Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 23 Feb 2022 08:03:42 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Guillaume Nault <gnault@redhat.com>,
        Kees Cook <keescook@chromium.org>
Cc:     Eric Dumazet <edumazet@google.com>,
        "Ziyang Xuan (William)" <william.xuanziyang@huawei.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        David Miller <davem@davemloft.net>,
        netdev <netdev@vger.kernel.org>,
        Vasily Averin <vvs@virtuozzo.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net] net: vlan: allow vlan device MTU change follow real
 device from smaller to bigger
Message-ID: <20220223080342.5cdd597c@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
In-Reply-To: <20220223112618.GA19531@debian.home>
References: <20220221124644.1146105-1-william.xuanziyang@huawei.com>
        <CANn89iKyWWCbAdv8W26HwGpM9q5+6rrk9E-Lbd2aujFkD3GMaQ@mail.gmail.com>
        <YhQ1KrtpEr3TgCwA@gondor.apana.org.au>
        <8248d662-8ea5-7937-6e34-5f1f8e19190f@huawei.com>
        <CANn89iLf2ira4XponYV91cbvcdK76ekU7fDW93fmuJ3iytFHcw@mail.gmail.com>
        <20220222103733.GA3203@debian.home>
        <20220222152815.1056ca24@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
        <20220223112618.GA19531@debian.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.