From mboxrd@z Thu Jan 1 00:00:00 1970 From: Flavio Leitner Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU Date: Tue, 2 Dec 2014 13:44:26 -0200 Message-ID: <20141202154425.GA5344@t520.home> References: <1417156385-18276-1-git-send-email-fan.du@intel.com> <1417158128.3268.2@smtp.corp.redhat.com> <5A90DA2E42F8AE43BC4A093BF0678848DED92B@SHSMSX104.ccr.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "'Jason Wang'" , "netdev@vger.kernel.org" , "davem@davemloft.net" , "fw@strlen.de" To: "Du, Fan" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43059 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753603AbaLBPok (ORCPT ); Tue, 2 Dec 2014 10:44:40 -0500 Content-Disposition: inline In-Reply-To: <5A90DA2E42F8AE43BC4A093BF0678848DED92B@SHSMSX104.ccr.corp.intel.com> Sender: netdev-owner@vger.kernel.org List-ID: On Sun, Nov 30, 2014 at 10:08:32AM +0000, Du, Fan wrote: > > > >-----Original Message----- > >From: Jason Wang [mailto:jasowang@redhat.com] > >Sent: Friday, November 28, 2014 3:02 PM > >To: Du, Fan > >Cc: netdev@vger.kernel.org; davem@davemloft.net; fw@strlen.de; Du, Fan > >Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU > > > > > > > >On Fri, Nov 28, 2014 at 2:33 PM, Fan Du wrote: > >> Test scenario: two KVM guests sitting in different hosts communicate > >> to each other with a vxlan tunnel. > >> > >> All interface MTU is default 1500 Bytes, from guest point of view, its > >> skb gso_size could be as bigger as 1448Bytes, however after guest skb > >> goes through vxlan encapuslation, individual segments length of a gso > >> packet could exceed physical NIC MTU 1500, which will be lost at > >> recevier side. > >> > >> So it's possible in virtualized environment, locally created skb len > >> after encapslation could be bigger than underlayer MTU. In such case, > >> it's reasonable to do GSO first, then fragment any packet bigger than > >> MTU as possible. > >> > >> +---------------+ TX RX +---------------+ > >> | KVM Guest | -> ... -> | KVM Guest | > >> +-+-----------+-+ +-+-----------+-+ > >> |Qemu/VirtIO| |Qemu/VirtIO| > >> +-----------+ +-----------+ > >> | | > >> v tap0 tap0 v > >> +-----------+ +-----------+ > >> | ovs bridge| | ovs bridge| > >> +-----------+ +-----------+ > >> | vxlan vxlan | > >> v v > >> +-----------+ +-----------+ > >> | NIC | <------> | NIC | > >> +-----------+ +-----------+ > >> > >> Steps to reproduce: > >> 1. Using kernel builtin openvswitch module to setup ovs bridge. > >> 2. Runing iperf without -M, communication will stuck. > > > >Is this issue specific to ovs or ipv4? Path MTU discovery should help in this case I > >believe. > > Problem here is host stack push local over-sized gso skb down to NIC, and perform GSO there > without any further ip segmentation. > > Reasonable behavior is do gso first at ip level, if gso-ed skb is bigger than MTU && df is set, > Then push ICMP_DEST_UNREACH/ICMP_FRAG_NEEDED message back to sender to adjust mtu. > > For PMTU to work, that's another issue I will try to address later on. > > >> > >> > >> Signed-off-by: Fan Du > >> --- > >> net/ipv4/ip_output.c | 7 ++++--- > >> 1 files changed, 4 insertions(+), 3 deletions(-) > >> > >> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c index > >> bc6471d..558b5f8 100644 > >> --- a/net/ipv4/ip_output.c > >> +++ b/net/ipv4/ip_output.c > >> @@ -217,9 +217,10 @@ static int ip_finish_output_gso(struct sk_buff > >> *skb) > >> struct sk_buff *segs; > >> int ret = 0; > >> > >> - /* common case: locally created skb or seglen is <= mtu */ > >> - if (((IPCB(skb)->flags & IPSKB_FORWARDED) == 0) || > >> - skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb)) > >> + /* Both locally created skb and forwarded skb could exceed > >> + * MTU size, so make a unified rule for them all. > >> + */ > >> + if (skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb)) > >> return ip_finish_output2(skb); Are you using kernel's vxlan device or openvswitch's vxlan device? Because for kernel's vxlan devices the MTU accounts for the header overhead so I believe your patch would work. However, the MTU is not visible for the ovs's vxlan devices, so that wouldn't work. fbl