netdev - Re: [PATCH] bridge: MTU auto tuning ignores IFLA

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Message-ID: <3dccacd8-4249-87f8-690c-6083374dc9d1@blackwall.org>
Date: Wed, 27 Sep 2023 11:10:50 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Trent Lloyd <trent.lloyd@...onical.com>, Roopa Prabhu <roopa@...dia.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Shuah Khan <shuah@...nel.org>
Cc: linux-kernel@...r.kernel.org, bridge@...ts.linux-foundation.org,
 netdev@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] bridge: MTU auto tuning ignores IFLA_MTU on NEWLINK

On 9/27/23 10:57, Trent Lloyd wrote:
> Commit 804b854d374e ("net: bridge: disable bridge MTU auto tuning if it
> was set manually") disabled auto-tuning of the bridge MTU when the MTU
> was explicitly set by the user, however that would only happen when the
> MTU was set after creation. This commit ensures auto-tuning is also
> disabled when the MTU is set during bridge creation.
> 
> Currently when the br_netdev_ops br_change_mtu function is called, the
> flag BROPT_MTU_SET_BY_USER is set. However this function is only called
> when the MTU is changed after interface creation and is not called if
> the MTU is specified during creation with IFLA_MTU (br_dev_newlink).
> 
> br_change_mtu also does not get called if the MTU is set to the same
> value it currently has, which makes it difficult to work around this
> issue (especially for the default MTU of 1500) as you have to first
> change the MTU to some other value and then back to the desired value.
> 

Yep, I think I also described this in the commit message of my patch.

> Add new selftests to ensure the bridge MTU is handled correctly:
>   - Bridge created with user-specified MTU (1500)
>   - Bridge created with user-specified MTU (2000)
>   - Bridge created without user-specified MTU
>   - Bridge created with user-specified MTU set after creation (2000)
> 
> Regression risk: Any workload which erroneously specified an MTU during
> creation but accidentally relied upon auto-tuning to a different value
> may be broken by this change.
> 

Hmm, you're right. There's a risk of regression. Also it acts 
differently when set to 1500 as you've mentioned. I think they should 
act the same, also bridge's fake rtable RTAX_MTU is not set.

> Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2034099
> Fixes: 804b854d374e ("net: bridge: disable bridge MTU auto tuning if it was set manually")
> Signed-off-by: Trent Lloyd <trent.lloyd@...onical.com>
> ---