From: Eric Dumazet <redacted>
Date: Wed, 21 Dec 2011 15:02:18 +0100

No idea on this +2 point.

I think I know, and I believe I instructed Alexey Kuznetsov to do
this.

When sendfile() is performed, we might start the SKB with the last few
bytes of one page, and end the SKB with the first few bytes of another
page.

In order to fit a full 64K frame into an SKB in this situation we have
to accomodate this case.