Re: [B.A.T.M.A.N.] [PATCH] batman-adv: Only copy skb data for multiple broadcasts

[Sven Eckelmann <sven.eckelmann@gmx.de>]

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Sven Eckelmann wrote:
> On Sunday 01 August 2010 17:28:53 Marek Lindner wrote:
> > On Saturday 24 July 2010 00:14:29 Sven Eckelmann wrote:
> > > batman-adv tries to resend broadcasts on all interfaces up to three
> > > times. For each round and each interface it must provide a skb which
> > > gets consumed by the sending function.
> > > 
> > > It is unnecessary to copy the data of each broadcast because the actual
> > > data is either not shared or already copied by
> > > add_bcast_packet_to_list. So it is enough to just copy the skb control
> > > data
> > 
> > I think the reason to call skb_copy() is the following dev_queue_xmit()
> > call which will consume the given skb. If we consider a case of having 3
> > interfaces all 3 cloned skbs point to the same data while going out via
> > different interfaces ? I wonder whether that can work ?!

We discussed that off the list. This part is addressed in
 [PATCH 1/7] batman-adv: Keep header writable and unshared

the problem we found was that our data is maybe still be queued somewhere 
after the dev_queue_xmit. This means that the header part is still be shared 
and we may not be able to change it in our new clone without side effects to 
the other skb. To prevent that we use skb_cow_head to ensure that our new 
added header doesn't interfere with skbs using the same data buffer.

This should reduce the overhead created by the skb_copy in many situations, 
but still allow us to freely write into the new pushed header area when the 
skb would otherwise be in a shared/cloned state.

Regards,
	Sven

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