On 7/18/20 11:58 AM, Stefano Brivio wrote: > On Sat, 18 Jul 2020 11:02:46 -0600 > David Ahern <[email protected]> wrote: > >> On 7/18/20 12:56 AM, Stefano Brivio wrote: >>> On Fri, 17 Jul 2020 09:04:51 -0600 >>> David Ahern <[email protected]> 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? > > And I'm laser pointing: check the bottom of that email ;) >
With this test case, the lookup fails: [ 144.689378] vxlan: vxlan_xmit_one: dev vxlan_a 10.0.1.1/57864 -> 10.0.0.0/4789 len 5010 gw 10.0.1.2 [ 144.692755] vxlan: skb_tunnel_check_pmtu: dst dev br0 skb dev vxlan_a skb len 5010 encap_mtu 4000 headroom 50 [ 144.697682] vxlan: skb_dst_update_pmtu_no_confirm: calling ip_rt_update_pmtu+0x0/0x160/ffffffff825ee850 for dev br0 mtu 3950 [ 144.703601] IPv4: __ip_rt_update_pmtu: dev br0 mtu 3950 old_mtu 5000 192.168.2.1 -> 192.168.2.2 [ 144.708177] IPv4: __ip_rt_update_pmtu: fib_lookup failed for 192.168.2.1 -> 192.168.2.2 Because the lookup fails, __ip_rt_update_pmtu skips creating the exception. This hack gets the lookup to succeed: fl4->flowi4_oif = dst->dev->ifindex; or fl4->flowi4_oif = 0; and the test passes.
