From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Received: from mail-ot0-f197.google.com (mail-ot0-f197.google.com [74.125.82.197])
	by kanga.kvack.org (Postfix) with ESMTP id 732C96B0033
	for <linux-mm@kvack.org>; Fri, 19 Jan 2018 12:30:28 -0500 (EST)
Received: by mail-ot0-f197.google.com with SMTP id q4so1457552oti.6
        for <linux-mm@kvack.org>; Fri, 19 Jan 2018 09:30:28 -0800 (PST)
Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65])
        by mx.google.com with SMTPS id d74sor3201134oig.194.2018.01.19.09.30.26
        for <linux-mm@kvack.org>
        (Google Transport Security);
        Fri, 19 Jan 2018 09:30:27 -0800 (PST)
Subject: Re: [Bug 198497] New: handle_mm_fault / xen_pmd_val /
 radix_tree_lookup_slot Null pointer
References: <bug-198497-27@https.bugzilla.kernel.org/>
 <20180118135518.639141f0b0ea8bb047ab6306@linux-foundation.org>
 <7ba7635e-249a-9071-75bb-7874506bd2b2@redhat.com>
 <20180119030447.GA26245@bombadil.infradead.org>
 <d38ff996-8294-81a6-075f-d7b2a60aa2f4@rimuhosting.com>
 <20180119132145.GB2897@bombadil.infradead.org>
From: Laura Abbott <labbott@redhat.com>
Message-ID: <9d2ddba4-3fb3-0fb4-a058-f2cfd1b05538@redhat.com>
Date: Fri, 19 Jan 2018 09:30:23 -0800
MIME-Version: 1.0
In-Reply-To: <20180119132145.GB2897@bombadil.infradead.org>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: owner-linux-mm@kvack.org
List-ID: <linux-mm.kvack.org>
To: Matthew Wilcox <willy@infradead.org>, xen@randomwebstuff.com
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, bugzilla-daemon@bugzilla.kernel.org

On 01/19/2018 05:21 AM, Matthew Wilcox wrote:
> On Fri, Jan 19, 2018 at 04:14:42PM +1300, xen@randomwebstuff.com wrote:
>>
>> On 19/01/18 4:04 PM, Matthew Wilcox wrote:
>>> On Thu, Jan 18, 2018 at 02:18:20PM -0800, Laura Abbott wrote:
>>>> On 01/18/2018 01:55 PM, Andrew Morton wrote:
>>>>>> [   24.647744] BUG: unable to handle kernel NULL pointer dereference at
>>>>>> 00000008
>>>>>> [   24.647801] IP: __radix_tree_lookup+0x14/0xa0
>>>>>> [   24.647811] *pdpt = 00000000253d6027 *pde = 0000000000000000
>>>>>> [   24.647828] Oops: 0000 [#1] SMP
>>>>>> [   24.647842] CPU: 5 PID: 3600 Comm: java Not tainted
>>>>>> 4.14.13-rh10-20180115190010.xenU.i386 #1
>>>>>> [   24.647855] task: e52518c0 task.stack: e4e7a000
>>>>>> [   24.647866] EIP: __radix_tree_lookup+0x14/0xa0
>>>>>> [   24.647876] EFLAGS: 00010286 CPU: 5
>>>>>> [   24.647884] EAX: 00000004 EBX: 00000007 ECX: 00000000 EDX: 00000000
> 
> If my understanding is right, EDX contains the index we're looking up.
> Which is zero.  So the swp_entry we got is one bit away from being NULL.
> Hmm.  Have you run memtest86 or some other memory tester on the system
> recently?
> 
>> PS: cannot recall seeing this issue on x86_64, just 32 bit.
> 
> Laura has 64-bit instances of this.
> 

The 64-bit backtraces reported in the bugzilla looked different,
I would consider it a different issue.

> PPS: reminder
>> this is on a Xen VM which per https://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html#PVH-Guest-Specific-Options
>> has "out of sync pagetables" if that is relevant (we do not set that option,
>> I am unsure what default is used).
> 
> Laura also has non-Xen instances of this.  They may not all be the same
> bug, of course.
> 

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>