netdev - Re: [Discussion] About over-MTU-sized skb in virtualized env

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Message-ID: <20141203105046.GE16959@breakpoint.cc>
Date:	Wed, 3 Dec 2014 11:50:46 +0100
From:	Florian Westphal <fw@...len.de>
To:	Du Fan <fengyuleidian0615@...il.com>
Cc:	Florian Westphal <fw@...len.de>, Thomas Graf <tgraf@...g.ch>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Jesse Gross <jesse@...ira.com>,
	Flavio Leitner <fbl@...hat.com>,
	"davem@...emloft.net" <davem@...emloft.net>, pshelar@...ira.com,
	netdev <netdev@...r.kernel.org>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	"Du, Fan" <fan.du@...el.com>
Subject: Re: [Discussion] About over-MTU-sized skb in virtualized env

Du Fan <fengyuleidian0615@...il.com> wrote:
> Sorry for resend this mail, because my company email is rejected by netdev.
> 
> 
> Hi Florian
> 
>  214 static int ip_finish_output_gso(struct sk_buff *skb)
>  215 {
>  216     netdev_features_t features;
>  217     struct sk_buff *segs;
>  218     int ret = 0;
>  219
>  220     /* common case: locally created skb or seglen is <= mtu */
>  221     if (((IPCB(skb)->flags & IPSKB_FORWARDED) == 0) ||
>  222           skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb))
>  223         return ip_finish_output2(skb);
> 
> Could you please state _concrete_ reason why locally created skb
> length is ＿always＿ fitting into MTU size? or why we needs this
> checking.

We don't "need" this checking.  Its just to avoid skb_gso_network_seglen()
computation for the common (local-out) case.

Locally generated GSO packet is not supposed to exceed dst_mtu, as that
is the PMTU discovery start point in absence of lower/learned value.
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