linux-kernel - Re: Broken commit: [NETFILTER]: ipt_REJECT: remove largely duplicate route

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Date:	Mon, 11 Dec 2006 17:33:57 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	kaber@...sh.net, khc@...waw.pl, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, netfilter-devel@...ts.netfilter.org
Subject: Re: Broken commit: [NETFILTER]: ipt_REJECT: remove largely
 duplicate route_reverse function

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Fri, 1 Dec 2006 15:37:55 +1100

> So in general when allocating packets we have two scenarios:
> 
> 1) The dst is known and fixed, i.e., all datagram protocols.  This is
> the easy case where the headroom is known exactly beforehand.
> 
> 2) The dst is unknown or may vary, this includes TCP, SCTP and DCCP.
> This is where we currently use MAX_HEADER plus some protocol-specific
> headroom.
> 
> Right now the normal (non-IPsec) dst output path always checks for
> sufficient headroom and reallocates if necessary (ip_finish_output2).
> I propose that we make IPsec do the same thing.

Agreed.

> For standard MTU-sized packets this discussion is moot since we have
> 2K of memory in each chunk.  However, for ACKs it could save a bit of
> memory.

For linear MTU-sized SKBs yes, but TCP data packets are going out %99
of the time with paged data these days and thus suffers from the same
set of issues and potential savings.
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