radix-tree: add radix_tree_prev_hole()

The counterpart of radix_tree_next_hole(). To be used by context readahead. Signed-off-by: Wu Fengguang <[email protected]> Cc: Vladislav Bolkhovitin <[email protected]> Cc: Jens Axboe <[email protected]> Cc: Jeff Moyer <[email protected]> Cc: Nick Piggin <[email protected]> Cc: Ying Han <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
TechNexion · Jun 17, 2009 · dc56612 · dc56612
1 parent d30a110
commit dc56612
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diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
@@ -167,6 +167,8 @@ radix_tree_gang_lookup_slot(struct radix_tree_root *root, void ***results,
 			unsigned long first_index, unsigned int max_items);
 unsigned long radix_tree_next_hole(struct radix_tree_root *root,
 				unsigned long index, unsigned long max_scan);
+unsigned long radix_tree_prev_hole(struct radix_tree_root *root,
+				unsigned long index, unsigned long max_scan);
 int radix_tree_preload(gfp_t gfp_mask);
 void radix_tree_init(void);
 void *radix_tree_tag_set(struct radix_tree_root *root,

diff --git a/lib/radix-tree.c b/lib/radix-tree.c
@@ -666,6 +666,43 @@ unsigned long radix_tree_next_hole(struct radix_tree_root *root,
 }
 EXPORT_SYMBOL(radix_tree_next_hole);
 
+/**
+ *	radix_tree_prev_hole    -    find the prev hole (not-present entry)
+ *	@root:		tree root
+ *	@index:		index key
+ *	@max_scan:	maximum range to search
+ *
+ *	Search backwards in the range [max(index-max_scan+1, 0), index]
+ *	for the first hole.
+ *
+ *	Returns: the index of the hole if found, otherwise returns an index
+ *	outside of the set specified (in which case 'index - return >= max_scan'
+ *	will be true). In rare cases of wrap-around, LONG_MAX will be returned.
+ *
+ *	radix_tree_next_hole may be called under rcu_read_lock. However, like
+ *	radix_tree_gang_lookup, this will not atomically search a snapshot of
+ *	the tree at a single point in time. For example, if a hole is created
+ *	at index 10, then subsequently a hole is created at index 5,
+ *	radix_tree_prev_hole covering both indexes may return 5 if called under
+ *	rcu_read_lock.
+ */
+unsigned long radix_tree_prev_hole(struct radix_tree_root *root,
+				   unsigned long index, unsigned long max_scan)
+{
+	unsigned long i;
+
+	for (i = 0; i < max_scan; i++) {
+		if (!radix_tree_lookup(root, index))
+			break;
+		index--;
+		if (index == LONG_MAX)
+			break;
+	}
+
+	return index;
+}
+EXPORT_SYMBOL(radix_tree_prev_hole);
+
 static unsigned int
 __lookup(struct radix_tree_node *slot, void ***results, unsigned long index,
 	unsigned int max_items, unsigned long *next_index)