linux-kernel - Re: divide error: bdi_dirty

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Message-ID: <50603829.9050904@linux.vnet.ibm.com>
Date:	Mon, 24 Sep 2012 16:08:33 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...64.org>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Fengguang Wu <fengguang.wu@...el.com>, Jan Kara <jack@...e.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Conny Seidel <conny.seidel@....com>
Subject: Re: divide error: bdi_dirty_limit+0x5a/0x9e

On 09/24/2012 03:53 PM, Borislav Petkov wrote:
> Hi all,
> 
> we're able to trigger the oops below when doing CPU hotplug tests.
> 

I hit this problem as well, which I reported here, a few days ago:
https://lkml.org/lkml/2012/9/13/222

<snip>

> 	...
> 
> and from looking at the register dump below, the dividend, which should
> be in %rdx:%rax is 0 and the divisor (denominator) we've got from
> bdi_writeout_fraction and is in %rdi is also 0. Which is strange because
> fprop_fraction_percpu guards for division by zero by setting denominator
> to 1 if it were zero but what about the case where den > num? Can that
> even happen?
> 
> And also, what happens if num is 0? Which it kinda is by looking at %rcx
> where there's copy of it.
>

Going by the usage of percpu_counter_read_positive() (which is used to get
both the values of num and den), the least value that num or den can have
is zero. So, the C code to guard against divide-by-zero looks OK to me.
Which unfortunately keeps the mystery unsolved :(

Regards,
Srivatsa S. Bhat
 

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