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

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Message-ID: <50DCE8C8.8050103@iskon.hr>
Date:	Fri, 28 Dec 2012 01:33:12 +0100
From:	Zlatko Calusic <zlatko.calusic@...on.hr>
To:	sedat.dilek@...il.com
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 28.12.2012 01:24, Sedat Dilek wrote:
> 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.
>

Thanks for the testing! And keep running it in case something 
interesting pops up. ;)

No need to revert cda73a10eb because it fixes another bug. And the patch 
you're now running fixes the new bug I introduced with a combination of 
my latest 2 patches. Nah, it gets complicated... :)

But, at least I found the culprit and as soon as Linus applies the fix, 
everything will be hunky dory again, at least on this front. :P

Thanks,
-- 
Zlatko
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