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

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Date:	Thu, 04 Dec 2014 15:48:46 +0800
From:	Du Fan <fengyuleidian0615@...il.com>
To:	Jesse Gross <jesse@...ira.com>, Pravin Shelar <pshelar@...ira.com>
CC:	Thomas Graf <tgraf@...g.ch>, "Michael S. Tsirkin" <mst@...hat.com>,
	"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>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>
Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

于 2014年12月04日 06:51, Jesse Gross 写道:
> My proposal would be something like this:
>   * For L2, reduce the VM MTU to the lowest common denominator on the segment.
>   * For L3, use path MTU discovery or fragment inner packet (i.e.
> normal routing behavior).
>   * As a last resort (such as if using an old version of virtio in the
> guest), fragment the tunnel packet.

After some investigation on OpenvSwitch package, it seems before this
commit: 06021dcb "datapath: compat: Fix compilation 3.11" OpenvSwitch
package is doing GSO on its own.

rpl_ip_local_out
   -> tnl_skb_gso_segment
       ^^^^^^^^^^^^^^^
          Perform GSO  in above function
     -> ip_local_out
           .
           .
         -> ip_finish_output

Which means, when over-MTU-sized skb enter ip_local_out, it's not a gso
type skb, so the stack perform ip fragmentation, and send them out.
So, over-MTU-sized skb did travel through stack into outside.

Why not dropping such OpenvSwitch level GSO operation after 3.10?



-- 
No zuo no die but I have to try.
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