netdev - Re: [PATCH net] ip[6]: don't register inet[6]dev when dev is down

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Date:   Fri, 7 Jul 2017 14:39:08 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Hongjun Li <hongjun.li@...nd.com>
Subject: Re: [PATCH net] ip[6]: don't register inet[6]dev when dev is down

Le 06/07/2017 à 20:16, Cong Wang a écrit :
> On Thu, Jul 6, 2017 at 5:08 AM, Nicolas Dichtel
> <nicolas.dichtel@...nd.com> wrote:
>> Le 06/07/2017 à 00:43, Cong Wang a écrit :
>>> On Wed, Jul 5, 2017 at 8:57 AM, Nicolas Dichtel
>>> <nicolas.dichtel@...nd.com> wrote:
>>>> When a device changes from one netns to another, it's first unregistered,
>>>> then the netns reference is updated and the dev is registered in the new
>>>> netns. Thus, when a slave moves to another netns, it is first
>>>> unregistered. This triggers a NETDEV_UNREGISTER event which is caught by
>>>> the bonding driver. The driver calls bond_release(), which calls
>>>> dev_set_mtu() and thus triggers NETDEV_CHANGEMTU (the device is still in
>>>> the old netns).
>>>
>>> I think in this special case it is meaningless to send
>>> NETDEV_CHANGEMTU, because the device is dying within
>>> its old netns, who still cares about its mtu change?
>>>
>>> Something like the attached patch...
>> Yes, your patch seems good and I hesitated with something like this.
>> But I don't see a valid case where the inet[6]dev must be created on a down
>> interface. I think the patch is valid, even with your patch.
> 
> Your patch is more risky because it affects normal CHANGEMTU path,
> I am not sure if it is correct to not to add idev when it is down either.
Why would it be needed to add this idev on a down interface?
If idev wasn't there I don't see why changing the mtu would justify to create
this idev.

> 
> This is a very unusual path, we don't have to take the risk.
I still think that this approach is better for two reasons:
 - we don't know if another path like this exists (need an audit) and it would
be easy to add one again by side effect in the future;
 - the patch is easy to backport in older kernel.


Regards,
Nicolas