netdev - Re: [PATCH V2] ipvlan: fix ipvlan MTU limits

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Date:   Thu, 11 Jan 2018 08:59:58 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     Jiri Benc <jbenc@...hat.com>
Cc:     liuqifa@...wei.com, David Miller <davem@...emloft.net>,
        dsahern@...il.com, mschiffer@...verse-factory.net,
        idosch@...lanox.com, fw@...len.de, kjlx@...pleofstupid.com,
        girish.moodalbail@...cle.com, sainath.grandhi@...el.com,
        linux-netdev <netdev@...r.kernel.org>, weiyongjun1@...wei.com,
        chenweilong@...wei.com, maowenan@...wei.com, wangyufen@...wei.com,
        yuehaibing@...wei.com, liujian56@...wei.com, liuzhe24@...wei.com,
        wangkefeng.wang@...wei.com, dingtianhong@...wei.com
Subject: Re: [PATCH V2] ipvlan: fix ipvlan MTU limits

On Thu, Jan 11, 2018 at 3:25 AM, Jiri Benc <jbenc@...hat.com> wrote:
> On Wed, 10 Jan 2018 18:09:50 -0800, Mahesh Bandewar (महेश बंडेवार) wrote:
>> I still prefer the approach I had mentioned that uses 'mtu_adj'. In
>> that approach you can leave those slaves which have changed their mtu
>> to be lower than masters' but if master's mtu changes to larger value
>> all other slaves will get updated mtu leaving behind the slaves who
>> have opted to change their mtu on their own. Also the same thing is
>> true when mtu get reduced at master.
>
> The problem with this magic behavior is, well, that it's magic. There's
> no way to tell what happens with a given slave when the master's MTU
> gets changed just by looking at the current configuration. There's also
> no way to switch the magic behavior back on once the slave's MTU is
> changed.
>
I guess the logic would be as simple as - if mtu_adj for a slave is
set to 0, then it's
following master otherwise not. By setting different mtu for a slave, you will
set this mtu_adj a positive number which would mean it's not following master.
So it's subjected to clamping when masters' mtu is reducing but should stay
otherwise. Also when slave decides to follow master again, it can set the mtu
to be same as masters' (making mtu_adj == 0) and then it would start following
master again.

Whether it's magic or not, it's the current behavior and I know several use
cases depend on this behavior which would be broken otherwise. The
approach I proposed keeps that going for those who depend on that while
adds an ability to set mtu per slave for the use case mentioned in this
patch-set too.

> At minimum, you'd need some kind of indication that the slave's MTU is
> following the master. And a way to toggle this back.
>
> Keefe's patch is much saner, the behavior is completely deterministic.
>
>  Jiri