[PATCH] vmscan: count writeback pages in nr_scanned

OOM kills have been observed with 70% of the pages in lowmem being in the
writeback state.  If we count those pages in sc->nr_scanned, the VM should
throttle and wait for IO completion, instead of OOM killing.

(akpm: this is how the code was designed to work - we broke it six months
ago).
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/vmscan.c

@@ -369,14 +369,14 @@ static int shrink_list(struct list_head *page_list, struct scan_control *sc)
 
 		BUG_ON(PageActive(page));
 
-		if (PageWriteback(page))
-			goto keep_locked;
-
 		sc->nr_scanned++;
 		/* Double the slab pressure for mapped and swapcache pages */
 		if (page_mapped(page) || PageSwapCache(page))
 			sc->nr_scanned++;
 
+		if (PageWriteback(page))
+			goto keep_locked;
+
 		referenced = page_referenced(page, 1, sc->priority <= 0);
 		/* In active use or really unfreeable?  Activate it. */
 		if (referenced && page_mapping_inuse(page))