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Subject: Re: [PATCH net] net: vlan: allow vlan device MTU change follow real
 device from smaller to bigger
To:     Herbert Xu <herbert@gondor.apana.org.au>,
        Eric Dumazet <edumazet@google.com>
CC:     David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Vasily Averin <vvs@virtuozzo.com>,
        Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20220221124644.1146105-1-william.xuanziyang@huawei.com>
 <CANn89iKyWWCbAdv8W26HwGpM9q5+6rrk9E-Lbd2aujFkD3GMaQ@mail.gmail.com>
 <YhQ1KrtpEr3TgCwA@gondor.apana.org.au>
From:   "Ziyang Xuan (William)" <william.xuanziyang@huawei.com>
Message-ID: <8248d662-8ea5-7937-6e34-5f1f8e19190f@huawei.com>
Date:   Tue, 22 Feb 2022 10:06:03 +0800
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> On Mon, Feb 21, 2022 at 07:43:18AM -0800, Eric Dumazet wrote:
>>
>> Herbert, do you recall why only a decrease was taken into consideration ?
> 
> Because we shouldn't override administrative settings of the MTU
> on the vlan device, unless we have to because of an MTU reduction
> on the underlying device.
> 
> Yes this is not perfect if the admin never set an MTU to start with
> but as we don't have a way of telling whether the admin has or has
> not changed the MTU setting, the safest course of action is to do
> nothing in that case.
If the admin has changed the vlan device MTU smaller than the underlying
device MTU firstly, then changed the underlying device MTU smaller than
the vlan device MTU secondly. The admin's configuration has been overridden.
Can we consider that the admin's configuration for the vlan device MTU has
been invalid and disappeared after the second change? I think so.

> 
> Thanks,
>