fault_in_iov_iter_writeable
fault_in_iov_iter_writeable(to, left);
window_size -= fault_in_iov_iter_writeable(to, window_size);
window_size -= fault_in_iov_iter_writeable(to, window_size);
if (fault_in_iov_iter_writeable(iter, left)) {
size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t bytes);
EXPORT_SYMBOL(fault_in_iov_iter_writeable);