zone_reclaim: do not unmap file backed pages

zone_reclaim should leave that to the real swapper. If a file backed
page is mapped to a process then its not easily reclaimable so leave
it alone.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.16-rc1-mm2/mm/vmscan.c
===================================================================
--- linux-2.6.16-rc1-mm2.orig/mm/vmscan.c	2006-01-23 10:02:23.000000000 -0800
+++ linux-2.6.16-rc1-mm2/mm/vmscan.c	2006-01-23 10:02:23.000000000 -0800
@@ -476,6 +476,12 @@ static int shrink_list(struct list_head 
 		 * processes. Try to unmap it here.
 		 */
 		if (page_mapped(page) && mapping) {
+			/*
+			 * No unmapping if we do not swap
+			 */
+			if (!sc->may_swap)
+				goto keep_locked;
+
 			switch (try_to_unmap(page, 0)) {
 			case SWAP_FAIL:
 				goto activate_locked;