linux-kernel - Re: [PATCH net] net: vlan: allow vlan device MTU change follow real device from smaller to bigger

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Message-ID: <20220223093749.6b33345a@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 23 Feb 2022 09:37:49 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "Ziyang Xuan (William)" <william.xuanziyang@...wei.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Vasily Averin <vvs@...tuozzo.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: vlan: allow vlan device MTU change follow real
 device from smaller to bigger

On Wed, 23 Feb 2022 17:58:36 +0100 Guillaume Nault wrote:
> On Wed, Feb 23, 2022 at 08:03:42AM -0800, Jakub Kicinski wrote:
> > I meant
> > 
> >   ip link set dev vlan0 mtu-policy blah
> > 
> > but also
> > 
> >   ip link set dev bond0 mtu-policy blah
> > 
> > and
> > 
> >   ip link set dev macsec0 mtu-policy blah2
> >   ip link set dev vxlan0 mtu-policy blah2
> > 
> > etc.  
> 
> Unless I'm missing something, that looks very much like what I proposed
> (these are all ARPHRD_ETHER devices). It's just a bit unclear whether
> "ip link set dev vlan0 mtu-policy blah" applies to vlan0 or to the vlans
> that might be stacked on top of it (given your other examples, I assume
> it's the later).

No, sorry I thought it would be clear, we need that neuralink ;)
It applies to the device on which it's configured. What I mean
is that bond, macsec, mpls etc have the same "should it follow 
the MTU of the lower device" problem, it's not vlan specific.
Or am I wrong about that?

> > To be honest I'm still not clear if this is a real problem.
> > The patch does not specify what the use case is.  
> 
> It's probably not a problem as long as we keep sane behaviour by
> default. Then we can let admins opt in for something more complex or
> loosely defined.

What I meant was - does anyone actually flip the MTU of their
interfaces back and forth while the system is running. Maybe
people do.