SBEFIFO_MAX_FFDC_SIZE
ffdc = vmalloc(SBEFIFO_MAX_FFDC_SIZE);
ffdc_iov.iov_len = SBEFIFO_MAX_FFDC_SIZE;
iov_iter_kvec(&ffdc_iter, ITER_DEST, &ffdc_iov, 1, SBEFIFO_MAX_FFDC_SIZE);
ffdc_sz = SBEFIFO_MAX_FFDC_SIZE - iov_iter_count(&ffdc_iter);