radix-tree: Store a pointer to the root in each node

Instead of having this mysterious private_data in each radix_tree_node,
store a pointer to the root, which can be useful for debugging.  This also
relieves the mm code from the duty of updating it.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

include/linux/radix-tree.h

@@ -96,7 +96,7 @@ struct radix_tree_node {
 	unsigned char	count;		/* Total entry count */
 	unsigned char	exceptional;	/* Exceptional entry count */
 	struct radix_tree_node *parent;		/* Used when ascending tree */
-	void *private_data;			/* For tree user */
+	struct radix_tree_root *root;		/* The tree we belong to */
 	union {
 		struct list_head private_list;	/* For tree user */
 		struct rcu_head	rcu_head;	/* Used when freeing node */

lib/radix-tree.c

@@ -374,6 +374,7 @@ static void ida_dump(struct ida *ida)
  */
 static struct radix_tree_node *
 radix_tree_node_alloc(gfp_t gfp_mask, struct radix_tree_node *parent,
+			struct radix_tree_root *root,
 			unsigned int shift, unsigned int offset,
 			unsigned int count, unsigned int exceptional)
 {
@@ -419,11 +420,12 @@ radix_tree_node_alloc(gfp_t gfp_mask, struct radix_tree_node *parent,
 out:
 	BUG_ON(radix_tree_is_internal_node(ret));
 	if (ret) {
-		ret->parent = parent;
 		ret->shift = shift;
 		ret->offset = offset;
 		ret->count = count;
 		ret->exceptional = exceptional;
+		ret->parent = parent;
+		ret->root = root;
 	}
 	return ret;
 }
@@ -631,7 +633,7 @@ static int radix_tree_extend(struct radix_tree_root *root, gfp_t gfp,
 
 	do {
 		struct radix_tree_node *node = radix_tree_node_alloc(gfp, NULL,
-								shift, 0, 1, 0);
+							root, shift, 0, 1, 0);
 		if (!node)
 			return -ENOMEM;
 
@@ -828,7 +830,7 @@ int __radix_tree_create(struct radix_tree_root *root, unsigned long index,
 		shift -= RADIX_TREE_MAP_SHIFT;
 		if (child == NULL) {
 			/* Have to add a child node.  */
-			child = radix_tree_node_alloc(gfp, node, shift,
+			child = radix_tree_node_alloc(gfp, node, root, shift,
 							offset, 0, 0);
 			if (!child)
 				return -ENOMEM;
@@ -1330,7 +1332,7 @@ int radix_tree_split(struct radix_tree_root *root, unsigned long index,
 
 	for (;;) {
 		if (node->shift > order) {
-			child = radix_tree_node_alloc(gfp, node,
+			child = radix_tree_node_alloc(gfp, node, root,
 					node->shift - RADIX_TREE_MAP_SHIFT,
 					offset, 0, 0);
 			if (!child)
@@ -2152,8 +2154,8 @@ void **idr_get_free(struct radix_tree_root *root,
 		shift -= RADIX_TREE_MAP_SHIFT;
 		if (child == NULL) {
 			/* Have to add a child node.  */
-			child = radix_tree_node_alloc(gfp, node, shift, offset,
-							0, 0);
+			child = radix_tree_node_alloc(gfp, node, root, shift,
+							offset, 0, 0);
 			if (!child)
 				return ERR_PTR(-ENOMEM);
 			all_tag_set(child, IDR_FREE);

mm/workingset.c

@@ -354,10 +354,8 @@ void workingset_update_node(struct radix_tree_node *node, void *private)
 	 * as node->private_list is protected by &mapping->tree_lock.
 	 */
 	if (node->count && node->count == node->exceptional) {
-		if (list_empty(&node->private_list)) {
-			node->private_data = mapping;
+		if (list_empty(&node->private_list))
 			list_lru_add(&shadow_nodes, &node->private_list);
-		}
 	} else {
 		if (!list_empty(&node->private_list))
 			list_lru_del(&shadow_nodes, &node->private_list);
@@ -435,7 +433,7 @@ static enum lru_status shadow_lru_isolate(struct list_head *item,
 	 */
 
 	node = container_of(item, struct radix_tree_node, private_list);
-	mapping = node->private_data;
+	mapping = container_of(node->root, struct address_space, page_tree);
 
 	/* Coming from the list, invert the lock order */
 	if (!spin_trylock(&mapping->tree_lock)) {