radix-tree: free up the bottom bit of exceptional entries for reuse

We are guaranteed that pointers to radix_tree_nodes always have the
bottom two bits clear (because they come from a slab cache, and slab
caches have a minimum alignment of sizeof(void *)), so we can redefine
'radix_tree_is_internal_node' to only return true if the bottom two bits
have value '01'. This frees up one quarter of the potential values for
use by the user.

Idea from Neil Brown.

Signed-off-by: Matthew Wilcox <[e-mail]>
Suggested-by: Neil Brown <[e-mail]>
Cc: Konstantin Khlebnikov <[e-mail]>
Cc: Kirill Shutemov <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Ross Zwisler <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/radix-tree.h

···
 #include <linux/rcupdate.h>
 
 /*
- * Entries in the radix tree have the low bit set if they refer to a
- * radix_tree_node.  If the low bit is clear then the entry is user data.
+ * The bottom two bits of the slot determine how the remaining bits in the
+ * slot are interpreted:
  *
- * We also use the low bit to indicate that the slot will be freed in the
- * next RCU idle period, and users need to re-walk the tree to find the
- * new slot for the index that they were looking for.  See the comment in
- * radix_tree_shrink() for details.
+ * 00 - data pointer
+ * 01 - internal entry
+ * 10 - exceptional entry
+ * 11 - locked exceptional entry
+ *
+ * The internal entry may be a pointer to the next level in the tree, a
+ * sibling entry, or an indicator that the entry in this slot has been moved
+ * to another location in the tree and the lookup should be restarted.  While
+ * NULL fits the 'data pointer' pattern, it means that there is no entry in
+ * the tree for this index (no matter what level of the tree it is found at).
+ * This means that you cannot store NULL in the tree as a value for the index.
  */
-#define RADIX_TREE_INTERNAL_NODE	1
+#define RADIX_TREE_ENTRY_MASK		3UL
+#define RADIX_TREE_INTERNAL_NODE	1UL
 
 /*
- * A common use of the radix tree is to store pointers to struct pages;
- * but shmem/tmpfs needs also to store swap entries in the same tree:
- * those are marked as exceptional entries to distinguish them.
+ * Most users of the radix tree store pointers but shmem/tmpfs stores swap
+ * entries in the same tree.  They are marked as exceptional entries to
+ * distinguish them from pointers to struct page.
  * EXCEPTIONAL_ENTRY tests the bit, EXCEPTIONAL_SHIFT shifts content past it.
  */
 #define RADIX_TREE_EXCEPTIONAL_ENTRY	2
 #define RADIX_TREE_EXCEPTIONAL_SHIFT	2
 
-static inline int radix_tree_is_internal_node(void *ptr)
+static inline bool radix_tree_is_internal_node(void *ptr)
 {
-	return (int)((unsigned long)ptr & RADIX_TREE_INTERNAL_NODE);
+	return ((unsigned long)ptr & RADIX_TREE_ENTRY_MASK) ==
+				RADIX_TREE_INTERNAL_NODE;
 }
 
 /*** radix-tree API starts here ***/
···
  */
 static inline int radix_tree_exception(void *arg)
 {
-	return unlikely((unsigned long)arg &
-		(RADIX_TREE_INTERNAL_NODE | RADIX_TREE_EXCEPTIONAL_ENTRY));
+	return unlikely((unsigned long)arg & RADIX_TREE_ENTRY_MASK);
 }
 
 /**