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

[Zlatko Calusic]

On 28.12.2012 01:24, Sedat Dilek wrote:
> On Fri, Dec 28, 2012 at 12:51 AM, Zlatko Calusic
> <zlatko.calusic@xxxxxxxx> wrote:
> > On 28.12.2012 00:42, Sedat Dilek wrote:
> > > On Fri, Dec 28, 2012 at 12:39 AM, Zlatko Calusic
> > > <zlatko.calusic@xxxxxxxx> 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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