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 <neilb@xxxxxxx>
Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxxxx>
---
include/linux/radix-tree.h | 38 +++++++++++++++++++++++---------------
1 file changed, 23 insertions(+), 15 deletions(-)
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index c2f69e2..cb4b7e8 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -29,28 +29,37 @@
#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.
- *
- * 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.
+ * The bottom two bits of the slot determine how the remaining bits in the
+ * slot are interpreted:
+ *
+ * 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 ***/
@@ -236,8 +245,7 @@ static inline int radix_tree_exceptional_entry(void *arg)
*/
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);
}
/**
--
2.8.0.rc3
--
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