bcachefs: btree_path

This splits btree_iter into two components: btree_iter is now the
externally visible componont, and it points to a btree_path which is now
reference counted.

This means we no longer have to clone iterators up front if they might
be mutated - btree_path can be shared by multiple iterators, and cloned
if an iterator would mutate a shared btree_path. This will help us use
iterators more efficiently, as well as slimming down the main long lived
state in btree_trans, and significantly cleans up the logic for iterator
lifetimes.

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

1 files changed, 1058 insertions, 1010 deletions

diff --git a/fs/bcachefs/btree_iter.c b/fs/bcachefs/btree_iter.c
index 7acec1e6db3d..06379f3e40a6 100644
--- a/fs/bcachefs/btree_iter.c
+++ b/fs/bcachefs/btree_iter.c
@@ -17,21 +17,30 @@
 
 #include <linux/prefetch.h>
 
-static void btree_iter_set_search_pos(struct btree_iter *, struct bpos);
-static inline void btree_trans_sort_iters(struct btree_trans *);
-static struct btree_iter *btree_iter_child_alloc(struct btree_trans *,
-					struct btree_iter *, unsigned long);
-static struct btree_iter *btree_trans_iter_alloc(struct btree_trans *,
-						 struct btree_iter *);
-static void btree_iter_copy(struct btree_trans *, struct btree_iter *, struct btree_iter *);
+static inline void btree_trans_sort_paths(struct btree_trans *);
 
-static inline int btree_iter_cmp(const struct btree_iter *l,
-				 const struct btree_iter *r)
+static inline void btree_path_list_remove(struct btree_trans *, struct btree_path *);
+static inline void btree_path_list_add(struct btree_trans *, struct btree_path *,
+				       struct btree_path *);
+
+static struct btree_path *btree_path_alloc(struct btree_trans *, struct btree_path *);
+
+static inline int __btree_path_cmp(const struct btree_path *l,
+				   enum btree_id	r_btree_id,
+				   bool			r_cached,
+				   struct bpos		r_pos,
+				   unsigned		r_level)
 {
-	return   cmp_int(l->btree_id,	r->btree_id) ?:
-		-cmp_int(l->cached,	r->cached) ?:
-		 bkey_cmp(l->real_pos,	r->real_pos) ?:
-		-cmp_int(l->level,	r->level);
+	return   cmp_int(l->btree_id,	r_btree_id) ?:
+		 cmp_int(l->cached,	r_cached) ?:
+		 bpos_cmp(l->pos,	r_pos) ?:
+		-cmp_int(l->level,	r_level);
+}
+
+static inline int btree_path_cmp(const struct btree_path *l,
+				 const struct btree_path *r)
+{
+	return __btree_path_cmp(l, r->btree_id, r->cached, r->pos, r->level);
 }
 
 static inline struct bpos bkey_successor(struct btree_iter *iter, struct bpos p)
@@ -60,10 +69,10 @@ static inline struct bpos bkey_predecessor(struct btree_iter *iter, struct bpos
 	return p;
 }
 
-static inline bool is_btree_node(struct btree_iter *iter, unsigned l)
+static inline bool is_btree_node(struct btree_path *path, unsigned l)
 {
 	return l < BTREE_MAX_DEPTH &&
-		(unsigned long) iter->l[l].b >= 128;
+		(unsigned long) path->l[l].b >= 128;
 }
 
 static inline struct bpos btree_iter_search_key(struct btree_iter *iter)
@@ -76,42 +85,42 @@ static inline struct bpos btree_iter_search_key(struct btree_iter *iter)
 	return pos;
 }
 
-static inline bool btree_iter_pos_before_node(struct btree_iter *iter,
+static inline bool btree_path_pos_before_node(struct btree_path *path,
 					      struct btree *b)
 {
-	return bpos_cmp(iter->real_pos, b->data->min_key) < 0;
+	return bpos_cmp(path->pos, b->data->min_key) < 0;
 }
 
-static inline bool btree_iter_pos_after_node(struct btree_iter *iter,
+static inline bool btree_path_pos_after_node(struct btree_path *path,
 					     struct btree *b)
 {
-	return bpos_cmp(b->key.k.p, iter->real_pos) < 0;
+	return bpos_cmp(b->key.k.p, path->pos) < 0;
 }
 
-static inline bool btree_iter_pos_in_node(struct btree_iter *iter,
+static inline bool btree_path_pos_in_node(struct btree_path *path,
 					  struct btree *b)
 {
-	return iter->btree_id == b->c.btree_id &&
-		!btree_iter_pos_before_node(iter, b) &&
-		!btree_iter_pos_after_node(iter, b);
+	return path->btree_id == b->c.btree_id &&
+		!btree_path_pos_before_node(path, b) &&
+		!btree_path_pos_after_node(path, b);
 }
 
 /* Btree node locking: */
 
 void bch2_btree_node_unlock_write(struct btree_trans *trans,
-			struct btree_iter *iter, struct btree *b)
+			struct btree_path *path, struct btree *b)
 {
-	bch2_btree_node_unlock_write_inlined(trans, iter, b);
+	bch2_btree_node_unlock_write_inlined(trans, path, b);
 }
 
 void __bch2_btree_node_lock_write(struct btree_trans *trans, struct btree *b)
 {
-	struct btree_iter *iter;
+	struct btree_path *linked;
 	unsigned readers = 0;
 
-	trans_for_each_iter(trans, iter)
-		if (iter->l[b->c.level].b == b &&
-		    btree_node_read_locked(iter, b->c.level))
+	trans_for_each_path(trans, linked)
+		if (linked->l[b->c.level].b == b &&
+		    btree_node_read_locked(linked, b->c.level))
 			readers++;
 
 	/*
@@ -136,21 +145,21 @@ void __bch2_btree_node_lock_write(struct btree_trans *trans, struct btree *b)
 }
 
 bool __bch2_btree_node_relock(struct btree_trans *trans,
-			      struct btree_iter *iter, unsigned level)
+			      struct btree_path *path, unsigned level)
 {
-	struct btree *b = btree_iter_node(iter, level);
-	int want = __btree_lock_want(iter, level);
+	struct btree *b = btree_path_node(path, level);
+	int want = __btree_lock_want(path, level);
 
-	if (!is_btree_node(iter, level))
+	if (!is_btree_node(path, level))
 		return false;
 
 	if (race_fault())
 		return false;
 
-	if (six_relock_type(&b->c.lock, want, iter->l[level].lock_seq) ||
-	    (btree_node_lock_seq_matches(iter, b, level) &&
+	if (six_relock_type(&b->c.lock, want, path->l[level].lock_seq) ||
+	    (btree_node_lock_seq_matches(path, b, level) &&
 	     btree_node_lock_increment(trans, b, level, want))) {
-		mark_btree_node_locked(iter, level, want);
+		mark_btree_node_locked(path, level, want);
 		return true;
 	} else {
 		return false;
@@ -158,88 +167,88 @@ bool __bch2_btree_node_relock(struct btree_trans *trans,
 }
 
 static bool bch2_btree_node_upgrade(struct btree_trans *trans,
-				    struct btree_iter *iter, unsigned level)
+				    struct btree_path *path, unsigned level)
 {
-	struct btree *b = iter->l[level].b;
+	struct btree *b = path->l[level].b;
 
-	EBUG_ON(btree_lock_want(iter, level) != BTREE_NODE_INTENT_LOCKED);
+	EBUG_ON(btree_lock_want(path, level) != BTREE_NODE_INTENT_LOCKED);
 
-	if (!is_btree_node(iter, level))
+	if (!is_btree_node(path, level))
 		return false;
 
-	if (btree_node_intent_locked(iter, level))
+	if (btree_node_intent_locked(path, level))
 		return true;
 
 	if (race_fault())
 		return false;
 
-	if (btree_node_locked(iter, level)
+	if (btree_node_locked(path, level)
 	    ? six_lock_tryupgrade(&b->c.lock)
-	    : six_relock_type(&b->c.lock, SIX_LOCK_intent, iter->l[level].lock_seq))
+	    : six_relock_type(&b->c.lock, SIX_LOCK_intent, path->l[level].lock_seq))
 		goto success;
 
-	if (btree_node_lock_seq_matches(iter, b, level) &&
+	if (btree_node_lock_seq_matches(path, b, level) &&
 	    btree_node_lock_increment(trans, b, level, BTREE_NODE_INTENT_LOCKED)) {
-		btree_node_unlock(iter, level);
+		btree_node_unlock(path, level);
 		goto success;
 	}
 
 	return false;
 success:
-	mark_btree_node_intent_locked(iter, level);
+	mark_btree_node_intent_locked(path, level);
 	return true;
 }
 
-static inline bool btree_iter_get_locks(struct btree_trans *trans,
-					struct btree_iter *iter,
+static inline bool btree_path_get_locks(struct btree_trans *trans,
+					struct btree_path *path,
 					bool upgrade, unsigned long trace_ip)
 {
-	unsigned l = iter->level;
+	unsigned l = path->level;
 	int fail_idx = -1;
 
 	do {
-		if (!btree_iter_node(iter, l))
+		if (!btree_path_node(path, l))
 			break;
 
 		if (!(upgrade
-		      ? bch2_btree_node_upgrade(trans, iter, l)
-		      : bch2_btree_node_relock(trans, iter, l))) {
+		      ? bch2_btree_node_upgrade(trans, path, l)
+		      : bch2_btree_node_relock(trans, path, l))) {
 			(upgrade
 			 ? trace_node_upgrade_fail
 			 : trace_node_relock_fail)(trans->ip, trace_ip,
-					iter->cached,
-					iter->btree_id, &iter->real_pos,
-					l, iter->l[l].lock_seq,
-					is_btree_node(iter, l)
+					path->cached,
+					path->btree_id, &path->pos,
+					l, path->l[l].lock_seq,
+					is_btree_node(path, l)
 					? 0
-					: (unsigned long) iter->l[l].b,
-					is_btree_node(iter, l)
-					? iter->l[l].b->c.lock.state.seq
+					: (unsigned long) path->l[l].b,
+					is_btree_node(path, l)
+					? path->l[l].b->c.lock.state.seq
 					: 0);
 			fail_idx = l;
-			btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+			btree_path_set_dirty(path, BTREE_ITER_NEED_TRAVERSE);
 		}
 
 		l++;
-	} while (l < iter->locks_want);
+	} while (l < path->locks_want);
 
