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

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Message-ID: <20220223080342.5cdd597c@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 23 Feb 2022 08:03:42 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>,
        Kees Cook <keescook@...omium.org>
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>,
        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 12:26:18 +0100 Guillaume Nault wrote:
> Do you mean something like:
> 
>   ip link set dev eth0 vlan-mtu-policy <policy-name>
> 
> that'd affect all existing (and future) vlans of eth0?

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.

> Then I think that for non-ethernet devices, we should reject this
> option and skip it when dumping config. But yes, that's another
> possibility.
> 
> I personnaly don't really mind, as long as we keep a clear behaviour.
> 
> What I'd really like to avoid is something like:
>   - By default it behaves this way.
>   - If you modified the MTU it behaves in another way
>   - But if you modified the MTU but later restored the
>     original MTU, then you're back to the default behaviour
>     (or not?), unless the MTU of the upper device was also
>     changed meanwhile, in which case ... to be continued ...
>   - BTW, you might not be able to tell how the VLAN's MTU is going to
>     behave by simply looking at its configuration, because that also
>     depends on past configurations.
>   - Well, and if your kernel is older than xxx, then you always get the
>     default behaviour.
>   - ... and we might modify the heuristics again in the future to
>     accomodate with situations or use cases we failed to consider.

To be honest I'm still not clear if this is a real problem.
The patch does not specify what the use case is.