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

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Date:	Wed, 03 Dec 2014 14:39:37 +0800
From:	Du Fan <fengyuleidian0615@...il.com>
To:	Florian Westphal <fw@...len.de>
CC:	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: [Discussion] About over-MTU-sized skb in virtualized env

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.
Especially in below scenario:
http://docs.openstack.org/admin-guide-cloud/content/figures/14/a/a/common/figures/under-the-hood-scenario-1-ovs-compute.png

As many people have discuss it in this thread:
http://marc.info/?t=141715669700004&r=1&w=2
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