Drain pages in batch units

drain_node_pages() currently drains the complete pageset of all pages. If there
are a large number of pages in the queues then we may hold off interrupts for
too long.

Duplicate the method used in free_hot_cold_page. Only drain pcp->batch pages
at one time.

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

Index: linux-2.6.19-rc5-mm2/mm/page_alloc.c
===================================================================
--- linux-2.6.19-rc5-mm2.orig/mm/page_alloc.c	2006-11-17 13:28:39.492284421 -0600
+++ linux-2.6.19-rc5-mm2/mm/page_alloc.c	2006-11-21 14:53:39.313626619 -0600
@@ -705,9 +705,15 @@ void drain_node_pages(int nodeid)
 
 			pcp = &pset->pcp[i];
 			if (pcp->count) {
+				int to_drain;
+
 				local_irq_save(flags);
-				free_pages_bulk(zone, pcp->count, &pcp->list, 0);
-				pcp->count = 0;
+				if (pcp->count >= pcp->batch)
+					to_drain = pcp->batch;
+				else
+					to_drain = pcp->count;
+				free_pages_bulk(zone, to_drain, &pcp->list, 0);
+				pcp->count -= to_drain;
 				local_irq_restore(flags);
 			}
 		}