do_iter_readv_writev
ret = do_iter_readv_writev(file, &iter, pos, READ, flags);
ret = do_iter_readv_writev(file, &iter, pos, WRITE, flags);
ret = do_iter_readv_writev(file, iter, ppos, READ, flags);
ret = do_iter_readv_writev(file, iter, ppos, WRITE, flags);