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Commit 3bcadd6f authored by Matthew Wilcox's avatar Matthew Wilcox Committed by Linus Torvalds
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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: default avatarMatthew Wilcox <willy@linux.intel.com>
Suggested-by: default avatarNeil Brown <neilb@suse.de>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Kirill Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jan Kara <jack@suse.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 78a9be0a

include/linux/radix-tree.h

+23 −15
Original line number Diff line number Diff line
@@ -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);

}



/**