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([2601:282:803:7700:a04e:6ffa:adfd:b8f]) by smtp.googlemail.com with ESMTPSA id o12sm13929901qtl.48.2020.07.18.10.02.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 18 Jul 2020 10:02:47 -0700 (PDT) Subject: Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu discovery on encap sockets To: Stefano Brivio Cc: Florian Westphal , netdev@vger.kernel.org, aconole@redhat.com References: <20200712200705.9796-1-fw@strlen.de> <20200712200705.9796-2-fw@strlen.de> <20200713003813.01f2d5d3@elisabeth> <20200713080413.GL32005@breakpoint.cc> <20200713140219.GM32005@breakpoint.cc> <20200714143327.2d5b8581@redhat.com> <20200715124258.GP32005@breakpoint.cc> <20200715153547.77dbaf82@elisabeth> <20200715143356.GQ32005@breakpoint.cc> <20200717142743.6d05d3ae@elisabeth> <89e5ec7b-845f-ab23-5043-73e797a29a14@gmail.com> <20200718085645.7420da02@elisabeth> From: David Ahern Message-ID: <9e47f521-b3dc-f116-658b-d6897b0ddf20@gmail.com> Date: Sat, 18 Jul 2020 11:02:46 -0600 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200718085645.7420da02@elisabeth> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 7/18/20 12:56 AM, Stefano Brivio wrote: > On Fri, 17 Jul 2020 09:04:51 -0600 > David Ahern wrote: > >> On 7/17/20 6:27 AM, Stefano Brivio wrote: >>>> >>>>> Note that this doesn't work as it is because of a number of reasons >>>>> (skb doesn't have a dst, pkt_type is not PACKET_HOST), and perhaps we >>>>> shouldn't be using icmp_send(), but at a glance that looks simpler. >>>> >>>> Yes, it also requires that the bridge has IP connectivity >>>> to reach the inner ip, which might not be the case. >>> >>> If the VXLAN endpoint is a port of the bridge, that needs to be the >>> case, right? Otherwise the VXLAN endpoint can't be reached. >>> >>>>> Another slight preference I have towards this idea is that the only >>>>> known way we can break PMTU discovery right now is by using a bridge, >>>>> so fixing the problem there looks more future-proof than addressing any >>>>> kind of tunnel with this problem. I think FoU and GUE would hit the >>>>> same problem, I don't know about IP tunnels, sticking that selftest >>>>> snippet to whatever other test in pmtu.sh should tell. >>>> >>>> Every type of bridge port that needs to add additional header on egress >>>> has this problem in the bridge scenario once the peer of the IP tunnel >>>> signals a PMTU event. >>> >>> Yes :( >> >> The vxlan/tunnel device knows it is a bridge port, and it knows it is >> going to push a udp and ip{v6} header. So why not use that information >> in setting / updating the MTU? That's what I was getting at on Monday >> with my comment about lwtunnel_headroom equivalent. > > If I understand correctly, you're proposing something similar to my > earlier draft from: > > <20200713003813.01f2d5d3@elisabeth> > https://lore.kernel.org/netdev/20200713003813.01f2d5d3@elisabeth/ > > the problem with it is that it wouldn't help: the MTU is already set to > the right value for both port and bridge in the case Florian originally > reported. I am definitely hand waving; I have not had time to create a setup showing the problem. Is there a reproducer using only namespaces? > > Also, given the implications on overriding configured MTUs, and > introducing (further) IP logic into the bridge, if Florian's idea of > injecting ICMP messages could be implemented in a generic function: > > On Wed, 15 Jul 2020 16:33:56 +0200 > Florian Westphal wrote: > >> Yes, it might be possible to move the proposed icmp inject into >> skb_tunnel_check_pmtu() -- it gets the needed headroom passed as arg, >> it could detect when device driver is in a bridge and it already knows >> when skb has no dst entry that it a pmtu change could be propagated to. > > I think that would be preferable: then it's fixed for all tunnels in a > generic, probably simpler way, without those two issues. > > But then again, we're talking about Linux bridge. Unfortunately this > doesn't fix the problem with Open vSwitch either. > Of course. (insert sarcastic jab at ovs here)