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

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Date:	Fri, 28 Dec 2012 01:24:22 +0100
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Zlatko Calusic <zlatko.calusic@...on.hr>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference at 0000000000000500

On Fri, Dec 28, 2012 at 12:51 AM, Zlatko Calusic
<zlatko.calusic@...on.hr> wrote:
> On 28.12.2012 00:42, Sedat Dilek wrote:
>>
>> On Fri, Dec 28, 2012 at 12:39 AM, Zlatko Calusic
>> <zlatko.calusic@...on.hr> wrote:
>>>
>>> On 28.12.2012 00:30, Sedat Dilek wrote:
>>>>
>>>>
>>>> Hi Zlatko,
>>>>
>>>> I am not sure if I hit the same problem as described in this thread.
>>>>
>>>> Under heavy load, while building a customized toolchain for the Freetz
>>>> router project I got a BUG || NULL pointer derefence || kswapd ||
>>>> zone_balanced || pgdat_balanced() etc. (details see my screenshot).
>>>>
>>>> I will try your patch from [1] ***only*** on top of my last
>>>> Linux-v3.8-rc1 GIT setup (post-v3.8-rc1 mainline + some net-fixes).
>>>>
>>>
>>> Yes, that's the same bug. It should be fixed with my latest patch, so I'd
>>> appreciate you testing it, to be on the safe side this time. There should
>>> be
>>> no difference if you apply it to anything newer than 3.8-rc1, so go for
>>> it.
>>> Thanks!
>>>
>>
>> Not sure how I can really reproduce this bug as one build worked fine
>> within my last v3.8-rc1 kernel.
>> I increased the parallel-make-jobs-number from "4" to "8" to stress a
>> bit harder.
>> Just building right now... and will report.
>>
>> If you have any test-case (script or whatever), please let me/us know.
>>
>
> Unfortunately not, I haven't reproduced it yet on my machines. But it seems
> that bug will hit only under heavy memory pressure. When close to OOM, or
> possibly with lots of writing to disk. It's also possible that fragmentation
> of memory zones could provoke it, that means testing it for a longer time.
>

I tested successfully by doing simultaneously...
- building Freetz with 8 parallel make-jobs
- building Linux GIT with 1 make-job
- 9 tabs open in firefox
- In one tab I ran YouTube music video
- etc.

I am reading [1] and [2] where another user reports success by reverting this...

commit cda73a10eb3f493871ed39f468db50a65ebeddce
"mm: do not sleep in balance_pgdat if there's no i/o congestion"

BTW, this machine has also 4GiB RAM (Ubuntu/precise AMD64).

Feel free to add a "Reported-by/Tested-by" if you think this is a
positive report.

- Sedat -

[1] http://marc.info/?l=linux-mm&m=135652835931174&w=2
[2] http://marc.info/?l=linux-mm&m=135653399800655&w=2

> --
> Zlatko
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