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Date: Tue, 2 Dec 2014 19:32:32 -0200
From: Flavio Leitner <fbl@...hat.com>
To: Jesse Gross <jesse@...ira.com>
Cc: "Du, Fan" <fan.du@...el.com>, Jason Wang <jasowang@...hat.com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"davem@...emloft.net" <davem@...emloft.net>,
"fw@...len.de" <fw@...len.de>
Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU
On Tue, Dec 02, 2014 at 10:06:53AM -0800, Jesse Gross wrote:
> On Tue, Dec 2, 2014 at 7:44 AM, Flavio Leitner <fbl@...hat.com> wrote:
> > On Sun, Nov 30, 2014 at 10:08:32AM +0000, Du, Fan wrote:
> >>
> >>
> >> >-----Original Message-----
> >> >From: Jason Wang [mailto:jasowang@...hat.com]
> >> >Sent: Friday, November 28, 2014 3:02 PM
> >> >To: Du, Fan
> >> >Cc: netdev@...r.kernel.org; davem@...emloft.net; fw@...len.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 <fan.du@...el.com> 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 <fan.du@...el.com>
> >> >> ---
> >> >> 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.
>
> This is being called after the tunnel code, so the MTU that is being
> looked at in all cases is the physical device's. Since the packet has
> already been encapsulated, tunnel header overhead is already accounted
> for in skb_gso_network_seglen() and this should be fine for both OVS
> and non-OVS cases.
Right, it didn't work on my first try and that explanation came to mind.
Anyway, I am testing this with containers instead of VMs, so I am using
veth and not Virtio-net.
This is the actual stack trace:
[...]
do_output
ovs_vport_send
vxlan_tnl_send
vxlan_xmit_skb
udp_tunnel_xmit_skb
iptunnel_xmit
\ skb_scrub_packet => skb->ignore_df = 0;
ip_local_out_sk
ip_output
ip_finish_output (_gso is inlined)
ip_fragment
and on ip_fragment() it does:
503 if (unlikely(((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) ||
504 (IPCB(skb)->frag_max_size &&
505 IPCB(skb)->frag_max_size > mtu))) {
506 IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
507 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
508 htonl(mtu));
509 kfree_skb(skb);
510 return -EMSGSIZE;
511 }
Since IP_DF is set and skb->ignore_df is reset to 0, in my case
the packet is dropped and an ICMP is sent back. The connection
remains stuck as before. Doesn't virtio-net set DF bit?
Thanks,
fbl
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