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

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Message-ID: <CAEP_g=-wh3rJ2g4Ly=+JGJGOTGa10hbfA5n=8ZS5FF+=XMsxTg@mail.gmail.com>
Date:	Tue, 2 Dec 2014 10:06:53 -0800
From:	Jesse Gross <jesse@...ira.com>
To:	"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 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.
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