diff --git a/share/man/man9/osd.9 b/share/man/man9/osd.9
index e8260fe46338..c27b5d9fa4bb 100644
--- a/share/man/man9/osd.9
+++ b/share/man/man9/osd.9
@@ -25,7 +25,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 5, 2011
+.Dd March 30, 2016
 .Dt OSD 9
 .Os
 .Sh NAME
@@ -33,6 +33,9 @@
 .Nm osd_register ,
 .Nm osd_deregister ,
 .Nm osd_set ,
+.Nm osd_reserve ,
+.Nm osd_set_reserved ,
+.Nm osd_free_reserved ,
 .Nm osd_get ,
 .Nm osd_del ,
 .Nm osd_call ,
@@ -63,6 +66,22 @@
 .Fa "void *value"
 .Fc
 .Ft void *
+.Fo osd_reserve
+.Fa "u_int slot"
+.Fc
+.Ft int
+.Fo osd_set_reserved
+.Fa "u_int type"
+.Fa "struct osd *osd"
+.Fa "u_int slot"
+.Fa "void *rsv"
+.Fa "void *value"
+.Fc
+.Ft void
+.Fo osd_free_reserved
+.Fa "void *rsv"
+.Fc
+.Ft void *
 .Fo osd_get
 .Fa "u_int type"
 .Fa "struct osd *osd"
@@ -198,6 +217,15 @@ argument points to a data object to associate with
 .Fa osd .
 .Pp
 The
+.Fn osd_set_reserved
+function does the same as
+.Fn osd_set ,
+but with an extra argument
+.Fa rsv
+that is internal-use memory previously allocated via
+.Fn osd_reserve .
+.Pp
+The
 .Fn osd_get
 function returns the data pointer associated with a kernel data structure's
 .Vt struct osd
@@ -324,6 +352,24 @@ will proceed without any
 .Xr realloc 9
 calls.
 .Pp
+It is possible for
+.Fn osd_set
+to fail to allocate this array.  To ensure that such allocation succeeds,
+.Fn osd_reserve
+may be called (in a non-blocking context), and it will pre-allocate the
+memory via
+.Xr malloc 9
+with M_WAITOK.
+Then this pre-allocated memory is passed to
+.Fn osd_set_reserved ,
+which will use it if necessary or otherwise discard it.
+The memory may also be explicitly discarded by calling
+.Fn osd_free_reserved .
+As this method always allocates memory whether or not it is ultimately needed,
+it should be used only rarely, such as in the unlikely event that
+.Fn osd_set
+fails.
+.Pp
 The
 .Nm
 API is geared towards slot identifiers storing pointers to the same underlying
@@ -359,15 +405,27 @@ the kernel including most fast paths.
 returns the slot identifier for the newly registered data type.
 .Pp
 .Fn osd_set
-returns zero on success or ENOMEM if the specified type/slot identifier pair
+and
+.Fn osd_set_reserved
+return zero on success or ENOMEM if the specified type/slot identifier pair
 triggered an internal
 .Xr realloc 9
-which failed.
+which failed
+.Fn ( osd_set_reserved
+will always succeed when
+.Fa rsv
+is non-NULL).
 .Pp
 .Fn osd_get
 returns the data pointer for the specified type/slot identifier pair, or NULL if
 the slot has not been initialised yet.
 .Pp
+.Fn osd_reserve
+returns a pointer suitable for passing to
+.Fn osd_set_reserved
+or
+.Fn osd_free_reserved .
+.Pp
 .Fn osd_call
 returns zero if no method is run or the method for each slot runs successfully.
 If a method for a slot returns non-zero,
diff --git a/sys/kern/kern_osd.c b/sys/kern/kern_osd.c
index ce3c3b6eebec..41d518ff59dd 100644
--- a/sys/kern/kern_osd.c
+++ b/sys/kern/kern_osd.c
@@ -54,7 +54,7 @@ struct osd_master {
 	struct sx		 osd_module_lock;
 	struct rmlock		 osd_object_lock;
 	struct mtx		 osd_list_lock;
-	LIST_HEAD(, osd)	 osd_list;		/* (m) */
+	LIST_HEAD(, osd)	 osd_list;		/* (l) */
 	osd_destructor_t	*osd_destructors;	/* (o) */
 	osd_method_t		*osd_methods;		/* (m) */
 	u_int			 osd_ntslots;		/* (m) */
@@ -197,6 +197,24 @@ osd_deregister(u_int type, u_int slot)
 
 int
 osd_set(u_int type, struct osd *osd, u_int slot, void *value)
+{
+
+	return (osd_set_reserved(type, osd, slot, NULL, value));
+}
+
+void *
+osd_reserve(u_int slot)
+{
+
+	KASSERT(slot > 0, ("Invalid slot."));
+
+	OSD_DEBUG("Reserving slot array (slot=%u).", slot);
+	return (malloc(sizeof(void *) * slot, M_OSD, M_WAITOK | M_ZERO));
+}
+
+int
+osd_set_reserved(u_int type, struct osd *osd, u_int slot, void *rsv,
+    void *value)
 {
 	struct rm_priotracker tracker;
 
