From: Wu Fengguang <wfg@mail.ustc.edu.cn>

Introduce a pair of functions to scan radix tree for hole/empty item.
	- radix_tree_scan_hole(root, index, max_scan)
	- radix_tree_scan_hole_backward(root, index, max_scan)

The implementation is dumb and obviously correct.
It will help to debug the smart ones to be introduced in future.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 include/linux/radix-tree.h |    4 +
 lib/radix-tree.c           |   71 +++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff -puN include/linux/radix-tree.h~radixtree-introduce-radix_tree_scan_hole include/linux/radix-tree.h
--- a/include/linux/radix-tree.h~radixtree-introduce-radix_tree_scan_hole
+++ a/include/linux/radix-tree.h
@@ -156,6 +156,10 @@ void *radix_tree_delete(struct radix_tre
 unsigned int
 radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
 			unsigned long first_index, unsigned int max_items);
+unsigned long radix_tree_scan_hole_backward(struct radix_tree_root *root,
+				unsigned long index, unsigned long max_scan);
+unsigned long radix_tree_scan_hole(struct radix_tree_root *root,
+				unsigned long index, unsigned long max_scan);
 int radix_tree_preload(gfp_t gfp_mask);
 void radix_tree_init(void);
 void *radix_tree_tag_set(struct radix_tree_root *root,
diff -puN lib/radix-tree.c~radixtree-introduce-radix_tree_scan_hole lib/radix-tree.c
--- a/lib/radix-tree.c~radixtree-introduce-radix_tree_scan_hole
+++ a/lib/radix-tree.c
@@ -598,6 +598,77 @@ int radix_tree_tag_get(struct radix_tree
 EXPORT_SYMBOL(radix_tree_tag_get);
 #endif
 
+static unsigned long radix_tree_scan_hole_dumb(struct radix_tree_root *root,
+				unsigned long index, unsigned long max_scan)
+{
+	unsigned long i;
+
+	for (i = 0; i < max_scan; i++) {
+		if (!radix_tree_lookup(root, index + i))
+			break;
+		if (index + i == ULONG_MAX)
+			break;
+	}
+
+	return index + i;
+}
+
+static unsigned long radix_tree_scan_hole_backward_dumb(
+				struct radix_tree_root *root,
+				unsigned long index, unsigned long max_scan)
+{
+	unsigned long i;
+
+	for (i = 0; i < max_scan; i++) {
+		if (!radix_tree_lookup(root, index - i))
+			break;
+		if (index - i == 0)
+			break;
+	}
+
+	return index - i;
+}
+
+/**
+ *	radix_tree_scan_hole    -    scan for hole
+ *	@root:		radix tree root
+ *	@index:		index key
+ *	@max_scan:      advice on max items to scan (it may scan a little more)
+ *
+ *      Scan forward from @index for a hole/empty item, stop when
+ *      - hit hole
+ *      - hit index ULONG_MAX
+ *      - @max_scan or more items scanned
+ *
+ *      One can be sure that (@returned_index >= @index).
+ */
+unsigned long radix_tree_scan_hole(struct radix_tree_root *root,
+				unsigned long index, unsigned long max_scan)
+{
+	return radix_tree_scan_hole_dumb(root, index, max_scan);
+}
+EXPORT_SYMBOL(radix_tree_scan_hole);
+
+/**
+ *	radix_tree_scan_hole_backward    -    scan backward for hole
+ *	@root:		radix tree root
+ *	@index:		index key
+ *	@max_scan:      advice on max items to scan (it may scan a little more)
+ *
+ *      Scan backward from @index for a hole/empty item, stop when
+ *      - hit hole
+ *      - hit index 0
+ *      - @max_scan or more items scanned
+ *
+ *      One can be sure that (@returned_index <= @index).
+ */
+unsigned long radix_tree_scan_hole_backward(struct radix_tree_root *root,
+				unsigned long index, unsigned long max_scan)
+{
+	return radix_tree_scan_hole_backward_dumb(root, index, max_scan);
+}
+EXPORT_SYMBOL(radix_tree_scan_hole_backward);
+
 static unsigned int
 __lookup(struct radix_tree_node *slot, void **results, unsigned long index,
 	unsigned int max_items, unsigned long *next_index)
_