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

This reverts commit 7c87758.

Anders and Philippe have reported that recent kernels occasionally hang
when used with NFS in readahead code.  The problem has been bisected to
7c87758 ("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 7c87758 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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 7c87758 ("readahead: properly shorten readahead when falling back to do_page_cache_ra()")
Reported-by: Anders Blomdell <[email protected]>
Reported-by: Philippe Troin <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Tested-by: Philippe Troin <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: jankara

mm/readahead.c

@@ -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:
-	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,