netdev - Re: [PATCH next] ipvlan: inherit MTU from master device

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Message-ID: <CAF2d9jitKK9dBkpAUYFJtTMsw2zCP0Z+U0fEs2W7bwj2VA1rUw@mail.gmail.com>
Date:	Thu, 28 Jan 2016 09:04:05 -0800
From:	Mahesh Bandewar <maheshb@...gle.com>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	Mahesh Bandewar <mahesh@...dewar.net>,
	David Miller <davem@...emloft.net>,
	Tim Hockin <thockin@...gle.com>,
	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tim Hockins <thockins@...gle.com>
Subject: Re: [PATCH next] ipvlan: inherit MTU from master device

On Thu, Jan 28, 2016 at 5:13 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> On Wed, Jan 27, 2016 at 11:33 PM, Mahesh Bandewar <mahesh@...dewar.net> wrote:
>> From: Mahesh Bandewar <maheshb@...gle.com>
>>
>> When we create IPvlan slave; we use ether_setup() and that
>> sets up default MTU to 1500 while the master device may have
>> lower / different MTU. Any subsequent changes to the masters'
>> MTU are reflected into the slaves' MTU setting. However if those
>> don't happen (most likely scenario), the slaves' MTU stays at
>> 1500 which could be bad.
>>
>> This change adds code to inherit MTU from the master device
>> instead of using the default value during the link initialization
>> phase.
>>
>> Signed-off-by: Mahesh Bandewar <maheshb@...gle.com>
>> CC: Eric Dumazet <eric.dumazet@...il.com>
>> CC: Tim Hockins <thockins@...gle.com>
>>
>
> Acked-by: Eric Dumazet <edumazet@...gle.com>
>
> Two points Mahesh :
>
> 1) ipvlan_adjust_mtu() could be static, it is only used from
> drivers/net/ipvlan/ipvlan_main.c
>
Correct, I'm working on a some other (generic) changes and I could
include this suggestion  as part of that / later patch or do you
prefer it to be part of this change?

> 2) mtu_adj does not seem to be used ?
mtu_adj is set to zero so that it follows the master. The idea behind
this field was that if enhancements / changes to the IPvlan driver
(some sort of built-in encapsulation etc.) is added, this can be set
to that magic value so that changes to master will still allow slaves
to follow it without compromising the built-in mechanism.