mm: unlockless reclaim

unlock_page is fairly expensive.  It can be avoided in page reclaim
success path.  By definition if we have any other references to the page
it would be a bug anyway.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -732,7 +732,14 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 		if (!mapping || !__remove_mapping(mapping, page))
 			goto keep_locked;
 
-		unlock_page(page);
+		/*
+		 * At this point, we have no other references and there is
+		 * no way to pick any more up (removed from LRU, removed
+		 * from pagecache). Can use non-atomic bitops now (and
+		 * we obviously don't have to worry about waking up a process
+		 * waiting on the page lock, because there are no references.
+		 */
+		__clear_page_locked(page);
 free_it:
 		nr_reclaimed++;
 		if (!pagevec_add(&freed_pvec, page)) {