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.

[akpm@linux-foundation.org: assert that irqs are disabled, for __inc_lruvec_page_state()]
[akpm@linux-foundation.org: s/WARN_ON_ONCE/VM_WARN_ON_ONCE/, per Johannes]
Link: http://lkml.kernel.org/r/20181009184732.762-4-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -161,6 +161,7 @@ enum node_stat_item {
 	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

@@ -1143,6 +1143,7 @@ const char * const vmstat_text[] = {
 	"nr_slab_unreclaimable",
 	"nr_isolated_anon",
 	"nr_isolated_file",
+	"workingset_nodes",
 	"workingset_refault",
 	"workingset_activate",
 	"workingset_restore",

mm/workingset.c

@@ -377,12 +377,20 @@ void workingset_update_node(struct radix_tree_node *node)
 	 * 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);
+		}
 	}
 }
 
@@ -473,6 +481,8 @@ static enum lru_status shadow_lru_isolate(struct list_head *item,
 	}
 
 	list_lru_isolate(lru, item);
+	__dec_lruvec_page_state(virt_to_page(node), WORKINGSET_NODES);
+
 	spin_unlock(lru_lock);
 
 	/*