linux-kernel - Re: [PATCH 01/11] pagewalk: update page table walker core

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Date:	Fri, 21 Feb 2014 01:43:20 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
CC:	linux-mm@...ck.org, akpm@...ux-foundation.org, mpm@...enic.com,
	cpw@....com, kosaki.motohiro@...fujitsu.com, hannes@...xchg.org,
	kamezawa.hiroyu@...fujitsu.com, mhocko@...e.cz,
	aneesh.kumar@...ux.vnet.ibm.com, xemul@...allels.com,
	riel@...hat.com, kirill.shutemov@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/11] pagewalk: update page table walker core

On 02/20/2014 10:20 PM, Naoya Horiguchi wrote:
> Hi Sasha,
> 
> On Thu, Feb 20, 2014 at 06:47:56PM -0500, Sasha Levin wrote:
>> Hi Naoya,
>>
>> This patch seems to trigger a NULL ptr deref here. I didn't have a change to look into it yet
>> but here's the spew:
> 
> Thanks for reporting.
> I'm not sure what caused this bug from the kernel message. But in my guessing,
> it seems that the NULL pointer is deep inside lockdep routine __lock_acquire(),
> so if we find out which pointer was NULL, it might be useful to bisect which
> the proble is (page table walker or lockdep, or both.)

This actually points to walk_pte_range() trying to lock a NULL spinlock. It happens when we call
pte_offset_map_lock() and get a NULL ptl out of pte_lockptr().

> BTW, just from curiousity, in my build environment many of kernel functions
> are inlined, so should not be shown in kernel message. But in your report
> we can see the symbols like walk_pte_range() and __lock_acquire() which never
> appear in my kernel. How did you do it? I turned off CONFIG_OPTIMIZE_INLINING,
> but didn't make it.

I'm really not sure. I've got a bunch of debug options enabled and it just seems to do the trick.

Try CONFIG_READABLE_ASM maybe?


Thanks,
Sasha

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