diff --git a/lib/libc/sys/swapon.2 b/lib/libc/sys/swapon.2
--- a/lib/libc/sys/swapon.2
+++ b/lib/libc/sys/swapon.2
@@ -28,7 +28,7 @@
 .\"     @(#)swapon.2	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd March 30, 2020
+.Dd December 2, 2021
 .Dt SWAPON 2
 .Os
 .Sh NAME
@@ -37,6 +37,8 @@
 .Sh LIBRARY
 .Lb libc
 .Sh SYNOPSIS
+.It vm/vm_param.h
+.It vm/swap_pager.h
 .In unistd.h
 .Ft int
 .Fn swapon "const char *special"
@@ -63,6 +65,40 @@
 system call disables paging and swapping on the given device.
 All associated swap metadata are deallocated, and the device
 is made available for other purposes.
+.Pp
+The
+.Fa special
+argument points to either the string that names the special device
+used for swapping or the following structure
+.Bd -literal
+struct swapoff_new_args {
+	const char *name_old_syscall;
+	const char *name;
+	u_int flags;
+};
+.Ed
+that extends swapoff's interface to include optional flags.
+The
+.Va name_old_syscall
+member must be initialized to
+.Dv NULL
+for the system to recognize the extended interface.
+It is recommended that the whole structure be zeroed for future
+compatibility, due to padding.
+.Pp
+The
+.Va name
+string is the special device name, same as legacy syscall argument.
+.The
+.Va flags
+argument takes the following flags:
+.Bl -tag -width SWAPOFF_FORCE
+.It Dv SWAPOFF_FORCE
+Overrides a very conservative check that prevents swapoff
+if the total amount of free memory and remaining swap
+devices space might be unsufficient for the system to continue
+operating.
+.El
 .Sh RETURN VALUES
 If an error has occurred, a value of -1 is returned and
 .Va errno
diff --git a/sbin/swapon/swapon.8 b/sbin/swapon/swapon.8
--- a/sbin/swapon/swapon.8
+++ b/sbin/swapon/swapon.8
@@ -28,7 +28,7 @@
 .\"     @(#)swapon.8	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd May 19, 2020
+.Dd November 29, 2021
 .Dt SWAPON 8
 .Os
 .Sh NAME
@@ -42,9 +42,9 @@
 .Nm swapoff
 .Oo Fl F Ar fstab
 .Oc
-.Fl aLq | Ar
+.Fl afLq | Ar
 .Nm swapctl
-.Op Fl AghklmsU
+.Op Fl AfghklmsU
 .Oo
 .Fl a Ar
 |
@@ -125,8 +125,14 @@
 written to standard output when a swap device is removed.
 Note that
 .Nm swapoff
-will fail and refuse to remove a swap device if there is insufficient
-VM (memory + remaining swap devices) to run the system.
+will fail and refuse to remove a swap device if a very conservative
+check does not conclude that there is sufficient VM (memory +
+remaining swap devices) to run the system.
+The
+.Fl f
+option turns off this check, which could deadlock the system
+if there is insufficient swap space remaining.
+.Pp
 The
 .Nm swapoff
 utility
diff --git a/sbin/swapon/swapon.c b/sbin/swapon/swapon.c
--- a/sbin/swapon/swapon.c
+++ b/sbin/swapon/swapon.c
@@ -51,6 +51,7 @@
 #include <sys/sysctl.h>
 #include <sys/wait.h>
 #include <vm/vm_param.h>
+#include <vm/swap_pager.h>
 
 #include <err.h>
 #include <errno.h>
@@ -78,7 +79,7 @@
 
 static enum { SWAPON, SWAPOFF, SWAPCTL } orig_prog, which_prog = SWAPCTL;
 
-static int Eflag, qflag;
+static int Eflag, fflag, qflag;
 
 int
 main(int argc, char **argv)
@@ -101,7 +102,7 @@
 	
 	doall = 0;
 	etc_fstab = NULL;
-	while ((ch = getopt(argc, argv, "AadEghklLmqsUF:")) != -1) {
+	while ((ch = getopt(argc, argv, "AadEfghklLmqsUF:")) != -1) {
 		switch(ch) {
 		case 'A':
 			if (which_prog == SWAPCTL) {
@@ -128,6 +129,12 @@
 			else
 				usage();
 			break;
+		case 'f':
+			if (which_prog == SWAPOFF)
+				fflag = 1;
+			else
+				usage();
+			break;
 		case 'g':
 			hflag = 'G';
 			break;
@@ -782,12 +789,18 @@
 static const char *
 swap_on_off_sfile(const char *name, int doingall)
 {
+	struct swapoff_new_args sa;
 	int error;
 
 	if (which_prog == SWAPON)
 		error = Eflag ? swapon_trim(name) : swapon(name);
-	else /* SWAPOFF */
-		error = swapoff(name);
+	else { /* SWAPOFF */
+		bzero(&sa, sizeof(sa));
+		sa.name = name;
+		if (fflag)
+			sa.flags |= SWAPOFF_FORCE;
+		error = swapoff((const char *)&sa);
+	}
 
