Re: [Xen-devel] RFH: Kernel OOPS in xen_netbk_rx_action / xenvif_gop_skb

[Philipp Hahn <hahn@xxxxxxxxxxxxx>]

Hello,

On 06.06.2014 12:58, Wei Liu wrote:
> On Fri, Jun 06, 2014 at 12:26:55PM +0200, Philipp Hahn wrote:
>> on one of our hosts (Xen-4.1.3 with Linux-3.10.26 + Debian patches)
>> running 16 Linux VMs (linux-3.2.39 and others) netback crashes during
>> the night when one of the VMs is rebooted by a cron-job:
>>> [38551.549615] Oops: 0000 [#1] SMP
> 
> Is there any more output above this line? Is it a NULL pointer
> dereference or something else?

Sorry, those lines got lost somehow during copy&paste:

[38551.547728] XXXlan0: port 9(vif26.0) entered disabled state
[38551.549365] BUG: unable to handle kernel paging request at
ffffc900108641d8
[38551.549461] IP: [<ffffffffa04147dc>] xen_netbk_rx_action+0x18b/0x6f0
[xen_netback]
[38551.549551] PGD 57e20067 PUD 57e21067 PMD 571a7067 PTE 0
[38551.549615] Oops: 0000 [#1] SMP

>>> [38551.550865] RIP: e030:[<ffffffffa04147dc>]  [<ffffffffa04147dc>]
>>> xen_netbk_rx_action+0x18b/0x6f0 [xen_netback]
> 
> Try addr2line?

Good to know, but since that host is already rebooted, I no longer know
the module load address, which seems to render addr2line useless.

>> The host itself is still alive and reachable by network, but all VMs are
>> no longer reachable.
>> The crash does not happen on every reboot: The VM was running fine for
>> 1½ week after a dom0 kernel update, but now crashed the following past
>> two nights.
>>
> 
> What's the Dom0 kernel version before upgrading? That would help us
> narrow down the range of changesets.

The previous kernel was 3.10.15. The update was performed to get another
bug fixed, which went into the Debian update between .11 and .26:

commit 0ff773f59ff375c42af2238457bda98ed4ddcd25
Author: David Vrabel <david.vrabel@xxxxxxxxxx>
Date:   Wed Sep 11 14:52:48 2013 +0100
    xen-netback: count number required slots for an skb more carefully
    [ Upstream commit 6e43fc04a6bc357d260583b8440882f28069207f ]

> The oops happens in guest receive path. Unfortunately that's a very
> complex function, it's hard to identify the problem by looking at the
> code.
> 
> And as you seem to be using a distro kernel, have your reported to
> Debian yet? I don't quite understand which Debian release has 3.10
> kernel though.

Actually this is "Univention Corporate Server" (UCS), which is
Debian-Squeeze based but with a newer Xen-4.1.3 and newer Linux-3.10 kernel.

> 3.7.0 is too old. There has been lots of changes since then.

Probably so, but thanks for the confirmation.

>> Running "objdump -Sl xen-netback.ko" shows the OOPs to happen here:
>>> /root/linux-3.10.11/drivers/net/xen-netback/netback.c:606
...
> You mentioned 3.10.26 at the beginning but now it's 3.10.11? I'm
> confused.

This has something to do with Debian patch policy: The first 3.10 kernel
was 3.10.11, so that number stays, even when it up-patched to 3.10.26.

> If it's dereferencing NULL pointer, skb_shinfo(skb) == NULL?

If I got the math right, it looks like it's crashing here:
>>> /root/linux-3.10.11/drivers/net/xen-netback/netback.c:611
>>>         meta->id = req->id;
>>>      7d8:       48 83 c2 08             add    $0x8,%rdx
>>>      7dc:       0f b7 34 d1             movzwl (%rcx,%rdx,8),%esi
>> 0x651 + 0x18B = 0x7DC

0x651 is the start of xen_netbk_rx_action() from objdump.

...
> There's one more patch that you can pick up from 3.10.y tree. I doubt it
> will make much difference though.
> 
> I think the first thing to do is to identify which line of code is
> causing the problem. If it is actually the line you're referring to in
> your analyse then we need to figure out why skb_shinfo(skb) is NULL...

I'll try to add some debug output to yell if skb_shinfo() is NULL, but
it might take some time until the bug manifests again.

> Wei.

Thank you for your feedback.

Philipp
-- 
Philipp Hahn
Open Source Software Engineer

Univention GmbH
be open.
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Tel.: +49 421 22232-0
Fax : +49 421 22232-99
hahn@xxxxxxxxxxxxx

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