 	/*
 	 * When we fail to get a lock, we have to ensure that any child nodes
-	 * can't be relocked so bch2_btree_iter_traverse has to walk back up to
+	 * can't be relocked so bch2_btree_path_traverse has to walk back up to
 	 * the node that we failed to relock:
 	 */
 	while (fail_idx >= 0) {
-		btree_node_unlock(iter, fail_idx);
-		iter->l[fail_idx].b = BTREE_ITER_NO_NODE_GET_LOCKS;
+		btree_node_unlock(path, fail_idx);
+		path->l[fail_idx].b = BTREE_ITER_NO_NODE_GET_LOCKS;
 		--fail_idx;
 	}
 
-	if (iter->uptodate == BTREE_ITER_NEED_RELOCK)
-		iter->uptodate = BTREE_ITER_UPTODATE;
+	if (path->uptodate == BTREE_ITER_NEED_RELOCK)
+		path->uptodate = BTREE_ITER_UPTODATE;
 
 	bch2_trans_verify_locks(trans);
 
-	return iter->uptodate < BTREE_ITER_NEED_RELOCK;
+	return path->uptodate < BTREE_ITER_NEED_RELOCK;
 }
 
 static struct bpos btree_node_pos(struct btree_bkey_cached_common *_b,
@@ -252,19 +261,20 @@ static struct bpos btree_node_pos(struct btree_bkey_cached_common *_b,
 
 /* Slowpath: */
 bool __bch2_btree_node_lock(struct btree_trans *trans,
-			    struct btree_iter *iter,
-			    struct btree *b, struct bpos pos, unsigned level,
+			    struct btree_path *path,
+			    struct btree *b,
+			    struct bpos pos, unsigned level,
 			    enum six_lock_type type,
 			    six_lock_should_sleep_fn should_sleep_fn, void *p,
 			    unsigned long ip)
 {
-	struct btree_iter *linked, *deadlock_iter = NULL;
+	struct btree_path *linked, *deadlock_path = NULL;
 	u64 start_time = local_clock();
 	unsigned reason = 9;
 	bool ret;
 
 	/* Check if it's safe to block: */
-	trans_for_each_iter(trans, linked) {
+	trans_for_each_path(trans, linked) {
 		if (!linked->nodes_locked)
 			continue;
 
@@ -282,25 +292,25 @@ bool __bch2_btree_node_lock(struct btree_trans *trans,
 		 */
 		if (type == SIX_LOCK_intent &&
 		    linked->nodes_locked != linked->nodes_intent_locked) {
-			deadlock_iter = linked;
+			deadlock_path = linked;
 			reason = 1;
 		}
 
-		if (linked->btree_id != iter->btree_id) {
-			if (linked->btree_id > iter->btree_id) {
-				deadlock_iter = linked;
+		if (linked->btree_id != path->btree_id) {
+			if (linked->btree_id > path->btree_id) {
+				deadlock_path = linked;
 				reason = 3;
 			}
 			continue;
 		}
 
 		/*
-		 * Within the same btree, cached iterators come before non
-		 * cached iterators:
+		 * Within the same btree, cached paths come before non
+		 * cached paths:
 		 */
-		if (linked->cached != iter->cached) {
-			if (iter->cached) {
-				deadlock_iter = linked;
+		if (linked->cached != path->cached) {
+			if (path->cached) {
+				deadlock_path = linked;
 				reason = 4;
 			}
 			continue;
@@ -308,11 +318,11 @@ bool __bch2_btree_node_lock(struct btree_trans *trans,
 
 		/*
 		 * Interior nodes must be locked before their descendants: if
-		 * another iterator has possible descendants locked of the node
+		 * another path has possible descendants locked of the node
 		 * we're about to lock, it must have the ancestors locked too:
 		 */
 		if (level > __fls(linked->nodes_locked)) {
-			deadlock_iter = linked;
+			deadlock_path = linked;
 			reason = 5;
 		}
 
@@ -320,19 +330,19 @@ bool __bch2_btree_node_lock(struct btree_trans *trans,
 		if (btree_node_locked(linked, level) &&
 		    bpos_cmp(pos, btree_node_pos((void *) linked->l[level].b,
 						 linked->cached)) <= 0) {
-			deadlock_iter = linked;
+			deadlock_path = linked;
 			reason = 7;
 		}
 	}
 
-	if (unlikely(deadlock_iter)) {
+	if (unlikely(deadlock_path)) {
 		trace_trans_restart_would_deadlock(trans->ip, ip,
 				trans->in_traverse_all, reason,
-				deadlock_iter->btree_id,
-				deadlock_iter->cached,
-				&deadlock_iter->real_pos,
-				iter->btree_id,
-				iter->cached,
+				deadlock_path->btree_id,
+				deadlock_path->cached,
+				&deadlock_path->pos,
+				path->btree_id,
+				path->cached,
 				&pos);
 		btree_trans_restart(trans);
 		return false;
@@ -342,9 +352,9 @@ bool __bch2_btree_node_lock(struct btree_trans *trans,
 		return true;
 
 #ifdef CONFIG_BCACHEFS_DEBUG
-	trans->locking_iter_idx = iter->idx;
+	trans->locking_path_idx = path->idx;
 	trans->locking_pos	= pos;
-	trans->locking_btree_id	= iter->btree_id;
+	trans->locking_btree_id	= path->btree_id;
 	trans->locking_level	= level;
 	trans->locking		= b;
 #endif
@@ -363,54 +373,57 @@ bool __bch2_btree_node_lock(struct btree_trans *trans,
 /* Btree iterator locking: */
 
 #ifdef CONFIG_BCACHEFS_DEBUG
-static void bch2_btree_iter_verify_locks(struct btree_iter *iter)
+
+static void bch2_btree_path_verify_locks(struct btree_path *path)
 {
 	unsigned l;
 
-	for (l = 0; btree_iter_node(iter, l); l++) {
-		if (iter->uptodate >= BTREE_ITER_NEED_RELOCK &&
-		    !btree_node_locked(iter, l))
+	for (l = 0; btree_path_node(path, l); l++) {
+		if (path->uptodate >= BTREE_ITER_NEED_RELOCK &&
+		    !btree_node_locked(path, l))
 			continue;
 
-		BUG_ON(btree_lock_want(iter, l) !=
-		       btree_node_locked_type(iter, l));
+		BUG_ON(btree_lock_want(path, l) !=
+		       btree_node_locked_type(path, l));
 	}
 }
 
 void bch2_trans_verify_locks(struct btree_trans *trans)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 
-	trans_for_each_iter(trans, iter)
-		bch2_btree_iter_verify_locks(iter);
+	trans_for_each_path(trans, path)
+		bch2_btree_path_verify_locks(path);
 }
 #else
-static inline void bch2_btree_iter_verify_locks(struct btree_iter *iter) {}
+static inline void bch2_btree_path_verify_locks(struct btree_path *path) {}
 #endif
 
+/* Btree path locking: */
+
 /*
  * Only for btree_cache.c - only relocks intent locks
  */
-bool bch2_btree_iter_relock_intent(struct btree_trans *trans,
-				   struct btree_iter *iter)
+bool bch2_btree_path_relock_intent(struct btree_trans *trans,
+				   struct btree_path *path)
 {
 	unsigned l;
 
-	for (l = iter->level;
-	     l < iter->locks_want && btree_iter_node(iter, l);
+	for (l = path->level;
+	     l < path->locks_want && btree_path_node(path, l);
 	     l++) {
-		if (!bch2_btree_node_relock(trans, iter, l)) {
+		if (!bch2_btree_node_relock(trans, path, l)) {
 			trace_node_relock_fail(trans->ip, _RET_IP_,
-					iter->cached,
-					iter->btree_id, &iter->real_pos,
-					l, iter->l[l].lock_seq,
-					is_btree_node(iter, l)
+					path->cached,
+					path->btree_id, &path->pos,
+					l, path->l[l].lock_seq,
+					is_btree_node(path, l)
 					? 0
-					: (unsigned long) iter->l[l].b,
-					is_btree_node(iter, l)
-					? iter->l[l].b->c.lock.state.seq
+					: (unsigned long) path->l[l].b,
+					is_btree_node(path, l)
+					? path->l[l].b->c.lock.state.seq
 					: 0);
-			btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+			btree_path_set_dirty(path, BTREE_ITER_NEED_TRAVERSE);
 			btree_trans_restart(trans);
 			return false;
 		}
@@ -420,27 +433,27 @@ bool bch2_btree_iter_relock_intent(struct btree_trans *trans,
 }
 
 __flatten
-static bool bch2_btree_iter_relock(struct btree_trans *trans,
-			struct btree_iter *iter, unsigned long trace_ip)
+static bool bch2_btree_path_relock(struct btree_trans *trans,
+			struct btree_path *path, unsigned long trace_ip)
 {
-	bool ret = btree_iter_get_locks(trans, iter, false, trace_ip);
+	bool ret = btree_path_get_locks(trans, path, false, trace_ip);
 
 	if (!ret)
 		btree_trans_restart(trans);
 	return ret;
 }
 
-bool __bch2_btree_iter_upgrade(struct btree_trans *trans,
-			       struct btree_iter *iter,
+bool __bch2_btree_path_upgrade(struct btree_trans *trans,
+			       struct btree_path *path,
 			       unsigned new_locks_want)
 {
-	struct btree_iter *linked;
+	struct btree_path *linked;
 
