Message-ID: <5306F588.10309@oracle.com>
Date: Fri, 21 Feb 2014 01:43:20 -0500
From: Sasha Levin <sasha.levin@oracle.com>
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To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
CC: linux-mm@kvack.org, akpm@linux-foundation.org, mpm@selenic.com,
        cpw@sgi.com, kosaki.motohiro@jp.fujitsu.com, hannes@cmpxchg.org,
        kamezawa.hiroyu@jp.fujitsu.com, mhocko@suse.cz,
        aneesh.kumar@linux.vnet.ibm.com, xemul@parallels.com, riel@redhat.com,
        kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/11] pagewalk: update page table walker core
References: <1392068676-30627-1-git-send-email-n-horiguchi@ah.jp.nec.com> <1392068676-30627-2-git-send-email-n-horiguchi@ah.jp.nec.com> <5306942C.2070902@gmail.com> <5306c629.012ce50a.6c48.ffff9844SMTPIN_ADDED_BROKEN@mx.google.com>
In-Reply-To: <5306c629.012ce50a.6c48.ffff9844SMTPIN_ADDED_BROKEN@mx.google.com>
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Sender: linux-kernel-owner@vger.kernel.org

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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