Here's a belated footnote to Paul Vixie's helpful posting of April 12:
> Date: Sun, 12 Apr 1998 16:38:02 -0700
> From: Paul A Vixie <paul@vix.com>
> Sender: owner-pgsql-hackers@hub.org
> Precedence: bulk
>
> mmap() is cool since it avoids copying data between kernel and user address
> spaces. However, mmap() is going to be either synchronous ("won't return 'til
> it has set up the page table stuff and maybe allocated backing store") or not
> ("will return immediately but your process will silently block if you try to
> access the address range before the back office work is done for the region").
> There is no callback facility and no way to poll for region readiness.
...
In the case of FreeBSD, there is no callback facility, this is true,
but you can poll for region readiness via mincore().