-	EBUG_ON(iter->locks_want >= new_locks_want);
+	EBUG_ON(path->locks_want >= new_locks_want);
 
-	iter->locks_want = new_locks_want;
+	path->locks_want = new_locks_want;
 
-	if (btree_iter_get_locks(trans, iter, true, _THIS_IP_))
+	if (btree_path_get_locks(trans, path, true, _THIS_IP_))
 		return true;
 
 	/*
@@ -448,7 +461,7 @@ bool __bch2_btree_iter_upgrade(struct btree_trans *trans,
 	 * iterators in the btree_trans here.
 	 *
 	 * On failure to upgrade the iterator, setting iter->locks_want and
-	 * calling get_locks() is sufficient to make bch2_btree_iter_traverse()
+	 * calling get_locks() is sufficient to make bch2_btree_path_traverse()
 	 * get the locks we want on transaction restart.
 	 *
 	 * But if this iterator was a clone, on transaction restart what we did
@@ -460,75 +473,67 @@ bool __bch2_btree_iter_upgrade(struct btree_trans *trans,
 	 *
 	 * The code below used to be needed to ensure ancestor nodes get locked
 	 * before interior nodes - now that's handled by
-	 * bch2_btree_iter_traverse_all().
+	 * bch2_btree_path_traverse_all().
 	 */
-	trans_for_each_iter(trans, linked)
-		if (linked != iter &&
-		    linked->cached == iter->cached &&
-		    linked->btree_id == iter->btree_id &&
+	trans_for_each_path(trans, linked)
+		if (linked != path &&
+		    linked->cached == path->cached &&
+		    linked->btree_id == path->btree_id &&
 		    linked->locks_want < new_locks_want) {
 			linked->locks_want = new_locks_want;
-			btree_iter_get_locks(trans, linked, true, _THIS_IP_);
+			btree_path_get_locks(trans, linked, true, _THIS_IP_);
 		}
 
-	if (iter->should_be_locked)
-		btree_trans_restart(trans);
 	return false;
 }
 
-void __bch2_btree_iter_downgrade(struct btree_iter *iter,
+void __bch2_btree_path_downgrade(struct btree_path *path,
 				 unsigned new_locks_want)
 {
 	unsigned l;
 
-	EBUG_ON(iter->locks_want < new_locks_want);
+	EBUG_ON(path->locks_want < new_locks_want);
 
-	iter->locks_want = new_locks_want;
+	path->locks_want = new_locks_want;
 
-	while (iter->nodes_locked &&
-	       (l = __fls(iter->nodes_locked)) >= iter->locks_want) {
-		if (l > iter->level) {
-			btree_node_unlock(iter, l);
+	while (path->nodes_locked &&
+	       (l = __fls(path->nodes_locked)) >= path->locks_want) {
+		if (l > path->level) {
+			btree_node_unlock(path, l);
 		} else {
-			if (btree_node_intent_locked(iter, l)) {
-				six_lock_downgrade(&iter->l[l].b->c.lock);
-				iter->nodes_intent_locked ^= 1 << l;
+			if (btree_node_intent_locked(path, l)) {
+				six_lock_downgrade(&path->l[l].b->c.lock);
+				path->nodes_intent_locked ^= 1 << l;
 			}
 			break;
 		}
 	}
 
-	bch2_btree_iter_verify_locks(iter);
+	bch2_btree_path_verify_locks(path);
 }
 
 void bch2_trans_downgrade(struct btree_trans *trans)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 
-	trans_for_each_iter(trans, iter)
-		bch2_btree_iter_downgrade(iter);
+	trans_for_each_path(trans, path)
+		bch2_btree_path_downgrade(path);
 }
 
 /* Btree transaction locking: */
 
-static inline bool btree_iter_should_be_locked(struct btree_iter *iter)
-{
-	return (iter->flags & BTREE_ITER_KEEP_UNTIL_COMMIT) ||
-		iter->should_be_locked;
-}
-
 bool bch2_trans_relock(struct btree_trans *trans)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 
 	if (unlikely(trans->restarted))
 		return false;
 
-	trans_for_each_iter(trans, iter)
-		if (btree_iter_should_be_locked(iter) &&
-		    !bch2_btree_iter_relock(trans, iter, _RET_IP_)) {
+	trans_for_each_path(trans, path)
+		if (path->should_be_locked &&
+		    !bch2_btree_path_relock(trans, path, _RET_IP_)) {
 			trace_trans_restart_relock(trans->ip, _RET_IP_,
-					iter->btree_id, &iter->real_pos);
+					path->btree_id, &path->pos);
 			BUG_ON(!trans->restarted);
 			return false;
 		}
@@ -537,37 +542,37 @@ bool bch2_trans_relock(struct btree_trans *trans)
 
 void bch2_trans_unlock(struct btree_trans *trans)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 
-	trans_for_each_iter(trans, iter)
-		__bch2_btree_iter_unlock(iter);
+	trans_for_each_path(trans, path)
+		__bch2_btree_path_unlock(path);
 }
 
 /* Btree iterator: */
 
 #ifdef CONFIG_BCACHEFS_DEBUG
 
-static void bch2_btree_iter_verify_cached(struct btree_trans *trans,
-					  struct btree_iter *iter)
+static void bch2_btree_path_verify_cached(struct btree_trans *trans,
+					  struct btree_path *path)
 {
 	struct bkey_cached *ck;
-	bool locked = btree_node_locked(iter, 0);
+	bool locked = btree_node_locked(path, 0);
 
-	if (!bch2_btree_node_relock(trans, iter, 0))
+	if (!bch2_btree_node_relock(trans, path, 0))
 		return;
 
-	ck = (void *) iter->l[0].b;
-	BUG_ON(ck->key.btree_id != iter->btree_id ||
-	       bkey_cmp(ck->key.pos, iter->pos));
+	ck = (void *) path->l[0].b;
+	BUG_ON(ck->key.btree_id != path->btree_id ||
+	       bkey_cmp(ck->key.pos, path->pos));
 
 	if (!locked)
-		btree_node_unlock(iter, 0);
+		btree_node_unlock(path, 0);
 }
 
-static void bch2_btree_iter_verify_level(struct btree_trans *trans,
-				struct btree_iter *iter, unsigned level)
+static void bch2_btree_path_verify_level(struct btree_trans *trans,
+				struct btree_path *path, unsigned level)
 {
-	struct btree_iter_level *l;
+	struct btree_path_level *l;
 	struct btree_node_iter tmp;
 	bool locked;
 	struct bkey_packed *p, *k;
@@ -577,25 +582,23 @@ static void bch2_btree_iter_verify_level(struct btree_trans *trans,
 	if (!bch2_debug_check_iterators)
 		return;
 
-	l	= &iter->l[level];
+	l	= &path->l[level];
 	tmp	= l->iter;
-	locked	= btree_node_locked(iter, level);
+	locked	= btree_node_locked(path, level);
 
-	if (iter->cached) {
+	if (path->cached) {
 		if (!level)
-			bch2_btree_iter_verify_cached(trans, iter);
+			bch2_btree_path_verify_cached(trans, path);
 		return;
 	}
 
-	BUG_ON(iter->level < iter->min_depth);
-
-	if (!btree_iter_node(iter, level))
+	if (!btree_path_node(path, level))
 		return;
 
-	if (!bch2_btree_node_relock(trans, iter, level))
+	if (!bch2_btree_node_relock(trans, path, level))
 		return;
 
-	BUG_ON(!btree_iter_pos_in_node(iter, l->b));
+	BUG_ON(!btree_path_pos_in_node(path, l->b));
 
 	bch2_btree_node_iter_verify(&l->iter, l->b);
 
@@ -606,29 +609,29 @@ static void bch2_btree_iter_verify_level(struct btree_trans *trans,
 	 * For extents, the iterator may have skipped past deleted keys (but not
 	 * whiteouts)
 	 */
-	p = level || btree_node_type_is_extents(iter->btree_id)
+	p = level || btree_node_type_is_extents(path->btree_id)
 		? bch2_btree_node_iter_prev(&tmp, l->b)
 		: bch2_btree_node_iter_prev_all(&tmp, l->b);
 	k = bch2_btree_node_iter_peek_all(&l->iter, l->b);
 
-	if (p && bkey_iter_pos_cmp(l->b, p, &iter->real_pos) >= 0) {
+	if (p && bkey_iter_pos_cmp(l->b, p, &path->pos) >= 0) {
 		msg = "before";
 		goto err;
 	}
 
-	if (k && bkey_iter_pos_cmp(l->b, k, &iter->real_pos) < 0) {
+	if (k && bkey_iter_pos_cmp(l->b, k, &path->pos) < 0) {
 		msg = "after";
 		goto err;
 	}
 
 	if (!locked)
-		btree_node_unlock(iter, level);
+		btree_node_unlock(path, level);
 	return;
 err:
 	strcpy(buf2, "(none)");
 	strcpy(buf3, "(none)");
 
-	bch2_bpos_to_text(&PBUF(buf1), iter->real_pos);
+	bch2_bpos_to_text(&PBUF(buf1), path->pos);
 
 	if (p) {
 		struct bkey uk = bkey_unpack_key(l->b, p);
@@ -640,20 +643,51 @@ err:
 		bch2_bkey_to_text(&PBUF(buf3), &uk);
 	}
 
-	panic("iterator should be %s key at level %u:\n"
-	      "iter pos %s\n"
+	panic("path should be %s key at level %u:\n"
+	      "path pos %s\n"
 	      "prev key %s\n"
 	      "cur  key %s\n",
 	      msg, level, buf1, buf2, buf3);
 }
 
-static void bch2_btree_iter_verify(struct btree_iter *iter)
+static void bch2_btree_path_verify(struct btree_trans *trans,
+				   struct btree_path *path)
 {
-	struct btree_trans *trans = iter->trans;
 	struct bch_fs *c = trans->c;
 	unsigned i;
 
