readahead: drop dead code in ondemand_readahead()

ondemand_readahead() scales up the readahead window if the current read
would hit the readahead mark placed by itself.  However the condition is
mostly dead code because:

a) In case of async readahead we always increase ra->start so ra->start
   == index is never true.

b) In case of sync readahead we either go through
   try_context_readahead() in which case ra->async_size == 1 < ra->size or
   we go through initial_readahead where ra->async_size == ra->size iff
   ra->size == max_pages.

So the only practical effect is reducing async_size for large initial
reads.  Make the code more obvious.

Link: https://lkml.kernel.org/r/20240625101909.12234-7-jack@suse.czSigned-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

@@ -549,7 +549,6 @@ static void ondemand_readahead(struct readahead_control *ractl,
 	struct backing_dev_info *bdi = inode_to_bdi(ractl->mapping->host);
 	struct file_ra_state *ra = ractl->ra;
 	unsigned long max_pages = ra->ra_pages;
-	unsigned long add_pages;
 	pgoff_t index = readahead_index(ractl);
 	pgoff_t expected, prev_index;
 	unsigned int order = folio ? folio_order(folio) : 0;
@@ -638,26 +637,10 @@ static void ondemand_readahead(struct readahead_control *ractl,
 initial_readahead:
 	ra->start = index;
 	ra->size = get_init_ra_size(req_size, max_pages);
-	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
+	ra->async_size = ra->size > req_size ? ra->size - req_size :
+					       ra->size >> 1;
 
 readit:
-	/*
-	 * Will this read hit the readahead marker made by itself?
-	 * If so, trigger the readahead marker hit now, and merge
-	 * the resulted next readahead window into the current one.
-	 * Take care of maximum IO pages as above.
-	 */
-	if (index == ra->start && ra->size == ra->async_size) {
-		add_pages = get_next_ra_size(ra, max_pages);
-		if (ra->size + add_pages <= max_pages) {
-			ra->async_size = add_pages;
-			ra->size += add_pages;
-		} else {
-			ra->size = max_pages;
-			ra->async_size = max_pages >> 1;
-		}
-	}
-
 	ractl->_index = ra->start;
 	page_cache_ra_order(ractl, ra, order);
 }