swapoff: Remove only late devices with -aL.

Currently, '/etc/rc.d/swaplate stop' removes all swap devices. This can be
very slow and may not even be possible if there is a lot of swap space in
use. However, removing swap devices is only needed for late swap devices
that may depend on daemons that subsequent shutdown steps stop. Normal swap
devices such as hard disk partitions will remain available throughout the
shutdown process and need not be removed.

In swapoff, interpret -aL to remove late swap devices only, and use this in
etc/rc.d/swaplate. The meaning of -aL in swapon remains unchanged (add all
swap devices, both normal and late).

PR:		187081
Reviewed by:	wblock (man page only), ngie
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D8126

etc/rc.d/swaplate

@@ -12,7 +12,7 @@
 name="swaplate"
 desc="Setup late swap space"
 start_cmd='/sbin/swapon -aLq'
-stop_cmd='/sbin/swapoff -aq'
+stop_cmd='/sbin/swapoff -aLq'
 
 load_rc_config swap
 run_rc_command "$1"

sbin/swapon/swapon.8

@@ -28,7 +28,7 @@
 .\"     @(#)swapon.8	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd October 2, 2016
+.Dd October 21, 2016
 .Dt SWAPON 8
 .Os
 .Sh NAME
@@ -98,6 +98,12 @@ will be removed, unless their
 .Dq noauto
 option is also set.
 If the
+.Fl L
+option is specified,
+only swap devices with the
+.Dq late
+option will be removed.
+If the
 .Fl q
 option is used,
 informational messages will not be

sbin/swapon/swapon.c

@@ -176,6 +176,10 @@ main(int argc, char **argv)
 				    strstr(fsp->fs_mntops, "late") &&
 				    late == 0)
 					continue;
+				if (which_prog == SWAPOFF &&
+				    strstr(fsp->fs_mntops, "late") == NULL &&
+				    late != 0)
+					continue;
 				swfile = swap_on_off(fsp->fs_spec, 1,
 				    fsp->fs_mntops);
 				if (swfile == NULL) {