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.  Unlike previous
attempt at this fix (commit 7c87758) which had to be reverted, we are
now careful to check there is indeed something to read so that we don't
submit negative-sized readahead.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jan Kara <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: jankara

mm/readahead.c

@@ -448,7 +448,8 @@ 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;
 	unsigned int min_order = mapping_min_folio_order(mapping);
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
@@ -506,12 +507,18 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	/*
 	 * If there were already pages in the page cache, then we may have
 	 * left some gaps.  Let the regular readahead code take care of this
-	 * situation.
+	 * situation below.
 	 */
 	if (!err)
 		return;
 fallback:
-	do_page_cache_ra(ractl, ra->size, ra->async_size);
+	/*
+	 * ->readahead() may have updated readahead window size so we have to
+	 * check there's still something to read.
+	 */
+	if (ra->size > index - start)
+		do_page_cache_ra(ractl, ra->size - (index - start),
+				 ra->async_size);
 }
 
 static unsigned long ractl_max_pages(struct readahead_control *ractl,