code: func Munmap(b []byte) (err error) {
  	err = syscall.Munmap(b)
  	if err == nil {
  		atomic.AddUint64(&mapCount, ^uint64(0)) // decrement
  	}
  	return err
  }
docs: // Munmap calls sycall.Munmap, and then decrements the global map count if there
  // was no error.