Index: sbin/swapon/swapon.c
===================================================================
--- sbin/swapon/swapon.c
+++ sbin/swapon/swapon.c
@@ -756,6 +756,24 @@
 	fd = open(name, O_WRONLY);
 	if (fd < 0)
 		errx(1, "Cannot open %s", name);
+
+	/*
+	 * Hack: see if the file is already in use as a swap device.  If so,
+	 * don't attempt to erase its blocks since we may end up clearing
+	 * swapped-out data.  Currently there is no surefire way to determine
+	 * whether a file is a swap device other than by attempting to enable it
+	 * as such, and observing EBUSY.
+	 */
+	error = swapon(name);
+	if (error != 0) {
+		error = errno;
+		(void)close(fd);
+		errno = error;
+		return (-1);
+	}
+	if (swapoff(name, SWAPOFF_FORCE) != 0)
+		err(1, "swapoff(%s)", name);
+
 	/* Find the device size. */
 	if (fstat(fd, &sb) < 0)
 		errx(1, "Cannot stat %s", name);