readahead: properly shorten readahead when falling back to do_page_cache_ra()

When we succeed in creating some folios in page_cache_ra_order() but then
need to fallback to single page folios, we don't shorten the amount to
read passed to do_page_cache_ra() by the amount we've already read.  This
then results in reading more and also in placing another readahead mark in
the middle of the readahead window which confuses readahead code.  Fix the
problem by properly reducing number of pages to read.

Link: https://lkml.kernel.org/r/20240625101909.12234-3-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

@@ -491,7 +491,8 @@ void page_cache_ra_order(struct readahead_control *ractl,
 		struct file_ra_state *ra, unsigned int new_order)
 {
 	struct address_space *mapping = ractl->mapping;
-	pgoff_t index = readahead_index(ractl);
+	pgoff_t start = readahead_index(ractl);
+	pgoff_t index = start;
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
 	unsigned int nofs;
@@ -544,7 +545,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	if (!err)
 		return;
 fallback:
-	do_page_cache_ra(ractl, ra->size, ra->async_size);
+	do_page_cache_ra(ractl, ra->size - (index - start), ra->async_size);
 }
 
 /*