btrfs: remove unnecessary next extent map search

At __tree_search(), and its single caller __lookup_extent_mapping(), there
is no point in finding the next extent map that starts after the search
offset if we were able to find the previous extent map that ends before
our search offset, because __lookup_extent_mapping() ignores the next
acceptable extent map if we were able to find the previous one.

So just return immediately if we were able to find the previous extent
map, therefore avoiding wasting time iterating the tree looking for the
next extent map which will not be used by __lookup_extent_mapping().

Signed-off-by: Filipe Manana <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: fdmanana

fs/btrfs/extent_map.c

@@ -141,17 +141,15 @@ static int tree_insert(struct rb_root_cached *root, struct extent_map *em)
  * it can't be found, try to find some neighboring extents
  */
 static struct rb_node *__tree_search(struct rb_root *root, u64 offset,
-				     struct rb_node **prev_ret,
-				     struct rb_node **next_ret)
+				     struct rb_node **prev_or_next_ret)
 {
 	struct rb_node *n = root->rb_node;
 	struct rb_node *prev = NULL;
 	struct rb_node *orig_prev = NULL;
 	struct extent_map *entry;
 	struct extent_map *prev_entry = NULL;
 
-	ASSERT(prev_ret);
-	ASSERT(next_ret);
+	ASSERT(prev_or_next_ret);
 
 	while (n) {
 		entry = rb_entry(n, struct extent_map, rb_node);
@@ -171,15 +169,23 @@ static struct rb_node *__tree_search(struct rb_root *root, u64 offset,
 		prev = rb_next(prev);
 		prev_entry = rb_entry(prev, struct extent_map, rb_node);
 	}
-	*prev_ret = prev;
-	prev = orig_prev;
 
+	/*
+	 * Previous extent map found, return as in this case the caller does not
+	 * care about the next one.
+	 */
+	if (prev) {
+		*prev_or_next_ret = prev;
+		return NULL;
+	}
+
+	prev = orig_prev;
 	prev_entry = rb_entry(prev, struct extent_map, rb_node);
 	while (prev && offset < prev_entry->start) {
 		prev = rb_prev(prev);
 		prev_entry = rb_entry(prev, struct extent_map, rb_node);
 	}
-	*next_ret = prev;
+	*prev_or_next_ret = prev;
 
 	return NULL;
 }
@@ -425,16 +431,13 @@ __lookup_extent_mapping(struct extent_map_tree *tree,
 {
 	struct extent_map *em;
 	struct rb_node *rb_node;
-	struct rb_node *prev = NULL;
-	struct rb_node *next = NULL;
+	struct rb_node *prev_or_next = NULL;
 	u64 end = range_end(start, len);
 
-	rb_node = __tree_search(&tree->map.rb_root, start, &prev, &next);
+	rb_node = __tree_search(&tree->map.rb_root, start, &prev_or_next);
 	if (!rb_node) {
-		if (prev)
-			rb_node = prev;
-		else if (next)
-			rb_node = next;
+		if (prev_or_next)
+			rb_node = prev_or_next;
 		else
 			return NULL;
 	}