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

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Message-ID: <20180112094833.70926efa@redhat.com>
Date:   Fri, 12 Jan 2018 09:48:33 +0100
From:   Jiri Benc <jbenc@...hat.com>
To:     "Mahesh Bandewar (महेश बंडेवार)" <maheshb@...gle.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 Fri, 12 Jan 2018 09:34:13 +0100, Jiri Benc wrote:
> I don't think this works currently. When someone (does not have to be
> you, it can be a management software running in background) sets the
> MTU to the current value, the magic behavior is lost without any way to
> restore it (unless I'm missing a way to restore it, see my question
> above). So any user that depends on the magic behavior is broken anyway
> even now.

Upon further inspection, it seems that currently, slaves always follow
master's MTU without a way to change it. Tough situation. Even
implementing user space toggleable mtu_adj could break users in the way
I described. But it seems to be the lesser evil, at least there would
be a way to unbreak the scripts with one line addition.

But it's absolute must to have this visible to the user space and
changeable. Something like this:

# ip a
123: ipvlan0: <FLAGS> mtu 1500 (auto) qdisc ...
# ip l s ipvlan0 mtu 1400
# ip a
123: ipvlan0: <FLAGS> mtu 1400 qdisc ...
# ip l s ipvlan0 mtu auto
# ip a
123: ipvlan0: <FLAGS> mtu 1500 (auto) qdisc ...

 Jiri