-	EBUG_ON(iter->btree_id >= BTREE_ID_NR);
+	EBUG_ON(path->btree_id >= BTREE_ID_NR);
+
+	for (i = 0; i < (!path->cached ? BTREE_MAX_DEPTH : 1); i++) {
+		if (!path->l[i].b) {
+			BUG_ON(c->btree_roots[path->btree_id].b->c.level > i);
+			break;
+		}
+
+		bch2_btree_path_verify_level(trans, path, i);
+	}
+
+	bch2_btree_path_verify_locks(path);
+}
+
+void bch2_trans_verify_paths(struct btree_trans *trans)
+{
+	struct btree_path *path;
+
+	if (!bch2_debug_check_iterators)
+		return;
+
+	trans_for_each_path(trans, path)
+		bch2_btree_path_verify(trans, path);
+}
+
+static void bch2_btree_iter_verify(struct btree_iter *iter)
+{
+	struct btree_trans *trans = iter->trans;
+
+	BUG_ON(iter->btree_id >= BTREE_ID_NR);
+
+	BUG_ON(!!(iter->flags & BTREE_ITER_CACHED) != iter->path->cached);
 
 	BUG_ON(!(iter->flags & BTREE_ITER_ALL_SNAPSHOTS) &&
 	       iter->pos.snapshot != iter->snapshot);
@@ -665,16 +699,7 @@ static void bch2_btree_iter_verify(struct btree_iter *iter)
 	       (iter->flags & BTREE_ITER_ALL_SNAPSHOTS) &&
 	       !btree_type_has_snapshots(iter->btree_id));
 
-	for (i = 0; i < (!iter->cached ? BTREE_MAX_DEPTH : 1); i++) {
-		if (!iter->l[i].b) {
-			BUG_ON(c->btree_roots[iter->btree_id].b->c.level > i);
-			break;
-		}
-
-		bch2_btree_iter_verify_level(trans, iter, i);
-	}
-
-	bch2_btree_iter_verify_locks(iter);
+	bch2_btree_path_verify(trans, iter->path);
 }
 
 static void bch2_btree_iter_verify_entry_exit(struct btree_iter *iter)
@@ -686,26 +711,19 @@ static void bch2_btree_iter_verify_entry_exit(struct btree_iter *iter)
 	       bkey_cmp(iter->pos, iter->k.p) > 0);
 }
 
-void bch2_trans_verify_iters(struct btree_trans *trans, struct btree *b)
-{
-	struct btree_iter *iter;
-
-	if (!bch2_debug_check_iterators)
-		return;
-
-	trans_for_each_iter_with_node(trans, b, iter)
-		bch2_btree_iter_verify_level(trans, iter, b->c.level);
-}
-
 #else
 
-static inline void bch2_btree_iter_verify_level(struct btree_trans *trans,
-					struct btree_iter *iter, unsigned l) {}
+static inline void bch2_btree_path_verify_level(struct btree_trans *trans,
+						struct btree_path *path, unsigned l) {}
+static inline void bch2_btree_path_verify(struct btree_trans *trans,
+					  struct btree_path *path) {}
 static inline void bch2_btree_iter_verify(struct btree_iter *iter) {}
 static inline void bch2_btree_iter_verify_entry_exit(struct btree_iter *iter) {}
 
 #endif
 
+/* Btree path: fixups after btree updates */
+
 static void btree_node_iter_set_set_pos(struct btree_node_iter *iter,
 					struct btree *b,
 					struct bset_tree *t,
@@ -723,38 +741,38 @@ static void btree_node_iter_set_set_pos(struct btree_node_iter *iter,
 	bch2_btree_node_iter_push(iter, b, k, btree_bkey_last(b, t));
 }
 
-static void __bch2_btree_iter_fix_key_modified(struct btree_iter *iter,
+static void __bch2_btree_path_fix_key_modified(struct btree_path *path,
 					       struct btree *b,
 					       struct bkey_packed *where)
 {
-	struct btree_iter_level *l = &iter->l[b->c.level];
+	struct btree_path_level *l = &path->l[b->c.level];
 
 	if (where != bch2_btree_node_iter_peek_all(&l->iter, l->b))
 		return;
 
-	if (bkey_iter_pos_cmp(l->b, where, &iter->real_pos) < 0)
+	if (bkey_iter_pos_cmp(l->b, where, &path->pos) < 0)
 		bch2_btree_node_iter_advance(&l->iter, l->b);
 }
 
-void bch2_btree_iter_fix_key_modified(struct btree_trans *trans,
+void bch2_btree_path_fix_key_modified(struct btree_trans *trans,
 				      struct btree *b,
 				      struct bkey_packed *where)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 
-	trans_for_each_iter_with_node(trans, b, iter) {
-		__bch2_btree_iter_fix_key_modified(iter, b, where);
-		bch2_btree_iter_verify_level(trans, iter, b->c.level);
+	trans_for_each_path_with_node(trans, b, path) {
+		__bch2_btree_path_fix_key_modified(path, b, where);
+		bch2_btree_path_verify_level(trans, path, b->c.level);
 	}
 }
 
-static void __bch2_btree_node_iter_fix(struct btree_iter *iter,
-				      struct btree *b,
-				      struct btree_node_iter *node_iter,
-				      struct bset_tree *t,
-				      struct bkey_packed *where,
-				      unsigned clobber_u64s,
-				      unsigned new_u64s)
+static void __bch2_btree_node_iter_fix(struct btree_path *path,
+				       struct btree *b,
+				       struct btree_node_iter *node_iter,
+				       struct bset_tree *t,
+				       struct bkey_packed *where,
+				       unsigned clobber_u64s,
+				       unsigned new_u64s)
 {
 	const struct bkey_packed *end = btree_bkey_last(b, t);
 	struct btree_node_iter_set *set;
@@ -772,7 +790,7 @@ static void __bch2_btree_node_iter_fix(struct btree_iter *iter,
 
 	/* didn't find the bset in the iterator - might have to readd it: */
 	if (new_u64s &&
-	    bkey_iter_pos_cmp(b, where, &iter->real_pos) >= 0) {
+	    bkey_iter_pos_cmp(b, where, &path->pos) >= 0) {
 		bch2_btree_node_iter_push(node_iter, b, where, end);
 		goto fixup_done;
 	} else {
@@ -787,7 +805,7 @@ found:
 		return;
 
 	if (new_u64s &&
-	    bkey_iter_pos_cmp(b, where, &iter->real_pos) >= 0) {
+	    bkey_iter_pos_cmp(b, where, &path->pos) >= 0) {
 		set->k = offset;
 	} else if (set->k < offset + clobber_u64s) {
 		set->k = offset + new_u64s;
@@ -814,7 +832,7 @@ fixup_done:
 	if (!bch2_btree_node_iter_end(node_iter) &&
 	    iter_current_key_modified &&
 	    (b->c.level ||
-	     btree_node_type_is_extents(iter->btree_id))) {
+	     btree_node_type_is_extents(path->btree_id))) {
 		struct bset_tree *t;
 		struct bkey_packed *k, *k2, *p;
 
@@ -842,7 +860,7 @@ fixup_done:
 }
 
 void bch2_btree_node_iter_fix(struct btree_trans *trans,
-			      struct btree_iter *iter,
+			      struct btree_path *path,
 			      struct btree *b,
 			      struct btree_node_iter *node_iter,
 			      struct bkey_packed *where,
@@ -850,26 +868,28 @@ void bch2_btree_node_iter_fix(struct btree_trans *trans,
 			      unsigned new_u64s)
 {
 	struct bset_tree *t = bch2_bkey_to_bset_inlined(b, where);
-	struct btree_iter *linked;
+	struct btree_path *linked;
 
-	if (node_iter != &iter->l[b->c.level].iter) {
-		__bch2_btree_node_iter_fix(iter, b, node_iter, t,
+	if (node_iter != &path->l[b->c.level].iter) {
+		__bch2_btree_node_iter_fix(path, b, node_iter, t,
 					   where, clobber_u64s, new_u64s);
 
 		if (bch2_debug_check_iterators)
 			bch2_btree_node_iter_verify(node_iter, b);
 	}
 
-	trans_for_each_iter_with_node(trans, b, linked) {
+	trans_for_each_path_with_node(trans, b, linked) {
 		__bch2_btree_node_iter_fix(linked, b,
 					   &linked->l[b->c.level].iter, t,
 					   where, clobber_u64s, new_u64s);
-		bch2_btree_iter_verify_level(trans, linked, b->c.level);
+		bch2_btree_path_verify_level(trans, linked, b->c.level);
 	}
 }
 
-static inline struct bkey_s_c __btree_iter_unpack(struct btree_iter *iter,
-						  struct btree_iter_level *l,
+/* Btree path level: pointer to a particular btree node and node iter */
+
+static inline struct bkey_s_c __btree_iter_unpack(struct bch_fs *c,
+						  struct btree_path_level *l,
 						  struct bkey *u,
 						  struct bkey_packed *k)
 {
@@ -894,50 +914,54 @@ static inline struct bkey_s_c __btree_iter_unpack(struct btree_iter *iter,
 	 * assertion here:
 	 */
 	if (bch2_debug_check_bkeys && !bkey_deleted(ret.k))
-		bch2_bkey_debugcheck(iter->trans->c, l->b, ret);
+		bch2_bkey_debugcheck(c, l->b, ret);
 
 	return ret;
 }
 
-/* peek_all() doesn't skip deleted keys */
-static inline struct bkey_s_c btree_iter_level_peek_all(struct btree_iter *iter,
-							struct btree_iter_level *l)
+static inline struct bkey_s_c btree_path_level_peek_all(struct bch_fs *c,
+							struct btree_path_level *l,
+							struct bkey *u)
 {
-	return __btree_iter_unpack(iter, l, &iter->k,
+	return __btree_iter_unpack(c, l, u,
 			bch2_btree_node_iter_peek_all(&l->iter, l->b));
 }
 
