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author | Matthew Wilcox <willy@linux.intel.com> | 2016-05-20 17:03:54 -0700 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2016-05-20 17:58:30 -0700 |
commit | 3bcadd6fa6c4fd07ace3626357c824eb532488a6 (patch) | |
tree | 4e0fee97bdc25e76f7a786e1049a9aab6208d518 | |
parent | 78a9be0a0a3367b94af242632c525d22b26f1a87 (diff) | |
download | linux-stable-3bcadd6fa6c4fd07ace3626357c824eb532488a6.tar.gz linux-stable-3bcadd6fa6c4fd07ace3626357c824eb532488a6.tar.bz2 linux-stable-3bcadd6fa6c4fd07ace3626357c824eb532488a6.zip |
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 <willy@linux.intel.com>
Suggested-by: Neil 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r-- | include/linux/radix-tree.h | 38 |
1 files changed, 23 insertions, 15 deletions
diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h index c2f69e25ba86..cb4b7e8cee81 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); } /** |