linux-kernel - Re: divide error: bdi_dirty

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Date:	Tue, 25 Sep 2012 00:24:41 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Borislav Petkov <bp@...64.org>
CC:	Fengguang Wu <fengguang.wu@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, 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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: divide error: bdi_dirty_limit+0x5a/0x9e

On 09/24/2012 06:26 PM, Borislav Petkov wrote:
> On Mon, Sep 24, 2012 at 08:29:00PM +0800, Fengguang Wu wrote:
>> On Mon, Sep 24, 2012 at 02:20:53PM +0200, Borislav Petkov wrote:
>>> On Mon, Sep 24, 2012 at 07:34:47PM +0800, Fengguang Wu wrote:
>>>> Will you test such a line? At least the generic do_div() only uses the
>>>> lower 32bits for division.
>>>>
>>>>         WARN_ON(!(den & 0xffffffff));
>>>
>>> But, but, the asm output says:
>>>
>>>   28:   48 89 c8                mov    %rcx,%rax
>>>   2b:*  48 f7 f7                div    %rdi     <-- trapping instruction
>>>   2e:   31 d2                   xor    %edx,%edx
>>>
>>> and this version of DIV does an unsigned division of RDX:RAX by the
>>> contents of a *64-bit register* ... in our case %rdi.
>>>
>>> Srivatsa's oops  shows the same:
>>>
>>>   28:   48 89 f0                mov    %rsi,%rax
>>>   2b:*  48 f7 f7                div    %rdi     <-- trapping instruction
>>>   2e:   41 8b 94 24 74 02 00    mov    0x274(%r12),%edx
>>>
>>> Right?
>>
>> Right, that's why I said "at least". As for x86, I'm as clueless as you..
> 
> Right, both oopses are on x86 so I don't think it is the bitness of the
> division.
> 
> Another thing those two have in common is that both happen when a CPU
> comes online. Srivatsa's is when CPU9 comes online (oops is detected on
> CPU9) and in our case CPU4 comes online but the oops says CPU0.
> 

I had posted another dump from one of my tests. That one triggers while
offlining a CPU (CPU 9).

https://lkml.org/lkml/2012/9/14/235

> So it has to be hotplug-related.

Regards,
Srivatsa S. Bhat


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