linux-kernel - Re: [PATCH] net:bridge:always disable auto-tuning when the user specified MTU

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Message-ID: <20190409193911.2d68a64d@shemminger-XPS-13-9360>
Date:   Tue, 9 Apr 2019 19:39:11 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Huang Rui <huangruippp@...il.com>
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        jakub.kicinski@...ronome.com, hawk@...nel.org,
        john.fastabend@...il.com, kafai@...com, songliubraving@...com,
        yhs@...com, jiri@...lanox.com, ecree@...arflare.com,
        idosch@...lanox.com, petrm@...lanox.com,
        alexander.h.duyck@...el.com, amritha.nambiar@...el.com,
        lirongqing@...du.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, xdp-newbies@...r.kernel.org,
        bpf@...r.kernel.org, roopa@...ulusnetworks.com,
        nikolay@...ulusnetworks.com, bridge@...ts.linux-foundation.org
Subject: Re: [PATCH] net:bridge:always disable auto-tuning when the user
 specified MTU

On Wed, 10 Apr 2019 02:32:08 +0000
Huang Rui <huangruippp@...il.com> wrote:

> For example.
> My purpose is to create a bridge br0 and join eth0 into br0.
> if we use this following way, the auto-tuning flag will not be disabled.
> 
> If eth0's mtu is 1200
> step 1.brctl addbr br0
> step 2.brctl addif br0 eth0
> step 3.ifconfig br0 mtu 1200
> step 4.ifconfig eth0 mtu 1500
> 
> Result:
> br0's MTU: 1500, eth0's MTU: 1500
> 
> Expected:
> br0's MTU: 1200, eth0's MTU: 1500
> 
> I have specified br0's MTU,  but auto-min policy works. So the MTU is
> not the result what we expected.
> As expected, if i have specified bridge's MTU, it will set the flag:
> BROPT_MTU_SET_BY_USER in net_bridge_opts disabled and auto-min/max
> policy will not work.But in this case, because the dev_set_mtu return
> early, the BROPT_MTU_SET_BY_USER flag will not be disabled and
> auto-min/max policy will still work.
> 
> Signed-off-by: Huang Rui <huangruiPPP@...il.com>

A bridge like this going to drop frames.
A frame received with MTU of 1200 will get dropped.
The proper way to do this is to change MTU of both interfaces to match.