-static inline struct bkey_s_c btree_iter_level_peek(struct btree_iter *iter,
-						    struct btree_iter_level *l)
+static inline struct bkey_s_c btree_path_level_peek(struct btree_trans *trans,
+						    struct btree_path *path,
+						    struct btree_path_level *l,
+						    struct bkey *u)
 {
-	struct bkey_s_c k = __btree_iter_unpack(iter, l, &iter->k,
+	struct bkey_s_c k = __btree_iter_unpack(trans->c, l, u,
 			bch2_btree_node_iter_peek(&l->iter, l->b));
 
-	iter->real_pos = k.k ? k.k->p : l->b->key.k.p;
-	iter->trans->iters_sorted = false;
+	path->pos = k.k ? k.k->p : l->b->key.k.p;
+	trans->paths_sorted = false;
 	return k;
 }
 
-static inline struct bkey_s_c btree_iter_level_prev(struct btree_iter *iter,
-						    struct btree_iter_level *l)
+static inline struct bkey_s_c btree_path_level_prev(struct btree_trans *trans,
+						    struct btree_path *path,
+						    struct btree_path_level *l,
+						    struct bkey *u)
 {
-	struct bkey_s_c k = __btree_iter_unpack(iter, l, &iter->k,
+	struct bkey_s_c k = __btree_iter_unpack(trans->c, l, u,
 			bch2_btree_node_iter_prev(&l->iter, l->b));
 
-	iter->real_pos = k.k ? k.k->p : l->b->data->min_key;
-	iter->trans->iters_sorted = false;
+	path->pos = k.k ? k.k->p : l->b->data->min_key;
+	trans->paths_sorted = false;
 	return k;
 }
 
-static inline bool btree_iter_advance_to_pos(struct btree_iter *iter,
-					     struct btree_iter_level *l,
+static inline bool btree_path_advance_to_pos(struct btree_path *path,
+					     struct btree_path_level *l,
 					     int max_advance)
 {
 	struct bkey_packed *k;
 	int nr_advanced = 0;
 
 	while ((k = bch2_btree_node_iter_peek_all(&l->iter, l->b)) &&
-	       bkey_iter_pos_cmp(l->b, k, &iter->real_pos) < 0) {
+	       bkey_iter_pos_cmp(l->b, k, &path->pos) < 0) {
 		if (max_advance > 0 && nr_advanced >= max_advance)
 			return false;
 
@@ -951,10 +975,10 @@ static inline bool btree_iter_advance_to_pos(struct btree_iter *iter,
 /*
  * Verify that iterator for parent node points to child node:
  */
-static void btree_iter_verify_new_node(struct btree_trans *trans,
-				       struct btree_iter *iter, struct btree *b)
+static void btree_path_verify_new_node(struct btree_trans *trans,
+				       struct btree_path *path, struct btree *b)
 {
-	struct btree_iter_level *l;
+	struct btree_path_level *l;
 	unsigned plevel;
 	bool parent_locked;
 	struct bkey_packed *k;
@@ -963,15 +987,15 @@ static void btree_iter_verify_new_node(struct btree_trans *trans,
 		return;
 
 	plevel = b->c.level + 1;
-	if (!btree_iter_node(iter, plevel))
+	if (!btree_path_node(path, plevel))
 		return;
 
-	parent_locked = btree_node_locked(iter, plevel);
+	parent_locked = btree_node_locked(path, plevel);
 
-	if (!bch2_btree_node_relock(trans, iter, plevel))
+	if (!bch2_btree_node_relock(trans, path, plevel))
 		return;
 
-	l = &iter->l[plevel];
+	l = &path->l[plevel];
 	k = bch2_btree_node_iter_peek_all(&l->iter, l->b);
 	if (!k ||
 	    bkey_deleted(k) ||
@@ -983,7 +1007,7 @@ static void btree_iter_verify_new_node(struct btree_trans *trans,
 		struct bkey uk = bkey_unpack_key(b, k);
 
 		bch2_dump_btree_node(trans->c, l->b);
-		bch2_bpos_to_text(&PBUF(buf1), iter->real_pos);
+		bch2_bpos_to_text(&PBUF(buf1), path->pos);
 		bch2_bkey_to_text(&PBUF(buf2), &uk);
 		bch2_bpos_to_text(&PBUF(buf3), b->data->min_key);
 		bch2_bpos_to_text(&PBUF(buf3), b->data->max_key);
@@ -991,20 +1015,20 @@ static void btree_iter_verify_new_node(struct btree_trans *trans,
 		      "iter pos %s %s\n"
 		      "iter key %s\n"
 		      "new node %s-%s\n",
-		      bch2_btree_ids[iter->btree_id], buf1,
+		      bch2_btree_ids[path->btree_id], buf1,
 		      buf2, buf3, buf4);
 	}
 
 	if (!parent_locked)
-		btree_node_unlock(iter, b->c.level + 1);
+		btree_node_unlock(path, b->c.level + 1);
 }
 
-static inline void __btree_iter_level_init(struct btree_iter *iter,
+static inline void __btree_path_level_init(struct btree_path *path,
 					   unsigned level)
 {
-	struct btree_iter_level *l = &iter->l[level];
+	struct btree_path_level *l = &path->l[level];
 
-	bch2_btree_node_iter_init(&l->iter, l->b, &iter->real_pos);
+	bch2_btree_node_iter_init(&l->iter, l->b, &path->pos);
 
 	/*
 	 * Iterators to interior nodes should always be pointed at the first non
@@ -1014,22 +1038,24 @@ static inline void __btree_iter_level_init(struct btree_iter *iter,
 		bch2_btree_node_iter_peek(&l->iter, l->b);
 }
 
-static inline void btree_iter_level_init(struct btree_trans *trans,
-					 struct btree_iter *iter,
+static inline void btree_path_level_init(struct btree_trans *trans,
+					 struct btree_path *path,
 					 struct btree *b)
 {
-	BUG_ON(iter->cached);
+	BUG_ON(path->cached);
 
-	btree_iter_verify_new_node(trans, iter, b);
+	btree_path_verify_new_node(trans, path, b);
 
-	EBUG_ON(!btree_iter_pos_in_node(iter, b));
+	EBUG_ON(!btree_path_pos_in_node(path, b));
 	EBUG_ON(b->c.lock.state.seq & 1);
 
-	iter->l[b->c.level].lock_seq = b->c.lock.state.seq;
-	iter->l[b->c.level].b = b;
-	__btree_iter_level_init(iter, b->c.level);
+	path->l[b->c.level].lock_seq = b->c.lock.state.seq;
+	path->l[b->c.level].b = b;
+	__btree_path_level_init(path, b->c.level);
 }
 
+/* Btree path: fixups after btree node updates: */
+
 /*
  * A btree node is being replaced - update the iterator to point to the new
  * node:
@@ -1037,37 +1063,37 @@ static inline void btree_iter_level_init(struct btree_trans *trans,
 void bch2_trans_node_add(struct btree_trans *trans, struct btree *b)
 {
 	enum btree_node_locked_type t;
-	struct btree_iter *iter;
+	struct btree_path *path;
 
-	trans_for_each_iter(trans, iter)
-		if (!iter->cached &&
-		    btree_iter_pos_in_node(iter, b)) {
+	trans_for_each_path(trans, path)
+		if (!path->cached &&
+		    btree_path_pos_in_node(path, b)) {
 			/*
-			 * bch2_trans_node_drop() has already been called -
+			 * bch2_btree_path_node_drop() has already been called -
 			 * the old node we're replacing has already been
 			 * unlocked and the pointer invalidated
 			 */
-			BUG_ON(btree_node_locked(iter, b->c.level));
+			BUG_ON(btree_node_locked(path, b->c.level));
 
-			t = btree_lock_want(iter, b->c.level);
+			t = btree_lock_want(path, b->c.level);
 			if (t != BTREE_NODE_UNLOCKED) {
 				six_lock_increment(&b->c.lock, (enum six_lock_type) t);
-				mark_btree_node_locked(iter, b->c.level, (enum six_lock_type) t);
+				mark_btree_node_locked(path, b->c.level, (enum six_lock_type) t);
 			}
 
-			btree_iter_level_init(trans, iter, b);
+			btree_path_level_init(trans, path, b);
 		}
 }
 
 void bch2_trans_node_drop(struct btree_trans *trans, struct btree *b)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 	unsigned level = b->c.level;
 
-	trans_for_each_iter(trans, iter)
-		if (iter->l[level].b == b) {
-			btree_node_unlock(iter, level);
-			iter->l[level].b = BTREE_ITER_NO_NODE_DROP;
+	trans_for_each_path(trans, path)
+		if (path->l[level].b == b) {
+			btree_node_unlock(path, level);
+			path->l[level].b = BTREE_ITER_NO_NODE_DROP;
 		}
 }
 
@@ -1077,12 +1103,14 @@ void bch2_trans_node_drop(struct btree_trans *trans, struct btree *b)
  */
 void bch2_trans_node_reinit_iter(struct btree_trans *trans, struct btree *b)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 
-	trans_for_each_iter_with_node(trans, b, iter)
-		__btree_iter_level_init(iter, b->c.level);
+	trans_for_each_path_with_node(trans, b, path)
+		__btree_path_level_init(path, b->c.level);
 }
 
+/* Btree path: traverse, set_pos: */
+
 static int lock_root_check_fn(struct six_lock *lock, void *p)
 {
 	struct btree *b = container_of(lock, struct btree, c.lock);
@@ -1091,38 +1119,38 @@ static int lock_root_check_fn(struct six_lock *lock, void *p)
 	return b == *rootp ? 0 : -1;
 }
 
-static inline int btree_iter_lock_root(struct btree_trans *trans,
-				       struct btree_iter *iter,
+static inline int btree_path_lock_root(struct btree_trans *trans,
+				       struct btree_path *path,
 				       unsigned depth_want,
 				       unsigned long trace_ip)
 {
 	struct bch_fs *c = trans->c;
-	struct btree *b, **rootp = &c->btree_roots[iter->btree_id].b;
+	struct btree *b, **rootp = &c->btree_roots[path->btree_id].b;
 	enum six_lock_type lock_type;
 	unsigned i;
 
