readahead() populates the page cache with data from a file so that subsequent
reads from that file will not block on disk I/O.
fd argument is a file descriptor identifying the file which is
to be read.
offset argument specifies the starting point from which data is to be read
count specifies the number of bytes to be read.
I/O is performed in whole pages, so that
offset is effectively rounded down to a page boundary
and bytes are read up to the next page boundary greater than or
readahead() does not read beyond the end of the file.
readahead() blocks until the specified data has been read.
The current file offset of the open file referred to by
fd is left unchanged.