Re: Headroom for bridging and wireless

> > > Apart from that, I've just changed the hard_header_len for my bridge box
> > > and the headroom did increase as it should, for packets originating from
> > > the bridge and going through br0.
> >
> > Nice, but it would be most useful to have the bridged packets use the
> > increased headroom.  I don't suppose there is a way to do that easily.  The
> > only thing I can think of right now is to have the skb functions always
> > reserve the maximum headroom for all devices in the system.  Seems a small
> > price to pay for a huge decrease in reallocs.
> > Note this is not just a bridge problem, it should also apply to routing.
> 
> I see include/linux/skbuff.h::__dev_alloc_skb() already allocates 16 more 
> bytes than needed. When looking at my own bridge box's head, data and mac.raw 
> skbuff pointers for bridged frames, they seem to always have 16 (well, 18) 
> bytes of spare space to play with. So, I think the only skbuffs that need a 
> realloc are those not allocated with dev_alloc_skb().
> Looks like in my case only the packets originating from the bridge box itself 
> don't have this extra 16 bytes head room. But that should be fixed with your 
> bridge patch.
> I'm not too familiar with these low level details, however.

Thanks for looking at the skb code.  I was not aware of the 16 bytes
reserved in dev_alloc_skb().  I will try to figure out where that extra
space goes when I get a chance.

-- 
It's easy to get a loan unless you need it.
        -- Norman Augustine

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