Index: sbin/mount/mount.c
===================================================================
--- sbin/mount/mount.c	(revision 187346)
+++ sbin/mount/mount.c	(working copy)
@@ -112,6 +112,7 @@
 	{ MNT_MULTILABEL,	"multilabel" },
 	{ MNT_ACLS,		"acls" },
 	{ MNT_GJOURNAL,		"gjournal" },
+	{ MNT_ORPHANED,		"orphaned" },
 	{ 0, NULL }
 };
 
Index: sys/ufs/ffs/ffs_vfsops.c
===================================================================
--- sys/ufs/ffs/ffs_vfsops.c	(revision 187346)
+++ sys/ufs/ffs/ffs_vfsops.c	(working copy)
@@ -601,6 +601,15 @@
  */
 static int sblock_try[] = SBLOCKSEARCH;
 
+static void
+ffs_orphan_callback(struct g_consumer *cp, void *user)
+{
+	struct mount *mp;
+
+	mp = (struct mount *)user;
+	vfs_orphan(mp);
+}
+
 /*
  * Common code for mount and mountroot
  */
@@ -626,9 +635,13 @@
 	cred = td ? td->td_ucred : NOCRED;
 
 	ronly = (mp->mnt_flag & MNT_RDONLY) != 0;
+	vfs_ref(mp);
 	DROP_GIANT();
 	g_topology_lock();
 	error = g_vfs_open(devvp, &cp, "ffs", ronly ? 0 : 1);
+	if (error == 0)
+		g_vfs_register_callback(cp, ffs_orphan_callback,
+		    mp, G_CB_ORPHAN);
 
 	/*
 	 * If we are a root mount, drop the E flag so fsck can do its magic.
@@ -639,8 +652,10 @@
 	g_topology_unlock();
 	PICKUP_GIANT();
 	VOP_UNLOCK(devvp, 0);
-	if (error)
+	if (error) {
+		vfs_rel(mp);
 		return (error);
+	}
 	if (devvp->v_rdev->si_iosize_max != 0)
 		mp->mnt_iosize_max = devvp->v_rdev->si_iosize_max;
 	if (mp->mnt_iosize_max > MAXPHYS)
@@ -900,6 +915,7 @@
 	(void) ufs_extattr_autostart(mp, td);
 #endif /* !UFS_EXTATTR_AUTOSTART */
 #endif /* !UFS_EXTATTR */
+
 	return (0);
 out:
 	if (bp)
@@ -910,6 +926,7 @@
 		g_vfs_close(cp);
 		g_topology_unlock();
 		PICKUP_GIANT();
+		vfs_rel(mp);
 	}
 	if (ump) {
 		mtx_destroy(UFS_MTX(ump));
@@ -1106,6 +1123,7 @@
 	g_vfs_close(ump->um_cp);
 	g_topology_unlock();
 	PICKUP_GIANT();
+	vfs_rel(mp);
 	vrele(ump->um_devvp);
 	mtx_destroy(UFS_MTX(ump));
 	if (mp->mnt_gjprovider != NULL) {
Index: sys/kern/vfs_syscalls.c
===================================================================
--- sys/kern/vfs_syscalls.c	(revision 187346)
+++ sys/kern/vfs_syscalls.c	(working copy)
@@ -326,6 +326,8 @@
 	sp->f_version = STATFS_VERSION;
 	sp->f_namemax = NAME_MAX;
 	sp->f_flags = mp->mnt_flag & MNT_VISFLAGMASK;
+	if (mp->mnt_kern_flag & MNTK_ORPHANED)
+		sp->f_flags |= MNT_ORPHANED;
 	error = VFS_STATFS(mp, sp, td);
 	if (error)
 		goto out;
@@ -415,6 +417,8 @@
 	sp->f_version = STATFS_VERSION;
 	sp->f_namemax = NAME_MAX;
 	sp->f_flags = mp->mnt_flag & MNT_VISFLAGMASK;
+	if (mp->mnt_kern_flag & MNTK_ORPHANED)
+		sp->f_flags |= MNT_ORPHANED;
 	error = VFS_STATFS(mp, sp, td);
 	if (error)
 		goto out;
@@ -515,6 +519,8 @@
 			sp->f_version = STATFS_VERSION;
 			sp->f_namemax = NAME_MAX;
 			sp->f_flags = mp->mnt_flag & MNT_VISFLAGMASK;
+			if (mp->mnt_kern_flag & MNTK_ORPHANED)
+				sp->f_flags |= MNT_ORPHANED;
 			/*
 			 * If MNT_NOWAIT or MNT_LAZY is specified, do not
 			 * refresh the fsstat cache. MNT_NOWAIT or MNT_LAZY
@@ -4654,6 +4660,8 @@
 	sp->f_version = STATFS_VERSION;
 	sp->f_namemax = NAME_MAX;
 	sp->f_flags = mp->mnt_flag & MNT_VISFLAGMASK;
+	if (mp->mnt_kern_flag & MNTK_ORPHANED)
+		sp->f_flags |= MNT_ORPHANED;
 	error = VFS_STATFS(mp, sp, td);
 	if (error == 0)
 		*buf = *sp;
Index: sys/kern/vfs_subr.c
===================================================================
--- sys/kern/vfs_subr.c	(revision 187346)
+++ sys/kern/vfs_subr.c	(working copy)
@@ -1062,7 +1062,7 @@
 #endif
 	MNT_ILOCK(mp);
 	if ((mp->mnt_kern_flag & MNTK_NOINSMNTQ) != 0 &&
-	    ((mp->mnt_kern_flag & MNTK_UNMOUNTF) != 0 ||
+	    ((mp->mnt_kern_flag & (MNTK_UNMOUNTF | MNTK_ORPHANED)) != 0 ||
 	     mp->mnt_nvnodelistsize == 0)) {
 		locked = VOP_ISLOCKED(vp);
 		if (!locked || (locked == LK_EXCLUSIVE &&
@@ -1070,6 +1070,8 @@
 			MNT_IUNLOCK(mp);
 			if (dtr != NULL)
 				dtr(vp, dtr_arg);
+			if ((mp->mnt_kern_flag & MNTK_ORPHANED) != 0)
+				return (ENXIO);
 			return (EBUSY);
 		}
 	}
@@ -2834,6 +2836,7 @@
 	MNT_KERN_FLAG(MNTK_MPSAFE);
 	MNT_KERN_FLAG(MNTK_NOKNOTE);
 	MNT_KERN_FLAG(MNTK_LOOKUP_SHARED);
+	MNT_KERN_FLAG(MNTK_ORPHANED);
 #undef MNT_KERN_FLAG
 	if (flags != 0) {
 		if (buf[0] != '\0')
@@ -4205,6 +4208,45 @@
 }
 
