netdev - Re: [PATCH net-next 2/2] net: Loosen constraints for recalculating checksum in skb

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Date:	Tue, 23 Apr 2013 16:43:58 -0700
From:	Jesse Gross <jesse@...ira.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	netdev <netdev@...r.kernel.org>,
	Jarno Rajahalme <jarno.rajahalme@....com>,
	Joseph Gasparakis <joseph.gasparakis@...el.com>,
	Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@...el.com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH net-next 2/2] net: Loosen constraints for recalculating
 checksum in skb_segment()

On Mon, Apr 22, 2013 at 7:19 PM, Simon Horman <horms@...ge.net.au> wrote:
> In the case where a non-MPLS GSO skb becomes an MPLS GSO skb, via
> Open vSwitch's push MPLS action it is desirable to provide segmentation
> in software. In this case the original protocol of the skb may have allowed
> its checksumming to be offloaded but this may no longer be supported now
> the skb is MPLS. Actually it seems to me that this is the likely case.
>
> In order to allow the checksum to be updated in this case loosen
> the rules for recalculating the checksum on in skb_segment().
>
> N.B.: I must confess that I am a little unsure of the details of
> the implementation of skb_checksum(). But I have observed that this
> is necessary as skb_checksum() hits the following:
>
>                 if (!hsize && i >= nfrags) {
>                         ...
>                         fskb = fskb->next;
>                         ...
>                 }
>                 ...
>                 if (fskb != skb_shinfo(skb)->frag_list)
>                         ...
>
> Signed-off-by: Simon Horman <horms@...ge.net.au>

I'm surprised at the need for changes here since neither vlans nor
tunneling protocols needed something similar. Do you know what is
special about MPLS that is triggering this?
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