Document some of the kernel vm_page API as well as the pbuf API.

Submitted by: Chad David <davidc@acns.ab.ca>

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+.\"
+.\" Copyright (C) 2001 Chad David <davidc@acns.ab.ca>. All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer as
+.\"    the first lines of this file unmodified other than the possible
+.\"    addition of one or more copyright notices.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice(s), this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
+.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+.\" DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
+.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
+.\" DAMAGE.
+.\"
+.\" $FreeBSD$
+.\"
+.Dd July 13, 2001
+.Dt VM_PAGE_HOLD 9
+.Os
+.Sh NAME
+.Nm vm_page_hold ,
+.Nm vm_page_unhold
+.Nd "updates a pages hold count"
+.Sh SYNOPSIS
+.In sys/param.h
+.In vm/vm.h
+.In vm/vm_page.h
+.Ft void
+.Fn vm_page_hold "vm_page_t m"
+.Ft void
+.Fn vm_page_unhold "vm_page_t m"
+.Sh DESCRIPTION
+The
+.Fn vm_page_hold
+function increases the hold count on a page.
+This prevents the page daemon from freeing the page.
+.Fn vm_page_hold
+should only be used for very temporary wiring of a page.
+If the page needs to be held for a longer period
+.Fn vm_page_write
+should be used.
+.Pp
+.Fn vm_page_unhold
+function reduces the hold count on a page.
+If the hold count is zero it is possible the page will be freed by the
+page daemon.
+.Sh SEE ALSO
+.Xr vm_page_wire 9
+.Xr wm_page_unwire 9
+.Sh AUTHORS
+This man page was written by
+.An Chad David Aq davidc@acns.ab.ca .