readahead: drop pointless index from force_page_cache_ra()

Current index to readahead is tracked in readahead_control and properly
updated by page_cache_ra_unbounded() (read_pages() in fact).  So there's
no need to track the index separately in force_page_cache_ra().

Link: https://lkml.kernel.org/r/20240625101909.12234-4-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Zhang Peng <zhangpengpeng0808@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/readahead.c

@@ -313,7 +313,7 @@ void force_page_cache_ra(struct readahead_control *ractl,
 	struct address_space *mapping = ractl->mapping;
 	struct file_ra_state *ra = ractl->ra;
 	struct backing_dev_info *bdi = inode_to_bdi(mapping->host);
-	unsigned long max_pages, index;
+	unsigned long max_pages;
 
 	if (unlikely(!mapping->a_ops->read_folio && !mapping->a_ops->readahead))
 		return;
@@ -322,18 +322,15 @@ void force_page_cache_ra(struct readahead_control *ractl,
 	 * If the request exceeds the readahead window, allow the read to
 	 * be up to the optimal hardware IO size
 	 */
-	index = readahead_index(ractl);
 	max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
 	nr_to_read = min_t(unsigned long, nr_to_read, max_pages);
 	while (nr_to_read) {
 		unsigned long this_chunk = (2 * 1024 * 1024) / PAGE_SIZE;
 
 		if (this_chunk > nr_to_read)
 			this_chunk = nr_to_read;
-		ractl->_index = index;
 		do_page_cache_ra(ractl, this_chunk, 0);
 
-		index += this_chunk;
 		nr_to_read -= this_chunk;
 	}
 }