Q: Given the following code slice:
```
1 void page_cache_ra_order(struct readahead_control *ractl,
2 		struct file_ra_state *ra, unsigned int new_order)
5 	pgoff_t index = readahead_index(ractl);
7 	pgoff_t mark = index + ra->size - ra->async_size;```
which has a CWE-457 vulnerability at line:
```
7 	pgoff_t mark = index + ra->size - ra->async_size;
```
Please analyze the root cause of the vulnerability.