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[204.195.22.127]) by smtp.gmail.com with ESMTPSA id c1sm40310406pgk.44.2019.04.09.19.39.14 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Tue, 09 Apr 2019 19:39:15 -0700 (PDT) Date: Tue, 9 Apr 2019 19:39:11 -0700 From: Stephen Hemminger To: Huang Rui Cc: davem@davemloft.net, ast@kernel.org, daniel@iogearbox.net, jakub.kicinski@netronome.com, hawk@kernel.org, john.fastabend@gmail.com, kafai@fb.com, songliubraving@fb.com, yhs@fb.com, jiri@mellanox.com, ecree@solarflare.com, idosch@mellanox.com, petrm@mellanox.com, alexander.h.duyck@intel.com, amritha.nambiar@intel.com, lirongqing@baidu.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, xdp-newbies@vger.kernel.org, bpf@vger.kernel.org, roopa@cumulusnetworks.com, nikolay@cumulusnetworks.com, bridge@lists.linux-foundation.org Subject: Re: [PATCH] net:bridge:always disable auto-tuning when the user specified MTU Message-ID: <20190409193911.2d68a64d@shemminger-XPS-13-9360> In-Reply-To: <20190410023208.25435-1-huangruiPPP@gmail.com> References: <20190410023208.25435-1-huangruiPPP@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 10 Apr 2019 02:32:08 +0000 Huang Rui 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 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.