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Date:   Wed, 10 Apr 2019 10:34:17 +0000
From:   Petr Machata <petrm@...lanox.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
CC:     Huang Rui <huangruippp@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        "hawk@...nel.org" <hawk@...nel.org>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kafai@...com" <kafai@...com>,
        "songliubraving@...com" <songliubraving@...com>,
        "yhs@...com" <yhs@...com>, Jiri Pirko <jiri@...lanox.com>,
        "ecree@...arflare.com" <ecree@...arflare.com>,
        Ido Schimmel <idosch@...lanox.com>,
        "alexander.h.duyck@...el.com" <alexander.h.duyck@...el.com>,
        "amritha.nambiar@...el.com" <amritha.nambiar@...el.com>,
        Li Rongqing <lirongqing@...du.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "xdp-newbies@...r.kernel.org" <xdp-newbies@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "roopa@...ulusnetworks.com" <roopa@...ulusnetworks.com>,
        "nikolay@...ulusnetworks.com" <nikolay@...ulusnetworks.com>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>
Subject: Re: [PATCH] net:bridge:always disable auto-tuning when the user
 specified MTU


Stephen Hemminger <stephen@...workplumber.org> writes:

> 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.

That's true even if above you set br0's MTU to 1201, but then the
auto-tuning is disabled as expected. The problem is that setting MTU to
1200 is perceived as a non-change, whereas it should instead be
perceived as a signal that the user takes over the MTU management.

> The proper way to do this is to change MTU of both interfaces to match.

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