linux-kernel - Re: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference at 0000000c IP: cfq_insert

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Date:	Wed, 20 Apr 2011 15:13:15 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference
 at  0000000c IP: cfq_insert_request+0x1d/0x3f5

On 2011-04-20 14:58, Michal Hocko wrote:
> Hi,
> I am not able to boot with the current git tree
> (2.6.39-rc4-00089-g2f666bc). 2.6.39-rc3 boots just fine.
> 
> The backtrace [1] looks really messy.
> 
> I have seen a similar backtrace at https://lkml.org/lkml/2011/3/25/169
> and tried the patch from https://lkml.org/lkml/2011/3/25/173 but it
> didn't help. The backtrace looks similar (much smaller) but the machine
> ends up dead as well with unbalanced preempt counter[2].
> 
> The config is attached.
> 
> Is this a known problem?
> 
> ---
> * [1] backtrace with the current git tree (2f666bc)
> 
> [   31.207283] BUG: unable to handle kernel NULL pointer dereference at 0000000c
> NULL pointer dereference[   31.207540] IP: at 0000000c
> [   31.207540] IP: [<c028dece>] cfq_insert_request+0x1d/0x3f5
> [   31.207720] *pde = 00000000  [<c028dece>] cfq_insert_request+0x1d/0x3f5
> NULL pointer dereference[   31.207720] *pde = 00000000
>  at 0000000c
> [   31.207888] Oops: 0000 [#1]
> PREEMPT [   31.207540] IP:[   31.207888] Oops: 0000 [#1] SMP PREEMPT DEBUG_PAGEALLOC [<c028dece>] cfq_insert_request+0x1d/0x3f5
> SMP DEBUG_PAGEALLOC[   31.207720] *pde = 00000000
> [   31.207888] Oops: 0000 [#1] PREEMPT [   31.208186] last sysfs file: /sys/devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0/block/sda/queue/scheduler

Ahh hang on, this may be a good clue. Does your boot scripts change the
IO scheduler?

-- 
Jens Axboe

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