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

This reverts commit 7c877586da3178974a8a94577b6045a48377ff25.

Anders and Philippe have reported that recent kernels occasionally hang when used with NFS in readahead code. The problem has been bisected to 7c877586da3 ("readahead: properly shorten readahead when falling back to do_page_cache_ra()"). The cause of the problem is that ra->size can be shrunk by read_pages() call and subsequently we end up calling do_page_cache_ra() with negative (read huge positive) number of pages. Let's revert 7c877586da3 for now until we can find a proper way how the logic in read_pages() and page_cache_ra_order() can coexist. This can lead to reduced readahead throughput due to readahead window confusion but that's better than outright hangs.

Reported-by Anders Blomdell <[email protected]>

Reported-by Philippe Troin <[email protected]>

CC [email protected]

Signed-off-by Jan Kara <[email protected]>

---
 mm/readahead.c | 5 ++---

 1 file changed, 2 insertions(+), 3 deletions(-)

mm/readahead.c +1 -2

index 8f1cf599b572..ea650b8b02fb 100644
@@ -458,8 +458,7 @@ 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 start = readahead_index(ractl);
-	pgoff_t index = start;
+	pgoff_t index = readahead_index(ractl);
  	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;

@@ -522,7 +521,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
  	if (!err)
  		return;

 fallback:

mm/readahead.c +1 -1

-	do_page_cache_ra(ractl, ra->size - (index - start), ra->async_size);
+	do_page_cache_ra(ractl, ra->size, ra->async_size);

 }

 static unsigned long ractl_max_pages(struct readahead_control *ractl,