netdev - Re: [PATCH net] vxlan: Restore initial MTU setting based on lower device

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Date:   Wed, 13 Dec 2017 15:19:48 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Matthias Schiffer <mschiffer@...verse-factory.net>,
        Junhan Yan <juyan@...hat.com>, Jiri Benc <jbenc@...hat.com>,
        Hangbin Liu <haliu@...hat.com>
Subject: Re: [PATCH net] vxlan: Restore initial MTU setting based on lower
 device

On Wed, 13 Dec 2017 23:37:00 +0100
Stefano Brivio <sbrivio@...hat.com> wrote:

> 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.
> 
> Restore the previous behaviour by calculating, for a new link,
> the MTU from the lower device, if present, and if no value is
> explicitly configured.
> 
> Reported-by: Junhan Yan <juyan@...hat.com>
> Fixes: a985343ba906 ("vxlan: refactor verification and application of configuration")
> Signed-off-by: Stefano Brivio <sbrivio@...hat.com>
> ---

Good catch.
Acked-by: Stephen Hemminger <stephen@...workplumber.org>