Man page 9 : page_cache_async_readahead

page_cache_async_readahead - file readahead for marked pages

void page_cache_async_readahead(struct address_space *  mapping , struct file_ra_state *  ra , struct file *  filp , struct page *  page , pgoff_t  offset , unsigned long  req_size );

mapping
    address_space which holds the pagecache and I/O vectors

ra
    file_ra_state which holds the readahead state

filp
    passed on to ->readpage and ->readpages

page
    the page at offset which has the PG_readahead flag set

offset
    start offset into mapping, in pagecache page-sized units

req_size
    hint: total size of the read which the caller is performing in pagecache pages

page_cache_async_readahead should be called when a page is used which has the PG_readahead flag; this is a marker to suggest that the application has used up enough of the readahead window that we should start pulling in more pages.