This patch reduces radix tree node memory usage by about a factor of 4
on many small files (< 64K) scenarios, and results in perfect packing of
the index range into 32 and 64 bits. There are pointer traversal and
memory usage costs for large files with dense pagecache.

Index: linux-2.6/lib/radix-tree.c
===================================================================
--- linux-2.6.orig/lib/radix-tree.c
+++ linux-2.6/lib/radix-tree.c
@@ -33,7 +33,7 @@
 
 
 #ifdef __KERNEL__
-#define RADIX_TREE_MAP_SHIFT	6
+#define RADIX_TREE_MAP_SHIFT	(CONFIG_BASE_SMALL ? 4 : 6)
 #else
 #define RADIX_TREE_MAP_SHIFT	3	/* For more stressful testing */
 #endif