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

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Date:   Tue, 22 Feb 2022 15:28:15 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     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>,
        Kees Cook <keescook@...omium.org>,
        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 Tue, 22 Feb 2022 11:37:33 +0100 Guillaume Nault wrote:
> What about an explicit option:
> 
>   ip link add link eth1 dev eth1.100 type vlan id 100 follow-parent-mtu
> 
> 
> Or for something more future proof, an option that can accept several
> policies:
> 
>   mtu-update <reduce-only,follow,...>
> 
>       reduce-only (default):
>         update vlan's MTU only if the new MTU is smaller than the
>         current one (current behaviour).
> 
>       follow:
>         always follow the MTU of the parent device.
> 
> Then if anyone wants more complex policies:
> 
>       follow-if-not-modified:
>         follow the MTU of the parent device as long as the VLAN's MTU
>         was not manually changed. Otherwise only adjust the VLAN's MTU
>         when the parent's one is set to a smaller value.
> 
>       follow-if-not-modified-but-not-quite:
>         like follow-if-not-modified but revert back to the VLAN's
>         last manually modified MTU, if any, whenever possible (that is,
>         when the parent device's MTU is set back to a higher value).
>         That probably requires the possibility to dump the last
>         modified MTU, so the administrator can anticipate the
>         consequences of modifying the parent device.
> 
>      yet-another-policy (because people have a lot of imagination):
>        for example, keep the MTU 4 bytes lower than the parent device,
>        to account for VLAN overhead.
> 
> Of course feel free to suggest better names and policies :).
> 
> This way, we can keep the current behaviour and avoid unexpected
> heuristics that are difficult to explain (and even more difficult for
> network admins to figure out on their own).

My $0.02 would be that if we want to make changes that require new uAPI
we should do it across uppers.