 /*
+ * Mark the filesystem as orphaned.  Usually called when the device
+ * that contained the filesystem goes away.
+ */
+void
+vfs_orphan(struct mount *mp)
+{
+	int error;
+	struct mount *tmp;
+
+	error = vfs_busy(mp, 0);
+	/* If the filesystem is being unmounted, do nothing. */
+	if (error)
+		return;
+	/* Prevent all future vnode operations from succeeding. */
+	MNT_ILOCK(mp);
+	mp->mnt_kern_flag |= (MNTK_ORPHANED | MNTK_NOINSMNTQ);
+	MNT_IUNLOCK(mp);
+
+	/*
+	 * Don't try to call vflush on a mount structure that is not
+	 * fully initialized yet.  Assume that the mount is initialized
+	 * if it can be found on the mountlist.
+	 */
+	mtx_lock(&mountlist_mtx);
+	TAILQ_FOREACH(tmp, &mountlist, mnt_list) {
+		if (tmp == mp)
+			break;
+	}
+	mtx_unlock(&mountlist_mtx);
+	if (tmp == NULL) {
+		vfs_unbusy(mp);
+		return;
+	}
+	vflush(mp, 0, FORCECLOSE, curthread);
+	vfs_unbusy(mp);
+	vfs_rel(mp); /* XXX: Why is this needed? */
+}
+
+/*
  * Mark for update the access time of the file if the filesystem
  * supports VA_MARK_ATIME.  This functionality is used by execve
  * and mmap, so we want to avoid the synchronous I/O implied by
Index: sys/fs/msdosfs/msdosfs_vfsops.c
===================================================================
--- sys/fs/msdosfs/msdosfs_vfsops.c	(revision 187346)
+++ sys/fs/msdosfs/msdosfs_vfsops.c	(working copy)
@@ -403,6 +403,15 @@
 	return (0);
 }
 
