linux-kernel - Re: Simple script that locks up my box with recent kernels

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Date:	Wed, 22 Nov 2006 12:00:59 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Linus Torvalds" <torvalds@...l.org>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Andrew Morton" <akpm@...l.org>
Subject: Re: Simple script that locks up my box with recent kernels

On 22/11/06, Linus Torvalds <torvalds@...l.org> wrote:
>
>
> On Wed, 22 Nov 2006, Jesper Juhl wrote:
> >
> > So it *seems* to be somehow related to running low on RAM and swap
> > starting to be used.
>
> Does it happen if you just do some simple "use all memory" script, eg run
> a few copies of
>
>         #define SIZE (100<<20)
>
>         char *buf = malloc(SIZE);
>         memset(buf, SIZE, 0);
>         sleep(100);
>
> on your box?
>

I'll try, when I get home from work. I'll let you know later.


> > The box has 2GB of RAM and 768MB swap.
>
> I wonder.. It _used_ to be true that we were pretty good at making swap be
> "extra" memory. But maybe we've lost some of that, and we have trouble
> with having more physical memory. We could end up in a situation where we
> allocate it all very quickly (because we don't actually page it out, we
> just allocate backing store for the pages), and we screw something up.
>
> But stupid bugs there should still leave us trivially able to do the SysRQ
> things, so..
>
Well, it's a fact that sysrq works just fine before the lockup but
does not work at all after a lockup, so...


> Is it highmem-related? Some bounce-buffering problem while having to swap?

I can try building a kernel without highmem support and see if I can
still cause it to lockup. Would be an interresting datapoint.

I'll also try reproducing the lockup without any swap active to see if
that makes a difference.


> What block device driver do you use for the swap device?
>
It's a swap partition on a IBM Ultra160 10K RPM SCSI disk. The
controller is an Adaptec 29160N. Using the SCSI_AIC7XXX driver.


> I don't think we use any irq-disable locking in the VM itself, but I could
> imagine some nasty situation with the block device layer getting into a
> deadlock with interrupts disabled when it runs out of queue entries and
> cannot allocate more memory..
>
Just let me know what you would like me to try/test to prove/disprove that.

-- 
Jesper Juhl <jesper.juhl@...il.com>
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