bcachefs: Optimize btree lookups in write path

This patch significantly reduces the number of btree lookups required in
the extent update path.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/btree_iter.c

@@ -1805,6 +1805,12 @@ inline struct bkey_s_c bch2_btree_path_peek_slot(struct btree_path *path, struct
 
 /* Btree iterators: */
 
+int __must_check
+__bch2_btree_iter_traverse(struct btree_iter *iter)
+{
+	return bch2_btree_path_traverse(iter->trans, iter->path, iter->flags);
+}
+
 int __must_check
 bch2_btree_iter_traverse(struct btree_iter *iter)
 {
@@ -2416,7 +2422,7 @@ static void __bch2_trans_iter_init(struct btree_trans *trans,
 	iter->path = bch2_path_get(trans,
 				   flags & BTREE_ITER_CACHED,
 				   btree_id,
-				   btree_iter_search_key(iter),
+				   iter->pos,
 				   locks_want,
 				   depth,
 				   flags & BTREE_ITER_INTENT);

fs/bcachefs/btree_iter.h

@@ -221,6 +221,7 @@ void bch2_trans_downgrade(struct btree_trans *);
 void bch2_trans_node_add(struct btree_trans *trans, struct btree *);
 void bch2_trans_node_reinit_iter(struct btree_trans *, struct btree *);
 
+int __must_check __bch2_btree_iter_traverse(struct btree_iter *iter);
 int __must_check bch2_btree_iter_traverse(struct btree_iter *);
 
 struct btree *bch2_btree_iter_peek_node(struct btree_iter *);

fs/bcachefs/btree_update_leaf.c

@@ -985,7 +985,14 @@ static int bch2_trans_update_extent(struct btree_trans *trans,
 		bch2_bkey_merge(c, bkey_i_to_s(insert), k);
 out:
 	if (!bkey_deleted(&insert->k)) {
-		bch2_btree_iter_set_pos(&iter, insert->k.p);
+		/*
+		 * Rewinding iterators is expensive: get a new one and the one
+		 * that points to the start of insert will be cloned from:
+		 */
+		bch2_trans_iter_exit(trans, &iter);
+		bch2_trans_iter_init(trans, &iter, btree_id, insert->k.p,
+				     BTREE_ITER_NOT_EXTENTS|
+				     BTREE_ITER_INTENT);
 		ret   = bch2_btree_iter_traverse(&iter) ?:
 			bch2_trans_update(trans, &iter, insert, flags);
 	}

fs/bcachefs/io.c

@@ -281,6 +281,16 @@ int bch2_extent_update(struct btree_trans *trans,
 	s64 i_sectors_delta = 0, disk_sectors_delta = 0;
 	int ret;
 
+	/*
+	 * This traverses us the iterator without changing iter->path->pos to
+	 * search_key() (which is pos + 1 for extents): we want there to be a
+	 * path already traversed at iter->pos because
+	 * bch2_trans_extent_update() will use it to attempt extent merging
+	 */
+	ret = __bch2_btree_iter_traverse(iter);
+	if (ret)
+		return ret;
+
 	ret = bch2_extent_trim_atomic(trans, iter, k);
 	if (ret)
 		return ret;