MFC r277649:

vmspace_release() may sleep if the last reference is being released,
   so add a WITNESS_WARN() to catch cases where it is called with a
   non-sleepable lock held.

   MFC after:    1 month
   Sponsored by: Sandvine Inc.

sys/vm/vm_map.c

@@ -342,6 +342,9 @@ void
 vmspace_free(struct vmspace *vm)
 {
 
+	WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
+	    "vmspace_free() called with non-sleepable lock held");
+
 	if (vm->vm_refcnt == 0)
 		panic("vmspace_free: attempt to free already freed vmspace");