+static void
+msdosfs_orphan_callback(struct g_consumer *cp, void *user)
+{
+	struct mount *mp;
+
+	mp = (struct mount *)user;
+	vfs_orphan(mp);
+}
+
 static int
 mountmsdosfs(struct vnode *devvp, struct mount *mp)
 {
@@ -421,14 +430,20 @@
 
 	ronly = (mp->mnt_flag & MNT_RDONLY) != 0;
 	/* XXX: use VOP_ACCESS to check FS perms */
+	vfs_ref(mp);
 	DROP_GIANT();
 	g_topology_lock();
 	error = g_vfs_open(devvp, &cp, "msdosfs", ronly ? 0 : 1);
+	if (error == 0)
+		g_vfs_register_callback(cp, msdosfs_orphan_callback,
+		    mp, G_CB_ORPHAN);
 	g_topology_unlock();
 	PICKUP_GIANT();
 	VOP_UNLOCK(devvp, 0);
-	if (error)
+	if (error) {
+		vfs_rel(mp);
 		return (error);
+	}
 
 	bo = &devvp->v_bufobj;
 	bp = NULL;		/* This and pmp both used in error_exit. */
@@ -756,6 +771,7 @@
 		g_vfs_close(cp);
 		g_topology_unlock();
 		PICKUP_GIANT();
+		vfs_rel(mp);
 	}
 	if (pmp) {
 		if (pmp->pm_inusemap)
@@ -826,6 +842,7 @@
 	g_vfs_close(pmp->pm_cp);
 	g_topology_unlock();
 	PICKUP_GIANT();
+	vfs_rel(mp);
 	vrele(pmp->pm_devvp);
 	free(pmp->pm_inusemap, M_MSDOSFSFAT);
 	if (pmp->pm_flags & MSDOSFS_LARGEFS)
Index: sys/geom/geom_vfs.c
===================================================================
--- sys/geom/geom_vfs.c	(revision 187346)
+++ sys/geom/geom_vfs.c	(working copy)
@@ -130,17 +130,56 @@
 	g_io_request(bip, cp);
 }
 
+struct g_vfs_cb {
+	struct g_consumer	*cb_consumer;
+	int			cb_event;
+	void			(*cb_callback)(struct g_consumer *, void *);
+	void			*cb_userptr;
+};
+
+void
+g_vfs_register_callback(struct g_consumer *cp,
+    void (callback)(struct g_consumer *, void *), void *userptr, int event)
+{
+	struct g_vfs_cb *cb;
+
+	g_topology_assert();
+
+	KASSERT(event >= 0 && event <= G_CB_LAST,
+	    ("invalid callback event flag"));
+	cb = cp->private;
+	KASSERT(cb[event].cb_callback == NULL,
+	    ("callback already registered"));
+
+	cb[event].cb_callback = callback;
+	cb[event].cb_userptr = userptr;
+	cb[event].cb_consumer = cp;
+	cb[event].cb_event = event;
+}
+
 static void
 g_vfs_orphan(struct g_consumer *cp)
 {
 	struct g_geom *gp;
 	struct bufobj *bo;
+	struct g_vfs_cb *cb;
 
 	g_topology_assert();
 
 	gp = cp->geom;
 	bo = gp->softc;
+	cb = cp->private;
+
 	g_trace(G_T_TOPOLOGY, "g_vfs_orphan(%p(%s))", cp, gp->name);
+
+	if (cb != NULL && cb[G_CB_ORPHAN].cb_callback != NULL) {
+		KASSERT(cb[G_CB_ORPHAN].cb_event == G_CB_ORPHAN,
+		    ("found callback for unknown event"));
+		KASSERT(cb[G_CB_ORPHAN].cb_consumer == cp,
+		    ("found callback for unknown consumer"));
+		(cb[G_CB_ORPHAN].cb_callback)(cp, cb[G_CB_ORPHAN].cb_userptr);
+	}
+
 	if (cp->acr > 0 || cp->acw > 0 || cp->ace > 0)
 		g_access(cp, -cp->acr, -cp->acw, -cp->ace);
 	g_detach(cp);
@@ -169,6 +208,8 @@
 	gp = g_new_geomf(&g_vfs_class, "%s.%s", fsname, pp->name);
 	cp = g_new_consumer(gp);
 	g_attach(cp, pp);
+	cp->private = g_malloc(sizeof(struct g_vfs_cb[G_CB_LAST + 1]),
+	    M_WAITOK | M_ZERO);
 	error = g_access(cp, 1, wr, 1);
 	if (error) {
 		g_wither_geom(gp, ENXIO);
@@ -195,6 +236,8 @@
 
 	g_topology_assert();
 
+	g_free(cp->private);
+	cp->private = NULL;
 	gp = cp->geom;
 	bo = gp->softc;
 	bufobj_invalbuf(bo, V_SAVE, 0, 0);
Index: sys/geom/geom_vfs.h
===================================================================
--- sys/geom/geom_vfs.h	(revision 187346)
+++ sys/geom/geom_vfs.h	(working copy)
@@ -35,8 +35,12 @@
 
 extern struct buf_ops *g_vfs_bufops;
 
