* David S. Miller [ref] 2005-01-27 16:31

The basic idea is that we stop trying to build TSO frames
in the actual transmit queue.  Instead, TSO packets are
built impromptu when we actually output packets on the
transmit queue.

Sound great.

static inline int tcp_skb_data_all_paged(struct sk_buff *skb)
{
	return (skb->len == skb->data_len);
}

You could also define this as (skb_headlen(skb) == 0)

The logic is simple because if TSO is being done we know
that all of the SKB data is paged (since SG+CSUM is a
requirement for TSO).  The one case where that
invariant might fail is due to a routing change (previous
device cannot do SG+CSUM, new device has full TSO capability)
and that is handled via the tcp_skb_data_all_paged() checks.

I assume the case when reroute changes oif to a device no
longer capable of SG+CSUM stays the same and the skb remains
paged until dev_queue_xmit?

My thinking is that whatever added expensive this new scheme
has, is offset by the simplifications the rest of the TCP
stack will have since it will no longer need to know anything
about multiple MSS values and packet counts.

I think the overhead is really worth the complexity that can
be removed with these changes.