From: Eric Dumazet <redacted>
Date: Fri, 27 Apr 2012 12:38:33 +0200

From: Eric Dumazet <edumazet@google.com>

TCP coalesce can check if skb to be merged has its skb->head mapped to a
page fragment, instead of a kmalloc() area.

We had to disable coalescing in this case, for performance reasons.

We 'upgrade' skb->head as a fragment in itself.

This reduces number of cache misses when user makes its copies, since a
less sk_buff are fetched.

This makes receive and ofo queues shorter and thus reduce cache line
misses in TCP stack.

This is a followup of patch "net: allow skb->head to be a page fragment"

Tested with tg3 nic, with GRO on or off. We can see "TCPRcvCoalesce"
counter being incremented.

Signed-off-by: Eric Dumazet <edumazet@google.com>

Applied.