-	EBUG_ON(iter->nodes_locked);
+	EBUG_ON(path->nodes_locked);
 
 	while (1) {
 		b = READ_ONCE(*rootp);
-		iter->level = READ_ONCE(b->c.level);
+		path->level = READ_ONCE(b->c.level);
 
-		if (unlikely(iter->level < depth_want)) {
+		if (unlikely(path->level < depth_want)) {
 			/*
 			 * the root is at a lower depth than the depth we want:
 			 * got to the end of the btree, or we're walking nodes
 			 * greater than some depth and there are no nodes >=
 			 * that depth
 			 */
-			iter->level = depth_want;
-			for (i = iter->level; i < BTREE_MAX_DEPTH; i++)
-				iter->l[i].b = NULL;
+			path->level = depth_want;
+			for (i = path->level; i < BTREE_MAX_DEPTH; i++)
+				path->l[i].b = NULL;
 			return 1;
 		}
 
-		lock_type = __btree_lock_want(iter, iter->level);
-		if (unlikely(!btree_node_lock(trans, iter, b, SPOS_MAX,
-					      iter->level, lock_type,
+		lock_type = __btree_lock_want(path, path->level);
+		if (unlikely(!btree_node_lock(trans, path, b, SPOS_MAX,
+					      path->level, lock_type,
 					      lock_root_check_fn, rootp,
 					      trace_ip))) {
 			if (trans->restarted)
@@ -1131,16 +1159,16 @@ static inline int btree_iter_lock_root(struct btree_trans *trans,
 		}
 
 		if (likely(b == READ_ONCE(*rootp) &&
-			   b->c.level == iter->level &&
+			   b->c.level == path->level &&
 			   !race_fault())) {
-			for (i = 0; i < iter->level; i++)
-				iter->l[i].b = BTREE_ITER_NO_NODE_LOCK_ROOT;
-			iter->l[iter->level].b = b;
-			for (i = iter->level + 1; i < BTREE_MAX_DEPTH; i++)
-				iter->l[i].b = NULL;
-
-			mark_btree_node_locked(iter, iter->level, lock_type);
-			btree_iter_level_init(trans, iter, b);
+			for (i = 0; i < path->level; i++)
+				path->l[i].b = BTREE_ITER_NO_NODE_LOCK_ROOT;
+			path->l[path->level].b = b;
+			for (i = path->level + 1; i < BTREE_MAX_DEPTH; i++)
+				path->l[i].b = NULL;
+
+			mark_btree_node_locked(path, path->level, lock_type);
+			btree_path_level_init(trans, path, b);
 			return 0;
 		}
 
@@ -1149,23 +1177,23 @@ static inline int btree_iter_lock_root(struct btree_trans *trans,
 }
 
 noinline
-static int btree_iter_prefetch(struct btree_trans *trans, struct btree_iter *iter)
+static int btree_path_prefetch(struct btree_trans *trans, struct btree_path *path)
 {
 	struct bch_fs *c = trans->c;
-	struct btree_iter_level *l = &iter->l[iter->level];
+	struct btree_path_level *l = path_l(path);
 	struct btree_node_iter node_iter = l->iter;
 	struct bkey_packed *k;
 	struct bkey_buf tmp;
 	unsigned nr = test_bit(BCH_FS_STARTED, &c->flags)
-		? (iter->level > 1 ? 0 :  2)
-		: (iter->level > 1 ? 1 : 16);
-	bool was_locked = btree_node_locked(iter, iter->level);
+		? (path->level > 1 ? 0 :  2)
+		: (path->level > 1 ? 1 : 16);
+	bool was_locked = btree_node_locked(path, path->level);
 	int ret = 0;
 
 	bch2_bkey_buf_init(&tmp);
 
 	while (nr && !ret) {
-		if (!bch2_btree_node_relock(trans, iter, iter->level))
+		if (!bch2_btree_node_relock(trans, path, path->level))
 			break;
 
 		bch2_btree_node_iter_advance(&node_iter, l->b);
@@ -1174,27 +1202,27 @@ static int btree_iter_prefetch(struct btree_trans *trans, struct btree_iter *ite
 			break;
 
 		bch2_bkey_buf_unpack(&tmp, c, l->b, k);
-		ret = bch2_btree_node_prefetch(c, trans, iter, tmp.k,
-					iter->btree_id, iter->level - 1);
+		ret = bch2_btree_node_prefetch(c, trans, path, tmp.k, path->btree_id,
+					       path->level - 1);
 	}
 
 	if (!was_locked)
-		btree_node_unlock(iter, iter->level);
+		btree_node_unlock(path, path->level);
 
 	bch2_bkey_buf_exit(&tmp, c);
 	return ret;
 }
 
 static noinline void btree_node_mem_ptr_set(struct btree_trans *trans,
-					    struct btree_iter *iter,
+					    struct btree_path *path,
 					    unsigned plevel, struct btree *b)
 {
-	struct btree_iter_level *l = &iter->l[plevel];
-	bool locked = btree_node_locked(iter, plevel);
+	struct btree_path_level *l = &path->l[plevel];
+	bool locked = btree_node_locked(path, plevel);
 	struct bkey_packed *k;
 	struct bch_btree_ptr_v2 *bp;
 
-	if (!bch2_btree_node_relock(trans, iter, plevel))
+	if (!bch2_btree_node_relock(trans, path, plevel))
 		return;
 
 	k = bch2_btree_node_iter_peek_all(&l->iter, l->b);
@@ -1204,60 +1232,61 @@ static noinline void btree_node_mem_ptr_set(struct btree_trans *trans,
 	bp->mem_ptr = (unsigned long)b;
 
 	if (!locked)
-		btree_node_unlock(iter, plevel);
+		btree_node_unlock(path, plevel);
 }
 
-static __always_inline int btree_iter_down(struct btree_trans *trans,
-					   struct btree_iter *iter,
+static __always_inline int btree_path_down(struct btree_trans *trans,
+					   struct btree_path *path,
+					   unsigned flags,
 					   unsigned long trace_ip)
 {
 	struct bch_fs *c = trans->c;
-	struct btree_iter_level *l = &iter->l[iter->level];
+	struct btree_path_level *l = path_l(path);
 	struct btree *b;
-	unsigned level = iter->level - 1;
-	enum six_lock_type lock_type = __btree_lock_want(iter, level);
+	unsigned level = path->level - 1;
+	enum six_lock_type lock_type = __btree_lock_want(path, level);
 	struct bkey_buf tmp;
 	int ret;
 
-	EBUG_ON(!btree_node_locked(iter, iter->level));
+	EBUG_ON(!btree_node_locked(path, path->level));
 
 	bch2_bkey_buf_init(&tmp);
 	bch2_bkey_buf_unpack(&tmp, c, l->b,
 			 bch2_btree_node_iter_peek(&l->iter, l->b));
 
-	b = bch2_btree_node_get(trans, iter, tmp.k, level, lock_type, trace_ip);
+	b = bch2_btree_node_get(trans, path, tmp.k, level, lock_type, trace_ip);
 	ret = PTR_ERR_OR_ZERO(b);
 	if (unlikely(ret))
 		goto err;
 
-	mark_btree_node_locked(iter, level, lock_type);
-	btree_iter_level_init(trans, iter, b);
+	mark_btree_node_locked(path, level, lock_type);
+	btree_path_level_init(trans, path, b);
 
 	if (tmp.k->k.type == KEY_TYPE_btree_ptr_v2 &&
 	    unlikely(b != btree_node_mem_ptr(tmp.k)))
-		btree_node_mem_ptr_set(trans, iter, level + 1, b);
+		btree_node_mem_ptr_set(trans, path, level + 1, b);
 
-	if (iter->flags & BTREE_ITER_PREFETCH)
-		ret = btree_iter_prefetch(trans, iter);
+	if (flags & BTREE_ITER_PREFETCH)
+		ret = btree_path_prefetch(trans, path);
 
-	if (btree_node_read_locked(iter, level + 1))
-		btree_node_unlock(iter, level + 1);
-	iter->level = level;
+	if (btree_node_read_locked(path, level + 1))
+		btree_node_unlock(path, level + 1);
+	path->level = level;
 
-	bch2_btree_iter_verify_locks(iter);
+	bch2_btree_path_verify_locks(path);
 err:
 	bch2_bkey_buf_exit(&tmp, c);
 	return ret;
 }
 
-static int btree_iter_traverse_one(struct btree_trans *,
-			struct btree_iter *, unsigned long);
+static int btree_path_traverse_one(struct btree_trans *, struct btree_path *,
+				   unsigned, unsigned long);
 
-static int __btree_iter_traverse_all(struct btree_trans *trans, int ret,
+static int __btree_path_traverse_all(struct btree_trans *trans, int ret,
 				     unsigned long trace_ip)
 {
 	struct bch_fs *c = trans->c;
-	struct btree_iter *iter, *prev = NULL;
+	struct btree_path *path, *prev = NULL;
 	int i;
 
 	if (trans->in_traverse_all)
@@ -1267,21 +1296,21 @@ static int __btree_iter_traverse_all(struct btree_trans *trans, int ret,
 retry_all:
 	trans->restarted = false;
 
-	trans_for_each_iter(trans, iter)
-		iter->should_be_locked = false;
+	trans_for_each_path(trans, path)
+		path->should_be_locked = false;
 
-	btree_trans_sort_iters(trans);
+	btree_trans_sort_paths(trans);
 
