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 <fengguang.wu@intel.com>
Cc: Vladislav Bolkhovitin <vst@vlnb.net>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 39 insertions, 0 deletions

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 355f6e80db0d..c5da74918096 100644
--- 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
index 4bb42a0344ec..5301a52cdb4d 100644
--- 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)