[PATCH 2/2] mm/readahead: simplify page_cache_ra_unbounded loop counter reset

[Frederick Mayle <fmayle@google.com>]

Minor cleanup, no behavior change intended.

`read_pages` ensures that `ractl->_nr_pages` is zero before it returns,
so the `ractl->_nr_pages` term in these expressions contributes nothing.
This seems to have been true since the statements were introduced in
commit f615bd5c4725f ("mm/readahead: Handle ractl nr_pages being
modified").

The new expression has an intuitive explanation. When filesystems
perform readahead, they increment `ractl->_index` by the number of pages
processed, so, after `read_pages` returns, `ractl->_index` points to the
first page after those already processed. `index` points to the first
page considered in the loop. So, `ractl->_index - index` is the number
of pages processed by the loop so far.

Signed-off-by: Frederick Mayle <fmayle@google.com>
---
 mm/readahead.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 23bec5497308..42f2f20633b0 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -281,7 +281,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 			 */
 			read_pages(ractl);
 			ractl->_index += min_nrpages;
-			i = ractl->_index + ractl->_nr_pages - index;
+			i = ractl->_index - index;
 			continue;
 		}
 
@@ -297,7 +297,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 				break;
 			read_pages(ractl);
 			ractl->_index += min_nrpages;
-			i = ractl->_index + ractl->_nr_pages - index;
+			i = ractl->_index - index;
 			continue;
 		}
 		if (i == mark)
-- 
2.54.0.563.g4f69b47b94-goog