
> On Wed, Jul 30, 2014 at 04:00:58PM -0700, Alexei Starovoitov wrote:
>> On Wed, Jul 30, 2014 at 3:51 PM, David Rientjes <rientjes@...gle.com> wrote:
>> > On Wed, 30 Jul 2014, Christoph Hellwig wrote:
>> >
>> >> On Wed, Jul 30, 2014 at 11:55:41AM +0800, Fengguang Wu wrote:
>> >> > Greetings,
>> >> >
>> >> > 0day kernel testing robot got the below dmesg and the first bad commit is
>> >>
>> >> How does this manage to trip over a 2 year old commit now?
>>
>> may be because all kernels are built with gcc 4.8.2 ?
>> Fengguang, did you recently switch to new compiler?
>> I think all older builds were with 4.6.3
>
> Alexei, I've been using gcc 4.8.2 for 4 months.
>
> Maybe it's because I test new randconfigs every day. And the problem
> may only show up with very specific kernel config?

It seems a lot of odd crashes suddenly showed up and I'm guessing it
may be related to compiler...
See commit 2062afb4f8 ("Fix gcc-4.9.0 miscompilation of load_balance()
 in scheduler")
it's a nasty gcc bug that affects 4.9 and 4.8.
I think it makes sense to apply that workaround before debugging much with 4.8
just to eliminate the possibility of miscompiled code.
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