netdev - Re: [PATCH v2 net-next 10/10] net: bridge: implement auto-normalization of MTU for hardware datapath

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Date:   Thu, 26 Mar 2020 21:41:25 +0200
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        Ido Schimmel <idosch@...sch.org>, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        murali.policharla@...adcom.com,
        Stephen Hemminger <stephen@...workplumber.org>,
        Jiri Pirko <jiri@...nulli.us>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 10/10] net: bridge: implement
 auto-normalization of MTU for hardware datapath

On 26/03/2020 20:49, Jakub Kicinski wrote:
> On Thu, 26 Mar 2020 14:38:57 +0200 Nikolay Aleksandrov wrote:
>> On 26/03/2020 14:25, Vladimir Oltean wrote:
>>> On Thu, 26 Mar 2020 at 14:19, Nikolay Aleksandrov
>>> <nikolay@...ulusnetworks.com> wrote:  
>>>>
>>>> On 26/03/2020 14:18, Vladimir Oltean wrote:  
>>>>> On Thu, 26 Mar 2020 at 14:06, Nikolay Aleksandrov
>>>>> <nikolay@...ulusnetworks.com> wrote:  
>>>>>>
>>>>>> On 26/03/2020 13:35, Ido Schimmel wrote:  
>>>>>>> On Thu, Mar 26, 2020 at 12:25:20PM +0200, Vladimir Oltean wrote:  
>>>>>>>> Hi Ido,
>>>>>>>>
>>>>>>>> On Thu, 26 Mar 2020 at 12:17, Ido Schimmel <idosch@...sch.org> wrote:  
>>>>>>>>>  
>>>>> [snip]  
>>>>>>>
>>>>>>> I think you should be more explicit about it. Did you consider listening
>>>>>>> to 'NETDEV_PRECHANGEMTU' notifications in relevant drivers and vetoing
>>>>>>> unsupported configurations with an appropriate extack message? If you
>>>>>>> can't veto (in order not to break user space), you can still emit an
>>>>>>> extack message.
>>>>>>>  
>>>>>>
>>>>>> +1, this sounds more appropriate IMO
>>>>>>  
>>>>>
>>>>> And what does vetoing gain me exactly? The practical inability to
>>>>> change the MTU of any interface that is already bridged and applies
>>>>> length check on RX?
>>>>>  
>>>>
>>>> I was referring to moving the logic to NETDEV_PRECHANGEMTU, the rest is up to you.
>>>>  
>>>
>>> If I'm not going to veto, then I don't see a lot of sense in listening
>>> on this particular notifier either. I can do the normalization just
>>> fine on NETDEV_CHANGEMTU.
>>
>> I should've been more explicit - I meant I agree that this change doesn't belong in
>> the bridge, and handling it in a notifier in the driver seems like a better place.
>> Yes - if it's not going to be a vetto, then CHANGEMTU is fine.
> 
> I'm not sure pushing behavior decisions like that out to the drivers 
> is ever a good idea. Linux should abstract HW differences after all,
> we don't want different drivers to perform different magic behind
> user's back. 
> 
> I'd think if HW is unable to apply given configuration vetoing is both
> correct and expected..
> 

This change implements a policy and makes it default for all HW-offloaded devices, but
not all of them have these restrictions or need it. Moreover MTU handling has always been
a vendor/driver-specific problem.
I do agree about the veto part, in my experience we've had countless problem reports
due to the bridge auto-MTU adjusting, but it seems to me it's the driver authors' right
to implement any policy they want as long as it doesn't affect everyone else.