mm: workingset: add vmstat counter for shadow nodes

Make it easier to catch bugs in the shadow node shrinker by adding a
counter for the shadow nodes in circulation.

[[e-mail]: assert that irqs are disabled, for __inc_lruvec_page_state()]
[[e-mail]: s/WARN_ON_ONCE/VM_WARN_ON_ONCE/, per Johannes]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Johannes Weiner <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/mmzone.h

···
 	NR_SLAB_UNRECLAIMABLE,
 	NR_ISOLATED_ANON,	/* Temporary isolated pages from anon lru */
 	NR_ISOLATED_FILE,	/* Temporary isolated pages from file lru */
+	WORKINGSET_NODES,
 	WORKINGSET_REFAULT,
 	WORKINGSET_ACTIVATE,
 	WORKINGSET_RESTORE,

mm/vmstat.c

···
 	"nr_slab_unreclaimable",
 	"nr_isolated_anon",
 	"nr_isolated_file",
+	"workingset_nodes",
 	"workingset_refault",
 	"workingset_activate",
 	"workingset_restore",

mm/workingset.c

···
 	 * already where they should be. The list_empty() test is safe
 	 * as node->private_list is protected by the i_pages lock.
 	 */
+	VM_WARN_ON_ONCE(!irqs_disabled());  /* For __inc_lruvec_page_state */
+
 	if (node->count && node->count == node->exceptional) {
-		if (list_empty(&node->private_list))
+		if (list_empty(&node->private_list)) {
 			list_lru_add(&shadow_nodes, &node->private_list);
+			__inc_lruvec_page_state(virt_to_page(node),
+						WORKINGSET_NODES);
+		}
 	} else {
-		if (!list_empty(&node->private_list))
+		if (!list_empty(&node->private_list)) {
 			list_lru_del(&shadow_nodes, &node->private_list);
+			__dec_lruvec_page_state(virt_to_page(node),
+						WORKINGSET_NODES);
+		}
 	}
 }
 
···
 	}
 
 	list_lru_isolate(lru, item);
+	__dec_lruvec_page_state(virt_to_page(node), WORKINGSET_NODES);
+
 	spin_unlock(lru_lock);
 
 	/*