diff --git a/share/man/man9/Makefile b/share/man/man9/Makefile
--- a/share/man/man9/Makefile
+++ b/share/man/man9/Makefile
@@ -401,6 +401,7 @@
 	vn_isdisk.9 \
 	vnet.9 \
 	vnode.9 \
+	vnode_pager_setsize.9 \
 	VOP_ACCESS.9 \
 	VOP_ACLCHECK.9 \
 	VOP_ADVISE.9 \
diff --git a/share/man/man9/vnode_pager_setsize.9 b/share/man/man9/vnode_pager_setsize.9
new file mode 100644
--- /dev/null
+++ b/share/man/man9/vnode_pager_setsize.9
@@ -0,0 +1,74 @@
+.\"
+.\" SPDX-License-Identifier: BSD-2-Clause-FreeBSD
+.\"
+.\" Copyright (c) 2021 The FreeBSD Foundation
+.\"
+.\" Portions of this software were developed by Ka Ho Ng
+.\" under sponsorship from the FreeBSD Foundation.
+.\"
+.\" 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, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, 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 AUTHOR AND CONTRIBUTORS ``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 AUTHOR OR CONTRIBUTORS 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.
+.\"
+.Dd April 6, 2021
+.Dt VNODE_PAGER_SETSIZE 9
+.Os
+.Sh NAME
+.Nm vnode_pager_setsize
+.Nd "notify the VM system about updates in the file size"
+.Sh SYNOPSIS
+.In sys/param.h
+.In vm/vm.h
+.In vm/vm_extern.h
+.Ft void
+.Fn vnode_pager_setsize "struct vnode *vp" "vm_ooffset_t nsize"
+.Sh DESCRIPTION
+.Nm
+lets the VM system know about a change in size for a file.
+Content beyond the new EOF specified by the
+.Fa nsize
+argument will be purged from the cache.
+This function is useful for use within file system code to implement
+truncation in
+.Xr VOP_SETATTR 9 .
+.Sh IMPLEMENTATION NOTES
+In case the new EOF specified by the
+.Fa nsize
+argument is not aligned to page boundaries,
+partial-page area starting beyond the EOF will be zeroed.
+In partial-page area,
+for content occupying whole blocks within block
+boundaries,
+the dirty bits for the corresponding blocks will be cleared.
+.Sh LOCKING
+Writer lock of the VM object of
+.Fa vp
+will be held within the function.
+.Sh SEE ALSO
+.Xr vnode 9
+.Sh HISTORY
+The
+.Nm
+manual page first appeared in
+.Fx 14 .
+.Sh AUTHORS
+This
+manual page was written by
+.An Ka Ho Ng Aq Mt khng@FreeBSD.org .