@@ -206,36 +224,34 @@ osd_set(u_int type, struct osd *osd, u_int slot, void *value)
 
 	rm_rlock(&osdm[type].osd_object_lock, &tracker);
 	if (slot > osd->osd_nslots) {
+		void *newptr;
+
 		if (value == NULL) {
 			OSD_DEBUG(
 			    "Not allocating null slot (type=%u, slot=%u).",
 			    type, slot);
 			rm_runlock(&osdm[type].osd_object_lock, &tracker);
+			if (rsv)
+				osd_free_reserved(rsv);
 			return (0);
-		} else if (osd->osd_nslots == 0) {
-			/*
-			 * First OSD for this object, so we need to allocate
-			 * space and put it onto the list.
-			 */
-			osd->osd_slots = malloc(sizeof(void *) * slot, M_OSD,
-			    M_NOWAIT | M_ZERO);
-			if (osd->osd_slots == NULL) {
-				rm_runlock(&osdm[type].osd_object_lock,
-				    &tracker);
-				return (ENOMEM);
-			}
-			osd->osd_nslots = slot;
-			mtx_lock(&osdm[type].osd_list_lock);
-			LIST_INSERT_HEAD(&osdm[type].osd_list, osd, osd_next);
-			mtx_unlock(&osdm[type].osd_list_lock);
-			OSD_DEBUG("Setting first slot (type=%u).", type);
-		} else {
-			void *newptr;
+		}
 
+		/*
+		 * Too few slots allocated here, so we need to extend or create
+		 * the array.
+		 */
+		if (rsv) {
 			/*
-			 * Too few slots allocated here, needs to extend
-			 * the array.
+			 * Use the reserve passed in (assumed to be
+			 * the right size).
 			 */
+			newptr = rsv;
+			if (osd->osd_nslots != 0) {
+				memcpy(newptr, osd->osd_slots,
+				    sizeof(void *) * osd->osd_nslots);
+				free(osd->osd_slots, M_OSD);
+			}
+		} else {
 			newptr = realloc(osd->osd_slots, sizeof(void *) * slot,
 			    M_OSD, M_NOWAIT | M_ZERO);
 			if (newptr == NULL) {
@@ -243,11 +259,22 @@ osd_set(u_int type, struct osd *osd, u_int slot, void *value)
 				    &tracker);
 				return (ENOMEM);
 			}
-			osd->osd_slots = newptr;
-			osd->osd_nslots = slot;
-			OSD_DEBUG("Growing slots array (type=%u).", type);
 		}
-	}
+		if (osd->osd_nslots == 0) {
+			/*
+			 * First OSD for this object, so we need to put it
+			 * onto the list.
+			 */
+			mtx_lock(&osdm[type].osd_list_lock);
+			LIST_INSERT_HEAD(&osdm[type].osd_list, osd, osd_next);
+			mtx_unlock(&osdm[type].osd_list_lock);
+			OSD_DEBUG("Setting first slot (type=%u).", type);
+		} else
+			OSD_DEBUG("Growing slots array (type=%u).", type);
+		osd->osd_slots = newptr;
+		osd->osd_nslots = slot;
+	} else if (rsv)
+		osd_free_reserved(rsv);
 	OSD_DEBUG("Setting slot value (type=%u, slot=%u, value=%p).", type,
 	    slot, value);
 	osd->osd_slots[slot - 1] = value;
@@ -255,6 +282,14 @@ osd_set(u_int type, struct osd *osd, u_int slot, void *value)
 	return (0);
 }
 
+void
+osd_free_reserved(void *rsv)
+{
+
+	OSD_DEBUG("Discarding reserved slot array.");
+	free(rsv, M_OSD);
+}
+
 void *
 osd_get(u_int type, struct osd *osd, u_int slot)
 {
diff --git a/sys/sys/osd.h b/sys/sys/osd.h
index 14316ae30cdb..820e0f49ce96 100644
--- a/sys/sys/osd.h
+++ b/sys/sys/osd.h
@@ -59,6 +59,10 @@ int osd_register(u_int type, osd_destructor_t destructor,
 void osd_deregister(u_int type, u_int slot);
 
 int osd_set(u_int type, struct osd *osd, u_int slot, void *value);
+void *osd_reserve(u_int slot);
+int osd_set_reserved(u_int type, struct osd *osd, u_int slot, void *rsv,
+    void *value);
+void osd_free_reserved(void *rsv);
 void *osd_get(u_int type, struct osd *osd, u_int slot);
 void osd_del(u_int type, struct osd *osd, u_int slot);
 int osd_call(u_int type, u_int method, void *obj, void *data);
@@ -71,6 +75,8 @@ void osd_exit(u_int type, struct osd *osd);
 	osd_deregister(OSD_THREAD, (slot))
 #define	osd_thread_set(td, slot, value)					\
 	osd_set(OSD_THREAD, &(td)->td_osd, (slot), (value))
+#define	osd_thread_set_reserved(td, slot, rsv, value)			\
+	osd_set_reserved(OSD_THREAD, &(td)->td_osd, (slot), (rsv), (value))
 #define	osd_thread_get(td, slot)					\
 	osd_get(OSD_THREAD, &(td)->td_osd, (slot))
 #define	osd_thread_del(td, slot)	do {				\
@@ -88,6 +94,8 @@ void osd_exit(u_int type, struct osd *osd);
 	osd_deregister(OSD_JAIL, (slot))
 #define	osd_jail_set(pr, slot, value)					\
 	osd_set(OSD_JAIL, &(pr)->pr_osd, (slot), (value))
+#define	osd_jail_set_reserved(pr, slot, rsv, value)			\
+	osd_set_reserved(OSD_JAIL, &(pr)->pr_osd, (slot), (rsv), (value))
 #define	osd_jail_get(pr, slot)						\
 	osd_get(OSD_JAIL, &(pr)->pr_osd, (slot))
 #define	osd_jail_del(pr, slot)						\