netdev - Re: [PATCH v3 net-next 2/2] net: deprecate eth_change

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Date:   Mon, 17 Oct 2016 16:22:28 -0400
From:   Jarod Wilson <jarod@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     kubakici@...pl, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH v3 net-next 2/2] net: deprecate eth_change_mtu, remove
 usage

On Mon, Oct 17, 2016 at 04:07:12PM -0400, Jarod Wilson wrote:
> On Mon, Oct 17, 2016 at 01:25:53PM -0400, David Miller wrote:
> > From: Jakub Kicinski <kubakici@...pl>
> > Date: Mon, 17 Oct 2016 18:20:49 +0100
> > 
> > > Hm.  I must be missing something really obvious.  I just booted
> > > net-next an hour ago and couldn't set MTU to anything larger than 1500
> > > on either nfp or igb.  As far as I can read the code it will set the
> > > max_mtu to 1500 in setup_ether() but none of the jumbo-capable drivers
> > > had been touched by Jarod so far...
> > 
> > Indeed.
> > 
> > Jarod, this doesn't work.
> > 
> > I guess the idea was that if the driver overrides ndo_change_mtu and
> > enforeced it's limits there, the driver would still work after your
> > changes.
> > 
> > But that isn't what is happening, look at the IGB example.
> > 
> > It uses ether_setup(), which sets those new defaults, but now when
> > the MTU is changed you enforce those default min/max before the
> > driver's ->ndo_change_mtu() has a change to step in front and make
> > the decision on it's own.
> > 
> > This means your changes pretty much did indeed break a lot of
> > drivers's ability to set larger than a 1500 byte MTU.
> 
> Argh. Yeah, I see it now. I was primarily operating with the follow-on
> patches also in play, which do touch all the ethernet drivers and set
> max_mtu to match current behavior, didn't consider the max_mtu case where
> only the initial patches were applied and the follow-on ones weren't. I've
> sent that set, which should theoretically make this problem go away, but I
> can also try to rework things if need be to restore intermediate jumbo
> frames functionality. (And there are actually non-ethernet devices that
> also call ether_setup and may or may not have larger than 1500 mtu that
> aren't yet addressed by that follow-on set).

Looks like the simplest thing to do is going to be to revert a52ad514, and
only make that change after all callers of ether_setup() are setting
min/max_mtu themselves as needed, then it can be reintroduced.

-- 
Jarod Wilson
jarod@...hat.com