linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference at 0000000000000500

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Date:	Fri, 28 Dec 2012 13:57:31 +0100
From:	Zlatko Calusic <zlatko.calusic@...on.hr>
To:	Zhouping Liu <zliu@...hat.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Johannes Weiner <jweiner@...hat.com>, mgorman@...e.de,
	hughd@...gle.com, Andrea Arcangeli <aarcange@...hat.com>,
	Hillf Danton <dhillf@...il.com>, sedat.dilek@...il.com
Subject: Re: BUG: unable to handle kernel NULL pointer dereference at 0000000000000500

On 28.12.2012 03:45, Zhouping Liu wrote:
>>
>> Thank you for the report Zhouping!
>>
>> Would you be so kind to test the following patch and report results?
>> Apply the patch to the latest mainline.
>
> Hello Zlatko,
>
> I have tested the below patch(applied it on mainline directly),
> but IMO, I'd like to say it maybe don't fix the issue completely.
>
> run the reproducer[1] on two machine, one machine has 2 numa nodes(8Gb RAM),
> another one has 4 numa nodes(8Gb RAM), then the system hung all the time, such as the dmesg log:
>
> [  713.066937] Killed process 6085 (oom01) total-vm:18880768kB, anon-rss:7915612kB, file-rss:4kB
> [  959.555269] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
> [  959.562144] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 1079.382018] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
> [ 1079.388872] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 1199.209709] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
> [ 1199.216562] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 1319.036939] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
> [ 1319.043794] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 1438.864797] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
> [ 1438.871649] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 1558.691611] INFO: task kworker/13:2:147 blocked for more than 120 seconds.
> [ 1558.698466] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> ......
>
> I'm not sure whether it's your patch triggering the hung task or not, but reverted cda73a10eb3,
> the reproducer(oom01) can PASS without both 'NULL pointer dereference at 0000000000000500' and hung task issues.
>
> but some time, it's possible that the reproducer(oom01) cause hung task on a box with large RAM(100Gb+), so I can't judge it...
>

Thanks for the test.

Yes, close to OOM things get quite unstable and it's hard to get 
reliable test results. Maybe you could run it a few times, and see if 
you can get any meaningful statistics out of a few runs. I need to check 
oom.c myself and see what it's doing. Thanks for the link.

Regards,
-- 
Zlatko
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