-	trans_for_each_iter_inorder_reverse(trans, iter, i) {
+	trans_for_each_path_inorder_reverse(trans, path, i) {
 		if (prev) {
-			if (iter->btree_id == prev->btree_id &&
-			    iter->locks_want < prev->locks_want)
-				__bch2_btree_iter_upgrade(trans, iter, prev->locks_want);
-			else if (!iter->locks_want && prev->locks_want)
-				__bch2_btree_iter_upgrade(trans, iter, 1);
+			if (path->btree_id == prev->btree_id &&
+			    path->locks_want < prev->locks_want)
+				__bch2_btree_path_upgrade(trans, path, prev->locks_want);
+			else if (!path->locks_want && prev->locks_want)
+				__bch2_btree_path_upgrade(trans, path, 1);
 		}
 
-		prev = iter;
+		prev = path;
 	}
 
 	bch2_trans_unlock(trans);
@@ -1308,27 +1337,27 @@ retry_all:
 	/* Now, redo traversals in correct order: */
 	i = 0;
 	while (i < trans->nr_sorted) {
-		iter = trans->iters + trans->sorted[i];
+		path = trans->paths + trans->sorted[i];
 
-		EBUG_ON(!(trans->iters_linked & (1ULL << iter->idx)));
+		EBUG_ON(!(trans->paths_allocated & (1ULL << path->idx)));
 
-		ret = btree_iter_traverse_one(trans, iter, _THIS_IP_);
+		ret = btree_path_traverse_one(trans, path, 0, _THIS_IP_);
 		if (ret)
 			goto retry_all;
 
-		EBUG_ON(!(trans->iters_linked & (1ULL << iter->idx)));
+		EBUG_ON(!(trans->paths_allocated & (1ULL << path->idx)));
 
-		if (iter->nodes_locked)
+		if (path->nodes_locked)
 			i++;
 	}
 
 	/*
 	 * BTREE_ITER_NEED_RELOCK is ok here - if we called bch2_trans_unlock()
-	 * and relock(), relock() won't relock since iter->should_be_locked
+	 * and relock(), relock() won't relock since path->should_be_locked
 	 * isn't set yet, which is all fine
 	 */
-	trans_for_each_iter(trans, iter)
-		BUG_ON(iter->uptodate >= BTREE_ITER_NEED_TRAVERSE);
+	trans_for_each_path(trans, path)
+		BUG_ON(path->uptodate >= BTREE_ITER_NEED_TRAVERSE);
 out:
 	bch2_btree_cache_cannibalize_unlock(c);
 
@@ -1338,36 +1367,36 @@ out:
 	return ret;
 }
 
-static int bch2_btree_iter_traverse_all(struct btree_trans *trans)
+static int bch2_btree_path_traverse_all(struct btree_trans *trans)
 {
-	return __btree_iter_traverse_all(trans, 0, _RET_IP_);
+	return __btree_path_traverse_all(trans, 0, _RET_IP_);
 }
 
-static inline bool btree_iter_good_node(struct btree_trans *trans,
-					struct btree_iter *iter,
+static inline bool btree_path_good_node(struct btree_trans *trans,
+					struct btree_path *path,
 					unsigned l, int check_pos)
 {
-	if (!is_btree_node(iter, l) ||
-	    !bch2_btree_node_relock(trans, iter, l))
+	if (!is_btree_node(path, l) ||
+	    !bch2_btree_node_relock(trans, path, l))
 		return false;
 
-	if (check_pos < 0 && btree_iter_pos_before_node(iter, iter->l[l].b))
+	if (check_pos < 0 && btree_path_pos_before_node(path, path->l[l].b))
 		return false;
-	if (check_pos > 0 && btree_iter_pos_after_node(iter, iter->l[l].b))
+	if (check_pos > 0 && btree_path_pos_after_node(path, path->l[l].b))
 		return false;
 	return true;
 }
 
-static inline unsigned btree_iter_up_until_good_node(struct btree_trans *trans,
-						     struct btree_iter *iter,
+static inline unsigned btree_path_up_until_good_node(struct btree_trans *trans,
+						     struct btree_path *path,
 						     int check_pos)
 {
-	unsigned l = iter->level;
+	unsigned l = path->level;
 
-	while (btree_iter_node(iter, l) &&
-	       !btree_iter_good_node(trans, iter, l, check_pos)) {
-		btree_node_unlock(iter, l);
-		iter->l[l].b = BTREE_ITER_NO_NODE_UP;
+	while (btree_path_node(path, l) &&
+	       !btree_path_good_node(trans, path, l, check_pos)) {
+		btree_node_unlock(path, l);
+		path->l[l].b = BTREE_ITER_NO_NODE_UP;
 		l++;
 	}
 
@@ -1383,53 +1412,54 @@ static inline unsigned btree_iter_up_until_good_node(struct btree_trans *trans,
  * On error, caller (peek_node()/peek_key()) must return NULL; the error is
  * stashed in the iterator and returned from bch2_trans_exit().
  */
-static int btree_iter_traverse_one(struct btree_trans *trans,
-				   struct btree_iter *iter,
+static int btree_path_traverse_one(struct btree_trans *trans,
+				   struct btree_path *path,
+				   unsigned flags,
 				   unsigned long trace_ip)
 {
-	unsigned l, depth_want = iter->level;
+	unsigned l, depth_want = path->level;
 	int ret = 0;
 
 	/*
-	 * Ensure we obey iter->should_be_locked: if it's set, we can't unlock
-	 * and re-traverse the iterator without a transaction restart:
+	 * Ensure we obey path->should_be_locked: if it's set, we can't unlock
+	 * and re-traverse the path without a transaction restart:
 	 */
-	if (iter->should_be_locked) {
-		ret = bch2_btree_iter_relock(trans, iter, trace_ip) ? 0 : -EINTR;
+	if (path->should_be_locked) {
+		ret = bch2_btree_path_relock(trans, path, trace_ip) ? 0 : -EINTR;
 		goto out;
 	}
 
-	if (iter->cached) {
-		ret = bch2_btree_iter_traverse_cached(trans, iter);
+	if (path->cached) {
+		ret = bch2_btree_path_traverse_cached(trans, path, flags);
 		goto out;
 	}
 
-	if (unlikely(iter->level >= BTREE_MAX_DEPTH))
+	if (unlikely(path->level >= BTREE_MAX_DEPTH))
 		goto out;
 
-	iter->level = btree_iter_up_until_good_node(trans, iter, 0);
+	path->level = btree_path_up_until_good_node(trans, path, 0);
 
 	/* If we need intent locks, take them too: */
-	for (l = iter->level + 1;
-	     l < iter->locks_want && btree_iter_node(iter, l);
+	for (l = path->level + 1;
+	     l < path->locks_want && btree_path_node(path, l);
 	     l++)
-		if (!bch2_btree_node_relock(trans, iter, l))
-			while (iter->level <= l) {
-				btree_node_unlock(iter, iter->level);
-				iter->l[iter->level].b = BTREE_ITER_NO_NODE_UP;
-				iter->level++;
+		if (!bch2_btree_node_relock(trans, path, l))
+			while (path->level <= l) {
+				btree_node_unlock(path, path->level);
+				path->l[path->level].b = BTREE_ITER_NO_NODE_UP;
+				path->level++;
 			}
 
 	/*
-	 * Note: iter->nodes[iter->level] may be temporarily NULL here - that
+	 * Note: path->nodes[path->level] may be temporarily NULL here - that
 	 * would indicate to other code that we got to the end of the btree,
 	 * here it indicates that relocking the root failed - it's critical that
-	 * btree_iter_lock_root() comes next and that it can't fail
+	 * btree_path_lock_root() comes next and that it can't fail
 	 */
-	while (iter->level > depth_want) {
-		ret = btree_iter_node(iter, iter->level)
-			? btree_iter_down(trans, iter, trace_ip)
-			: btree_iter_lock_root(trans, iter, depth_want, trace_ip);
+	while (path->level > depth_want) {
+		ret = btree_path_node(path, path->level)
+			? btree_path_down(trans, path, flags, trace_ip)
+			: btree_path_lock_root(trans, path, depth_want, trace_ip);
 		if (unlikely(ret)) {
 			if (ret == 1) {
 				/*
@@ -1440,74 +1470,405 @@ static int btree_iter_traverse_one(struct btree_trans *trans,
 				goto out;
 			}
 
-			__bch2_btree_iter_unlock(iter);
-			iter->level = depth_want;
+			__bch2_btree_path_unlock(path);
+			path->level = depth_want;
 
-			if (ret == -EIO) {
-				iter->flags |= BTREE_ITER_ERROR;
-				iter->l[iter->level].b =
+			if (ret == -EIO)
+				path->l[path->level].b =
 					BTREE_ITER_NO_NODE_ERROR;
-			} else {
-				iter->l[iter->level].b =
+			else
+				path->l[path->level].b =
 					BTREE_ITER_NO_NODE_DOWN;
-			}
 			goto out;
 		}
 	}
 
-	iter->uptodate = BTREE_ITER_UPTODATE;
+	path->uptodate = BTREE_ITER_UPTODATE;
 out:
 	BUG_ON((ret == -EINTR) != !!trans->restarted);
 	trace_iter_traverse(trans->ip, trace_ip,
-			    iter->cached,
-			    iter->btree_id, &iter->real_pos, ret);
-	bch2_btree_iter_verify(iter);
+			    path->cached,
+			    path->btree_id, &path->pos, ret);
+	bch2_btree_path_verify(trans, path);
 	return ret;
 }
 
-static int __must_check __bch2_btree_iter_traverse(struct btree_iter *iter)
+static int __btree_path_traverse_all(struct btree_trans *, int, unsigned long);
+
+int __must_check bch2_btree_path_traverse(struct btree_trans *trans,
+					  struct btree_path *path, unsigned flags)
 {
-	struct btree_trans *trans = iter->trans;
 	int ret;
 
