mmap device-drivers

[Manuel Stühn]

Hi,
is it possible for a device driver to keep track if there are still
active mmap() pointers pointing to that device?
  
Linux does have something like
  
struct vm_operations_struct vm_ops =
{
                .close = vm_close,
};
  
where vm_close gets called when any mmapped pointer goes out of scope and the
driver can refcount the mappers.
  
Is there something similar in freebsd?