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Message-ID: <10569cc5-1624-4edf-88b9-ec9110c11890@kernel.org>
Date: Sat, 16 Dec 2023 11:19:46 -0700
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Subject: Re: [PATCH] RFC: net: ipconfig: temporarily bring interface down when
 changing MTU.
Content-Language: en-US
To: Graeme Smecher <gsmecher@threespeedlogic.com>
Cc: "David S. Miller" <davem@davemloft.net>,
 Eric Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>,
 Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org,
 claudiu.beznea@tuxon.dev, nicolas.ferre@microchip.com, mdf@kernel.org
References: <58519bfa-260c-4745-a145-fdca89b4e9d1@kernel.org>
 <20231216010431.84776-1-gsmecher@threespeedlogic.com>
From: David Ahern <dsahern@kernel.org>
In-Reply-To: <20231216010431.84776-1-gsmecher@threespeedlogic.com>
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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;