Index: share/man/man9/Makefile
===================================================================
--- share/man/man9/Makefile
+++ share/man/man9/Makefile
@@ -414,6 +414,7 @@
 MLINKS=	unr.9 alloc_unr.9 \
 	unr.9 alloc_unrl.9 \
 	unr.9 alloc_unr_specific.9 \
+	unr.9 clear_unrhdr.9 \
 	unr.9 delete_unrhdr.9 \
 	unr.9 free_unr.9 \
 	unr.9 new_unrhdr.9
Index: share/man/man9/unr.9
===================================================================
--- share/man/man9/unr.9
+++ share/man/man9/unr.9
@@ -24,11 +24,12 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 5, 2010
+.Dd October 4, 2017
 .Dt UNR 9
 .Os
 .Sh NAME
 .Nm new_unrhdr ,
+.Nm clear_unrhdr ,
 .Nm delete_unrhdr ,
 .Nm alloc_unr ,
 .Nm alloc_unr_specific ,
@@ -39,6 +40,8 @@
 .Ft "struct unrhdr *"
 .Fn new_unrhdr "int low" "int high" "struct mtx *mutex"
 .Ft void
+.Fn clear_unrhdr "struct unrhdr *uh"
+.Ft void
 .Fn delete_unrhdr "struct unrhdr *uh"
 .Ft int
 .Fn alloc_unr "struct unrhdr *uh"
@@ -70,8 +73,14 @@
 .Dv NULL ,
 it is used for locking when allocating and freeing units.
 Otherwise, internal mutex is used.
+.It Fn clear_unrhdr uh
+Clear all units from the specified unit number allocator entity. This function
+resets the entity as if it were just initialized with
+.Fn new_unrhdr .
 .It Fn delete_unrhdr uh
-Destroy specified unit number allocator entity.
+Delete specified unit number allocator entity. This function frees the memory
+associated with the entity, it does not free any units. To free all units use
+.Fn clear_unrhdr .
 .It Fn alloc_unr uh
 Return a new unit number.
 The lowest free number is always allocated.
Index: sys/fs/tmpfs/tmpfs_subr.c
===================================================================
--- sys/fs/tmpfs/tmpfs_subr.c
+++ sys/fs/tmpfs/tmpfs_subr.c
@@ -362,7 +362,8 @@
 		panic("tmpfs_free_node: type %p %d", node, (int)node->tn_type);
 	}
 
-	free_unr(tmp->tm_ino_unr, node->tn_id);
+	if (!detach)
+		free_unr(tmp->tm_ino_unr, node->tn_id);
 	uma_zfree(tmp->tm_node_pool, node);
 	TMPFS_LOCK(tmp);
 	tmpfs_free_tmp(tmp);
Index: sys/fs/tmpfs/tmpfs_vfsops.c
===================================================================
--- sys/fs/tmpfs/tmpfs_vfsops.c
+++ sys/fs/tmpfs/tmpfs_vfsops.c
@@ -317,6 +317,8 @@
 			TMPFS_NODE_UNLOCK(node);
 	}
 
+	clear_unrhdr(tmp->tm_ino_unr);
+
 	mp->mnt_data = NULL;
 	tmpfs_free_tmp(tmp);
 	vfs_write_resume(mp, VR_START_WRITE);
Index: sys/kern/subr_unit.c
===================================================================
--- sys/kern/subr_unit.c
+++ sys/kern/subr_unit.c
@@ -366,6 +366,27 @@
 	Free(uh);
 }
 
+void
+clear_unrhdr(struct unrhdr *uh)
+{
+	struct unr *up, *uq;
+
+	up = TAILQ_FIRST(&uh->head);
+	while (up != NULL) {
+		uq = TAILQ_NEXT(up, list);
+		if (up->ptr != uh) {
+			Free(up->ptr);
+		}
+		Free(up);
+		up = uq;
+	}
+	TAILQ_INIT(&uh->head);
+	uh->busy = 0;
+	uh->alloc = 0;
+	KASSERT(TAILQ_EMPTY(&uh->ppfree),
+	    ("unrhdr has postponed item for free"));
+}
+
 static __inline int
 is_bitmap(struct unrhdr *uh, struct unr *up)
 {
Index: sys/sys/systm.h
===================================================================
--- sys/sys/systm.h
+++ sys/sys/systm.h
@@ -450,6 +450,7 @@
 struct unrhdr *new_unrhdr(int low, int high, struct mtx *mutex);
 void init_unrhdr(struct unrhdr *uh, int low, int high, struct mtx *mutex);
 void delete_unrhdr(struct unrhdr *uh);
+void clear_unrhdr(struct unrhdr *uh);
 void clean_unrhdr(struct unrhdr *uh);
 void clean_unrhdrl(struct unrhdr *uh);
 int alloc_unr(struct unrhdr *uh);