Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

From: Flavio Leitner <[email-masked]>
Date: Tue, 2 Dec 2014 13:44:26 -0200

On Sun, Nov 30, 2014 at 10:08:32AM +0000, Du, Fan wrote:
> 
> 
> >-----Original Message-----
> >From: Jason Wang [mailto:[email-masked]]
> >Sent: Friday, November 28, 2014 3:02 PM
> >To: Du, Fan
> >Cc: [email-masked]; [email-masked]; [email-masked]; 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 <[email-masked]> 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 <[email-masked]>
> >> ---
> >>  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