[PATCH 2/2] readahead: properly shorten readahead when falling back to do_page_cache_ra()

[Jan Kara <jack@suse.cz>]

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 7c877586da31) 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.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 mm/readahead.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 78d7f4db9966..006954c76652 100644
--- a/mm/readahead.c
+++ b/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,
-- 
2.35.3