[ovs-dev] [PATCH] gre: Improve headroom calculation to reducing copying.

[Ben Pfaff]

On Fri, Jun 11, 2010 at 03:40:25PM -0700, Jesse Gross wrote:
> The first change is use skb_header_cloned() instead of (skb_cloned()
>  && skb_clone_writable()).  While these are effectively the same
> checks, skb_clone_writable() does not exist until 2.6.23 and relies
> on information not available in earlier kernels and therefore the
> compat version always returns false.  Since GRE only adds data to
> the front of the packet and this is on the transmit path (meaning
> there is no important data in the SKB head) it is safe to use
> skb_header_cloned().  This avoids a copy for locally generated
> TCP packets on the affected kernels.
>
> The second change is to always allocate 64 bytes of headroom when
> a copy needs to be made.  There is no cost to allocating extra
> space once we have decided to make a copy and can avoid another
> copy later if we need to add a VLAN tag.

Looks good.

Should we use max(headroom, 64) just in case some situation calls for
more than 64 bytes of headroom, or do we know that that cannot happen?