Re: Broken commit: [NETFILTER]: ipt_REJECT: remove largely duplicate route_reverse function

[David Miller <davem@davemloft.net>]

From: Herbert Xu <herbert@gondor.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.