netdev - Re: [PATCH v2] vxlan: restore dev->mtu setting based on lower device

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Date:   Thu, 14 Dec 2017 19:27:13 +0100
From:   Stefano Brivio <sbrivio@...hat.com>
To:     Alexey Kodanev <alexey.kodanev@...cle.com>
Cc:     netdev@...r.kernel.org,
        Matthias Schiffer <mschiffer@...verse-factory.net>,
        David Miller <davem@...emloft.net>,
        Junhan Yan <juyan@...hat.com>, Jiri Benc <jbenc@...hat.com>,
        Hangbin Liu <haliu@...hat.com>
Subject: Re: [PATCH v2] vxlan: restore dev->mtu setting based on lower
 device

On Thu, 14 Dec 2017 20:20:00 +0300
Alexey Kodanev <alexey.kodanev@...cle.com> wrote:

> Stefano Brivio says:
>     Commit a985343ba906 ("vxlan: refactor verification and
>     application of configuration") introduced a change in the
>     behaviour of initial MTU setting: earlier, the MTU for a link
>     created on top of a given lower device, without an initial MTU
>     specification, was set to the MTU of the lower device minus
>     headroom as a result of this path in vxlan_dev_configure():
> 
> 	if (!conf->mtu)
> 		dev->mtu = lowerdev->mtu -
> 			   (use_ipv6 ? VXLAN6_HEADROOM : VXLAN_HEADROOM);
> 
>     which is now gone. Now, the initial MTU, in absence of a
>     configured value, is simply set by ether_setup() to ETH_DATA_LEN
>     (1500 bytes).
> 
>     This breaks userspace expectations in case the MTU of
>     the lower device is higher than 1500 bytes minus headroom.
> 
> This patch restores the previous behaviour on newlink operation. Since
> max_mtu can be negative and we update dev->mtu directly, also check it
> for valid minimum.
> 
> Reported-by: Junhan Yan <juyan@...hat.com>
> Fixes: a985343ba906 ("vxlan: refactor verification and application of configuration")
> Signed-off-by: Alexey Kodanev <alexey.kodanev@...cle.com>

Thanks for re-posting this.

Acked-by: Stefano Brivio <sbrivio@...hat.com>

Or even (but I don't know what's the current netdev practice for these
particular cases):

Signed-off-by: Stefano Brivio <sbrivio@...hat.com>

-- 
Stefano