 	if (error == -1) {
 		switch (errno) {
@@ -820,7 +833,7 @@
 	    fprintf(stderr, "[-F fstab] -aLq | [-E] file ...\n");
 	    break;
 	case SWAPOFF:
-	    fprintf(stderr, "[-F fstab] -aLq | file ...\n");
+	    fprintf(stderr, "[-F fstab] -afLq | file ...\n");
 	    break;
 	case SWAPCTL:
 	    fprintf(stderr, "[-AghklmsU] [-a file ... | -d file ...]\n");
diff --git a/sys/vm/swap_pager.h b/sys/vm/swap_pager.h
--- a/sys/vm/swap_pager.h
+++ b/sys/vm/swap_pager.h
@@ -69,6 +69,16 @@
 #define	SW_UNMAPPED	0x01
 #define	SW_CLOSING	0x04
 
+struct swapoff_new_args {
+	const char *name_old_syscall;
+	const char *name;
+	u_int flags;
+	u_int pad0;
+	uintptr_t pad1[8];
+};
+
+#define	SWAPOFF_FORCE	0x00000001
+
 #ifdef _KERNEL
 
 extern int swap_pager_avail;
diff --git a/sys/vm/swap_pager.c b/sys/vm/swap_pager.c
--- a/sys/vm/swap_pager.c
+++ b/sys/vm/swap_pager.c
@@ -470,7 +470,7 @@
 static int	swapongeom(struct vnode *);
 static int	swaponvp(struct thread *, struct vnode *, u_long);
 static int	swapoff_one(struct swdevt *sp, struct ucred *cred,
-		    bool ignore_check);
+		    u_int flags);
 
 /*
  * Swap bitmap functions
@@ -2340,10 +2340,6 @@
 };
 #endif
 
-/*
- * MPSAFE
- */
-/* ARGSUSED */
 int
 sys_swapon(struct thread *td, struct swapon_args *uap)
 {
@@ -2489,25 +2485,44 @@
 };
 #endif
 
-/*
- * MPSAFE
- */
-/* ARGSUSED */
 int
 sys_swapoff(struct thread *td, struct swapoff_args *uap)
 {
 	struct vnode *vp;
 	struct nameidata nd;
 	struct swdevt *sp;
-	int error;
+	struct swapoff_new_args sa;
+	int error, probe_byte;
 
 	error = priv_check(td, PRIV_SWAPOFF);
 	if (error)
 		return (error);
 
+	/*
+	 * Detect old vs. new-style swapoff(2) syscall.  The first
+	 * pointer in the memory pointed to by uap->name is NULL for
+	 * the new variant.
+	 */
+	probe_byte = fubyte(uap->name);
+	switch (probe_byte) {
+	case -1:
+		return (EFAULT);
+	case 0:
+		error = copyin(uap->name, &sa, sizeof(sa));
+		if (error != 0)
+			return (error);
+		if ((sa.flags & ~(SWAPOFF_FORCE)) != 0)
+			return (EINVAL);
+		break;
+	default:
+		bzero(&sa, sizeof(sa));
+		sa.name = uap->name;
+		break;
+	}
+
 	sx_xlock(&swdev_syscall_lock);
 
-	NDINIT(&nd, LOOKUP, FOLLOW | AUDITVNODE1, UIO_USERSPACE, uap->name);
+	NDINIT(&nd, LOOKUP, FOLLOW | AUDITVNODE1, UIO_USERSPACE, sa.name);
 	error = namei(&nd);
 	if (error)
 		goto done;
@@ -2524,14 +2539,14 @@
 		error = EINVAL;
 		goto done;
 	}
-	error = swapoff_one(sp, td->td_ucred, false);
+	error = swapoff_one(sp, td->td_ucred, sa.flags);
 done:
 	sx_xunlock(&swdev_syscall_lock);
 	return (error);
 }
 
 static int
-swapoff_one(struct swdevt *sp, struct ucred *cred, bool ignore_check)
+swapoff_one(struct swdevt *sp, struct ucred *cred, u_int flags)
 {
 	u_long nblks;
 #ifdef MAC
@@ -2561,7 +2576,7 @@
 	 * means that we can lose swap data when filesystems go away,
 	 * which is arguably worse.
 	 */
-	if (!ignore_check &&
+	if ((flags & SWAPOFF_FORCE) == 0 &&
 	    vm_free_count() + swap_pager_avail < nblks + nswap_lowat)
 		return (ENOMEM);
 
@@ -2612,7 +2627,7 @@
 			devname = devtoname(sp->sw_vp->v_rdev);
 		else
 			devname = "[file]";
-		error = swapoff_one(sp, thread0.td_ucred, true);
+		error = swapoff_one(sp, thread0.td_ucred, SWAPOFF_FORCE);
 		if (error != 0) {
 			printf("Cannot remove swap device %s (error=%d), "
 			    "skipping.\n", devname, error);