+#define	G_CB_ORPHAN	1
+#define	G_CB_LAST	G_CB_ORPHAN
+
 void g_vfs_strategy(struct bufobj *bo, struct buf *bp);
 int g_vfs_open(struct vnode *vp, struct g_consumer **cpp, const char *fsname, int wr);
 void g_vfs_close(struct g_consumer *cp);
-
+void g_vfs_register_callback(struct g_consumer *cp,
+    void (callback)(struct g_consumer *, void *), void *user, int event);
 #endif /* _GEOM_GEOM_VFS_H_ */
Index: sys/sys/mount.h
===================================================================
--- sys/sys/mount.h	(revision 187346)
+++ sys/sys/mount.h	(working copy)
@@ -243,14 +243,17 @@
 #define	MNT_EXPUBLIC	0x20000000	/* public export (WebNFS) */
 
 /*
- * Flags set by internal operations,
- * but visible to the user.
- * XXX some of these are not quite right.. (I've never seen the root flag set)
+ * Flags set by internal operations, but visible to the user.
+ * Note that MNT_ORPHANED flag is never actually set on mnt_flag field
+ * in struct mount; it's only set on f_flags in struct statfs when
+ * MNTK_ORPHANED is set.  We cannot use MNT_ORPHANED instead of MNTK_ORPHANED
+ * due to missing locking of mnt_flag.
  */
 #define	MNT_LOCAL	0x00001000	/* filesystem is stored locally */
 #define	MNT_QUOTA	0x00002000	/* quotas are enabled on filesystem */
 #define	MNT_ROOTFS	0x00004000	/* identifies the root filesystem */
 #define	MNT_USER	0x00008000	/* mounted by a user */
+#define	MNT_ORPHANED	0x00020000	/* MNTK_ORPHANED is set */
 #define	MNT_IGNORE	0x00800000	/* do not show entry in df */
 
 /*
@@ -266,7 +269,8 @@
 			MNT_ROOTFS	| MNT_NOATIME	| MNT_NOCLUSTERR| \
 			MNT_NOCLUSTERW	| MNT_SUIDDIR	| MNT_SOFTDEP	| \
 			MNT_IGNORE	| MNT_EXPUBLIC	| MNT_NOSYMFOLLOW | \
-			MNT_GJOURNAL	| MNT_MULTILABEL | MNT_ACLS)
+			MNT_GJOURNAL	| MNT_MULTILABEL | MNT_ACLS | \
+			MNT_ORPHANED)
 
 /* Mask of flags that can be updated. */
 #define	MNT_UPDATEMASK (MNT_NOSUID	| MNT_NOEXEC	| \
@@ -282,6 +286,8 @@
  * XXX: These are not STATES and really should be somewhere else.
  * XXX: MNT_BYFSID collides with MNT_ACLS, but because MNT_ACLS is only used for
  *      mount(2) and MNT_BYFSID is only used for unmount(2) it's harmless.
+ * XXX: MNT_DELEXPORT collides with MNT_ORPHANED, but MNT_DELEXPORT is never
+ *      used in mnt_flag, only for ex_flags.
  */
 #define	MNT_UPDATE	0x00010000	/* not a real mount, just an update */
 #define	MNT_DELEXPORT	0x00020000	/* delete export host lists */
@@ -317,6 +323,7 @@
 #define MNTK_NOINSMNTQ	0x00000008	/* insmntque is not allowed */
 #define	MNTK_DRAINING	0x00000010	/* lock draining is happening */
 #define	MNTK_REFEXPIRE	0x00000020	/* refcount expiring is happening */
+#define	MNTK_ORPHANED	0x00000040	/* device is gone */
 #define MNTK_UNMOUNT	0x01000000	/* unmount in progress */
 #define	MNTK_MWAIT	0x02000000	/* waiting for unmount to finish */
 #define	MNTK_SUSPEND	0x08000000	/* request write suspension */
@@ -730,6 +737,7 @@
 int	vfs_suser(struct mount *, struct thread *);
 void	vfs_unbusy(struct mount *);
 void	vfs_unmountall(void);
+void	vfs_orphan(struct mount *);
 extern	TAILQ_HEAD(mntlist, mount) mountlist;	/* mounted filesystem list */
 extern	struct mtx mountlist_mtx;
 extern	struct nfs_public nfs_pub;