netdev - Re: [PATCH net-next] net: macb: allow MTU change when the interface is up

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Message-ID: <20240325185647.0ad674e9@kernel.org>
Date: Mon, 25 Mar 2024 18:56:47 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Simon Horman <horms@...nel.org>, thomas.perrot@...tlin.com
Cc: Nicolas Ferre <nicolas.ferre@...rochip.com>, Claudiu Beznea
 <claudiu.beznea@...on.dev>, "David S . Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: macb: allow MTU change when the interface
 is up

On Mon, 25 Mar 2024 20:54:01 +0000 Simon Horman wrote:
> I'm not sure that it is expected behaviour for an interface
> to reset like this when a change of MTU is requested.
> While conversely I think it is common (if not entirely desirable)
> to prohibit changing the MTU when an interface is up.
> What is the problem being addressed here?

Right..

> >  	dev->mtu = new_mtu;
> >  
> > +	if (reset)
> > +		macb_open(dev);

.. imagine admin does this over SSH on a remote box and system 
is under memory pressure. Even ignoring the fact you're not checking
the return value, the result of changing MTU should be either having 
the requested MTU (success) or having the old MTU (failure).
Not "machine drops off the network" :(