netdev - Re: [PATCH] RFC: net: ipconfig: temporarily bring interface down when changing MTU.

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Date: Sat, 16 Dec 2023 11:19:46 -0700
From: David Ahern <dsahern@...nel.org>
To: Graeme Smecher <gsmecher@...eespeedlogic.com>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 claudiu.beznea@...on.dev, nicolas.ferre@...rochip.com, mdf@...nel.org
Subject: Re: [PATCH] RFC: net: ipconfig: temporarily bring interface down when
 changing MTU.

On 12/15/23 5:04 PM, Graeme Smecher wrote:
\> diff --git a/net/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
> index c56b6fe6f0d7..69c2a41393a0 100644
> --- a/net/ipv4/ipconfig.c
> +++ b/net/ipv4/ipconfig.c
> @@ -396,9 +396,21 @@ static int __init ic_setup_if(void)
>  	 */
>  	if (ic_dev_mtu != 0) {
>  		rtnl_lock();
> -		if ((err = dev_set_mtu(ic_dev->dev, ic_dev_mtu)) < 0)
> -			pr_err("IP-Config: Unable to set interface mtu to %d (%d)\n",
> -			       ic_dev_mtu, err);
> +		/* Some Ethernet adapters only allow MTU to change when down. */
> +		if((err = dev_change_flags(ic_dev->dev, ic_dev->dev->flags | IFF_UP, NULL)))

IFF_UP?

> +			pr_err("IP-Config: About to set MTU, but failed to "
> +				 "bring interface %s down! (%d)\n",
> +				 ic_dev->dev->name, err);
> +		else {
> +			if ((err = dev_set_mtu(ic_dev->dev, ic_dev_mtu)) < 0)

try to set the MTU even if DOWN fails otherwise a regression.

> +				pr_err("IP-Config: Unable to set interface mtu to %d (%d)\n",
> +				       ic_dev_mtu, err);
> +
> +			if((err = dev_change_flags(ic_dev->dev, ic_dev->dev->flags | IFF_UP, NULL)))
> +				pr_err("IP-Config: Trying to set MTU, but unable "
> +					 "to bring interface %s back up! (%d)\n",

This step is not "trying to set MTU"; the last one did:

pr_err("IP-Config: Failed to bring interface %s up after changing MTU
(%d)\n",

> +					 ic_dev->dev->name, err);
> +		}
>  		rtnl_unlock();
>  	}
>  	return 0;