bcachefs: reserve path idx 0 for sentinal

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_iter.c

@@ -2515,7 +2515,7 @@ static void btree_trans_verify_sorted_refs(struct btree_trans *trans)
 	struct btree_path *path;
 	unsigned i;
 
-	BUG_ON(trans->nr_sorted != bitmap_weight(trans->paths_allocated, BTREE_ITER_MAX));
+	BUG_ON(trans->nr_sorted != bitmap_weight(trans->paths_allocated, BTREE_ITER_MAX) - 1);
 
 	trans_for_each_path(trans, path) {
 		BUG_ON(path->sorted_idx >= trans->nr_sorted);
@@ -2896,6 +2896,8 @@ struct btree_trans *__bch2_trans_get(struct bch_fs *c, unsigned fn_idx)
 		atomic_inc_not_zero(&c->journal_keys.ref);
 	closure_init_stack(&trans->ref);
 
+	trans->paths_allocated[0] = 1;
+
 	s = btree_trans_stats(trans);
 	if (s && s->max_mem) {
 		unsigned expected_mem_bytes = roundup_pow_of_two(s->max_mem);

fs/bcachefs/btree_iter.h

@@ -80,7 +80,7 @@ __trans_next_path(struct btree_trans *trans, unsigned idx)
 	     _path = __trans_next_path((_trans), (_path)->idx + 1))
 
 #define trans_for_each_path(_trans, _path)				\
-	trans_for_each_path_from(_trans, _path, 0)
+	trans_for_each_path_from(_trans, _path, 1)
 
 static inline struct btree_path *
 __trans_next_path_safe(struct btree_trans *trans, unsigned *idx)
@@ -103,7 +103,7 @@ __trans_next_path_safe(struct btree_trans *trans, unsigned *idx)
 	     _idx++)
 
 #define trans_for_each_path_safe(_trans, _path, _idx)			\
-	trans_for_each_path_safe_from(_trans, _path, _idx, 0)
+	trans_for_each_path_safe_from(_trans, _path, _idx, 1)
 
 static inline struct btree_path *next_btree_path(struct btree_trans *trans, struct btree_path *path)
 {
@@ -155,7 +155,7 @@ __trans_next_path_with_node(struct btree_trans *trans, struct btree *b,
 }
 
 #define trans_for_each_path_with_node(_trans, _b, _path)		\
-	for (_path = __trans_next_path_with_node((_trans), (_b), 0);	\
+	for (_path = __trans_next_path_with_node((_trans), (_b), 1);	\
 	     (_path);							\
 	     _path = __trans_next_path_with_node((_trans), (_b),	\
 						 (_path)->idx + 1))