netdev - Re: Question about an assignment in handle

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Message-ID: <20100525102603.GA11494@gondor.apana.org.au>
Date:	Tue, 25 May 2010 20:26:03 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	jamal <hadi@...erus.ca>
Cc:	Jiri Pirko <jpirko@...hat.com>, netdev@...r.kernel.org,
	davem@...emloft.net, kaber@...sh.net
Subject: Re: Question about an assignment in handle_ing()

On Tue, May 25, 2010 at 05:51:07AM -0400, jamal wrote:
> The code is correct. 
> 
> Main reason for the else condn is driven by optimization:
> If you are running tcpdump or other af packet type code, the "if" condn
> is hit and matching actions are not allowed trample on that same packet
> data. They have to make a private copy; otherwise, the "else" is hit and
> (for optimization reason) you give ok to the actions that follow to
> munge the packet. Essentially, you dont want actions to alloc/copy every
> single time when you are not running tcpdump for example; reason is that
> most of the time you run tcpdump it is for debugging.
> [I had seen very observable differences on some old mips board back in
> the day on whether you avoided copy every time vs when debugging by
> running tcpdump and copied every packet.]

In that case you should be checking whether the skb is cloned.
After all, tcpdump might have simply filtered the packet out.

BTW, this is the case whenever you run a DHCP client/server.  So
on most boxes your optimisation will never kick in as is.  Also
the skb may still be cloned even if there is no AF_PACKET listener.
In that case your optimisation may be incorrect.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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