Fairness and limited I/O bandwidth

How does this impact fairness calculations? On a heavily loaded system, where there is contention for filesystem reads (either because of bandwidth, or seek time), suppose a process is parsimoniously reading just the first hundred bytes of a file. But the kernel's readahead slurps the whole first megabyte. Will the I/O scheduler penalize that process for the whole I/O performed, or does the accounting work only on what was requested?