netdev - Re: [PATCH] net: ip_finish_output_gso: If skb_gso_network_seglen exceeds MTU, allow segmentation for gre tunneled skbs

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Message-ID: <a2d2b7bb-0366-b044-6895-6b5a7790fdf6@ucloud.cn>
Date:	Tue, 16 Aug 2016 15:12:19 +0800
From:	wenxu <wenxu@...oud.cn>
To:	Shmulik Ladkani <shmulik.ladkani@...il.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	David Miller <davem@...emloft.net>, kuznet@....inr.ac.ru,
	jmorris@...ei.org, kaber@...sh.net, yoshfuji@...ux-ipv6.org,
	netdev@...r.kernel.org, wenx05124561@....com
Subject: Re: [PATCH] net: ip_finish_output_gso: If skb_gso_network_seglen
 exceeds MTU, allow segmentation for gre tunneled skbs

Hi,

> Hi,
>
> On Fri, 12 Aug 2016 13:11:50 +0200, hannes@...essinduktion.org wrote:
>> I really would not like to see this expanded to gre and other protocols.
>> All switches drop packets where the packets are exceeding the MTU,
>> bridges and also openvswitch should behave the same.
>>
>> Unfortunately we already had this loophole in the kernel that vxlan udp
>> output path could fragment the packet again, even in case of switches.
>> But this stopped working for GSO packets, which violates another rule in
>> the kernel, GSO should always be transparent and user space should never
>> have to care if a packet is GSO or not.
>>
>> Because we couldn't a) roll back the change that we fragment packets in
>> UDP output paths and b) should not violate GSO transparency rule, I
>> strongly believed it would be better too only change the kernel in a way
>> that it transparently works with GSO, too. If we argue that a VTEP is
>> its own UDP endpoint which is set up after the bridge, I still can sleep
>> well. :)
>>
>> My understanding was that GRE failed consistently, GSO as well as
>> non-GSO packets are dropped, which would be the correct behavior for me.
>> I don't want to change this. A good argument against this would be if we
>> violate the GSO transparency rule again. But when I looked into the code
>> I couldn't see that.
> I completely agree with your arguments.
>
> I think we may run into the same GSO vs Non-GSO anomaly if one uses
> a "nopmtudisc" tunnel, or a gre tunnel in "collect_md" mode, where the
> encapsulating iphdr 'df' is derived from 'tun_flags&TUNNEL_DONT_FRAGMENT'
> (e.g. in case DF is not set).
>
> I suspect OvS's vport-gre does exactly that, so I assume this is the
> reason why the change was suggested.
>
> Maybe we can change our criteria in the following manner:
>  
> -	if (skb_iif && proto == IPPROTO_UDP) {
> +	if (skb_iif && !(df & htons(IP_DF))) {
> 		IPCB(skb)->flags |= IPSKB_FRAG_SEGS;
>
> This way, any tunnel explicitly providing DF is NOT allowed to
> further fragment the resulting segments (leading to tx segments being
> dropped).
> Others tunnels, that do not care (e.g. vxlan and geneve, and probably
> ovs vport-gre, or other ovs encap vports, in df_default=false mode),
> will behave same for gso and non-gso.
>
> WDYT? Am I missing something here?
>
> Thanks,
> Shmulik

I think the criteria 'skb_iif && !(df & htons(IP_DF)' is suitable.   'nopmtudisc' tunnel and ovs-gre tunnel can clear the DF through df_default=false.
In this situation both gso(final segment) or no-gso packet can be fragment(if the packet size is more than mtu).

Thanks
Wenxu