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: Wed, 29 Nov 2006 17:51:46 +1100

> I'm just emphasising that LL_MAX_HEADER is by no means the *maximum*
> header size in a Linux system.

But it is the maximum "link level" singular header size.

It is MAX_HEADER which is the hack and the main issue.

What MAX_HEADER's setting is trying to do is optimistically allocate
enough for a single level of tunnelling.  It does not handle nested
tunneling at all, of course.

> As to getting rid of those ifdefs, here is one idea.  We keep a
> read-mostly global variable that represents the actual current
> maximum LL header size.  Everytime a new device appears (or if
> its hard header size changes) we update this variable if needed.
> 
> Hmm, we don't actually update the hard header size should the
> underlying device change for tunnels.  Good thing the tunnels
> only use that as a hint and reallocate if necessary :)
> 
> This is not optimal in that it never decreases, but it's certainly
> better than a compile-time constant (e.g., people using distribution
> kernels don't necessarily use tunnels).

I like this idea for the most part.  It also deals nicely with, as you
alude to, how the MAX_HEADER scheme uses the space even if you don't
configure any tunnels at all.

Actually, I wonder how antiquated this all is.  I bet we could get rid
of MAX_HEADER, then if we have to realloc headroom, we adjust some
per-device header thing which will behave like your global value idea
does.  On the next allocation, we'll do the right thing.  Although I
cannot come up with a scheme that works without reintroducing another
net_device pointer to sk_buff, which seems necessary to handle arbitrary
nesting. :-/