radix-tree: introduce radix_tree_load_root()

All the tree walking functions start with some variant of this code;
centralise it in one place so we're not chasing subtly different bugs
everywhere.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Kirill Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jan Kara <jack@suse.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/radix-tree.c

@@ -405,6 +405,29 @@ static inline unsigned long radix_tree_maxindex(unsigned int height)
 	return height_to_maxindex[height];
 }
 
+static inline unsigned long node_maxindex(struct radix_tree_node *node)
+{
+	return radix_tree_maxindex(node->path & RADIX_TREE_HEIGHT_MASK);
+}
+
+static unsigned radix_tree_load_root(struct radix_tree_root *root,
+		struct radix_tree_node **nodep, unsigned long *maxindex)
+{
+	struct radix_tree_node *node = rcu_dereference_raw(root->rnode);
+
+	*nodep = node;
+
+	if (likely(radix_tree_is_indirect_ptr(node))) {
+		node = indirect_to_ptr(node);
+		*maxindex = node_maxindex(node);
+		return (node->path & RADIX_TREE_HEIGHT_MASK) *
+			RADIX_TREE_MAP_SHIFT;
+	}
+
+	*maxindex = 0;
+	return 0;
+}
+
 /*
  *	Extend a radix tree so it can store key @index.
  */