linux-kernel - Re: divide error: bdi_dirty

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Date:	Mon, 24 Sep 2012 14:56:32 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Fengguang Wu <fengguang.wu@...el.com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.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>
Subject: Re: divide error: bdi_dirty_limit+0x5a/0x9e

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.

So it has to be hotplug-related.

-- 
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Boris.

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