+	if (path->uptodate < BTREE_ITER_NEED_RELOCK)
+		return 0;
+
 	ret =   bch2_trans_cond_resched(trans) ?:
-		btree_iter_traverse_one(trans, iter, _RET_IP_);
-	if (unlikely(ret) && hweight64(trans->iters_linked) == 1) {
-		ret = __btree_iter_traverse_all(trans, ret, _RET_IP_);
+		btree_path_traverse_one(trans, path, flags, _RET_IP_);
+	if (unlikely(ret) && hweight64(trans->paths_allocated) == 1) {
+		ret = __btree_path_traverse_all(trans, ret, _RET_IP_);
 		BUG_ON(ret == -EINTR);
 	}
 
 	return ret;
 }
 
-/*
- * Note:
- * bch2_btree_iter_traverse() is for external users, btree_iter_traverse() is
- * for internal btree iterator users
- *
- * bch2_btree_iter_traverse sets iter->real_pos to iter->pos,
- * btree_iter_traverse() does not:
- */
-static inline int __must_check
-btree_iter_traverse(struct btree_iter *iter)
+static void btree_path_copy(struct btree_trans *trans, struct btree_path *dst,
+			    struct btree_path *src)
+{
+	unsigned i, offset = offsetof(struct btree_path, pos);
+
+	memcpy((void *) dst + offset,
+	       (void *) src + offset,
+	       sizeof(struct btree_path) - offset);
+
+	for (i = 0; i < BTREE_MAX_DEPTH; i++)
+		if (btree_node_locked(dst, i))
+			six_lock_increment(&dst->l[i].b->c.lock,
+					   __btree_lock_want(dst, i));
+
+	trans->paths_sorted = false;
+}
+
+struct btree_path * __must_check
+__bch2_btree_path_make_mut(struct btree_trans *trans,
+			 struct btree_path *path, bool intent)
+{
+	struct btree_path *new = btree_path_alloc(trans, path);
+
+	btree_path_copy(trans, new, path);
+	__btree_path_get(new, intent);
+	__btree_path_put(path, intent);
+	path = new;
+	path->preserve = false;
+#ifdef CONFIG_BCACHEFS_DEBUG
+	path->ip_allocated = _RET_IP_;
+#endif
+	return path;
+}
+
+static struct btree_path * __must_check
+__bch2_btree_path_set_pos(struct btree_trans *trans,
+			  struct btree_path *path, struct bpos new_pos,
+			  bool intent, int cmp)
+{
+#ifdef CONFIG_BCACHEFS_DEBUG
+	struct bpos old_pos = path->pos;
+#endif
+	unsigned l = path->level;
+
+	EBUG_ON(trans->restarted);
+	EBUG_ON(!path->ref);
+
+	path = bch2_btree_path_make_mut(trans, path, intent);
+
+	path->pos		= new_pos;
+	path->should_be_locked	= false;
+	trans->paths_sorted	= false;
+
+	if (unlikely(path->cached)) {
+		btree_node_unlock(path, 0);
+		path->l[0].b = BTREE_ITER_NO_NODE_CACHED;
+		btree_path_set_dirty(path, BTREE_ITER_NEED_TRAVERSE);
+		goto out;
+	}
+
+	l = btree_path_up_until_good_node(trans, path, cmp);
+
+	if (btree_path_node(path, l)) {
+		/*
+		 * We might have to skip over many keys, or just a few: try
+		 * advancing the node iterator, and if we have to skip over too
+		 * many keys just reinit it (or if we're rewinding, since that
+		 * is expensive).
+		 */
+		if (cmp < 0 ||
+		    !btree_path_advance_to_pos(path, &path->l[l], 8))
+			__btree_path_level_init(path, l);
+
+		/* Don't leave it locked if we're not supposed to: */
+		if (btree_lock_want(path, l) == BTREE_NODE_UNLOCKED)
+			btree_node_unlock(path, l);
+	}
+
+	if (l != path->level)
+		btree_path_set_dirty(path, BTREE_ITER_NEED_TRAVERSE);
+out:
+	bch2_btree_path_verify(trans, path);
+#ifdef CONFIG_BCACHEFS_DEBUG
+	trace_path_set_pos(trans->ip, _RET_IP_, path->btree_id,
+			   &old_pos, &new_pos, l);
+#endif
+	return path;
+}
+
+static inline struct btree_path * __must_check
+btree_path_set_pos(struct btree_trans *trans,
+		   struct btree_path *path, struct bpos new_pos,
+		   bool intent)
+{
+	int cmp = bpos_cmp(new_pos, path->pos);
+
+	return cmp
+		? __bch2_btree_path_set_pos(trans, path, new_pos, intent, cmp)
+		: path;
+}
+
+/* Btree path: main interface: */
+
+static struct btree_path *have_path_at_pos(struct btree_trans *trans, struct btree_path *path)
+{
+	struct btree_path *next;
+
+	next = prev_btree_path(trans, path);
+	if (next && !btree_path_cmp(next, path))
+		return next;
+
+	next = next_btree_path(trans, path);
+	if (next && !btree_path_cmp(next, path))
+		return next;
+
+	return NULL;
+}
+
+static bool have_node_at_pos(struct btree_trans *trans, struct btree_path *path)
+{
+	struct btree_path *next;
+
+	next = prev_btree_path(trans, path);
+	if (next && path_l(next)->b == path_l(path)->b)
+		return true;
+
+	next = next_btree_path(trans, path);
+	if (next && path_l(next)->b == path_l(path)->b)
+		return true;
+
+	return false;
+}
+
+static inline void __bch2_path_free(struct btree_trans *trans, struct btree_path *path)
 {
-	return iter->uptodate >= BTREE_ITER_NEED_RELOCK
-		? __bch2_btree_iter_traverse(iter)
-		: 0;
+	__bch2_btree_path_unlock(path);
+	btree_path_list_remove(trans, path);
+	trans->paths_allocated &= ~(1ULL << path->idx);
 }
 
+void bch2_path_put(struct btree_trans *trans, struct btree_path *path, bool intent)
+{
+	struct btree_path *dup;
+
+	EBUG_ON(trans->paths + path->idx != path);
+	EBUG_ON(!path->ref);
+
+	if (!__btree_path_put(path, intent))
+		return;
+
+	/*
+	 * Perhaps instead we should check for duplicate paths in traverse_all:
+	 */
+	if (path->preserve &&
+	    (dup = have_path_at_pos(trans, path))) {
+		dup->preserve = true;
+		path->preserve = false;
+	}
+
+	if (!path->preserve &&
+	    have_node_at_pos(trans, path))
+		__bch2_path_free(trans, path);
+}
+
+noinline __cold
+void bch2_dump_trans_paths_updates(struct btree_trans *trans)
+{
+	struct btree_path *path;
+	struct btree_insert_entry *i;
+	unsigned idx;
+	char buf[300];
+
+	btree_trans_sort_paths(trans);
+
+	trans_for_each_path_inorder(trans, path, idx)
+		printk(KERN_ERR "path: idx %u ref %u:%u%s btree %s pos %s %pS\n",
+		       path->idx, path->ref, path->intent_ref,
+		       path->preserve ? " preserve" : "",
+		       bch2_btree_ids[path->btree_id],
+		       (bch2_bpos_to_text(&PBUF(buf), path->pos), buf),
+#ifdef CONFIG_BCACHEFS_DEBUG
+		       (void *) path->ip_allocated
+#else
+		       NULL
+#endif
+		       );
+
+	trans_for_each_update(trans, i)
+		printk(KERN_ERR "update: btree %s %s %pS\n",
+		       bch2_btree_ids[i->btree_id],
+		       (bch2_bkey_val_to_text(&PBUF(buf), trans->c, bkey_i_to_s_c(i->k)), buf),
+		       (void *) i->ip_allocated);
+}
+
+static struct btree_path *btree_path_alloc(struct btree_trans *trans,
+					   struct btree_path *pos)
+{
+	struct btree_path *path;
+	unsigned idx;
+
+	if (unlikely(trans->paths_allocated ==
+		     ~((~0ULL << 1) << (BTREE_ITER_MAX - 1)))) {
+		bch2_dump_trans_paths_updates(trans);
+		panic("trans path oveflow\n");
+	}
+
+	idx = __ffs64(~trans->paths_allocated);
+	trans->paths_allocated |= 1ULL << idx;
+
+	path = &trans->paths[idx];
+
+	path->idx		= idx;
+	path->ref		= 0;
+	path->intent_ref	= 0;
+	path->nodes_locked	= 0;
+	path->nodes_intent_locked = 0;
+
+	btree_path_list_add(trans, pos, path);
+	return path;
+}
+
+struct btree_path *bch2_path_get(struct btree_trans *trans, bool cached,
+				 enum btree_id btree_id, struct bpos pos,
+				 unsigned locks_want, unsigned level,
+				 bool intent)
+{
+	struct btree_path *path, *best = NULL;
+	struct bpos pos_min = POS_MIN;
+	int i;
+
+	BUG_ON(trans->restarted);
+
+	trans_for_each_path(trans, path) {
+		if (path->cached != cached ||
+		    path->btree_id != btree_id ||
+		    path->level != level)
+			continue;
+
+		if (best) {
+			int cmp = bkey_cmp(bpos_diff(best->pos, pos),
+					   bpos_diff(path->pos, pos));
+
+			if (cmp < 0 ||
+			    ((cmp == 0 && (path->ref || path->preserve))))
+				continue;
+		}
+
+		best = path;
+	}
+
+	if (best) {
+		__btree_path_get(best, intent);
+		path = btree_path_set_pos(trans, best, pos, intent);
+		path->preserve = true;
+	} else {
+		path = btree_path_alloc(trans, NULL);
+
+		__btree_path_get(path, intent);
+		path->pos			= pos;
+		path->btree_id			= btree_id;
+		path->cached			= cached;
+		path->preserve			= true;
+		path->uptodate			= BTREE_ITER_NEED_TRAVERSE;
+		path->should_be_locked		= false;
+		path->level			= level;
+		path->locks_want		= locks_want;
+		path->nodes_locked		= 0;
+		path->nodes_intent_locked	= 0;
+		for (i = 0; i < ARRAY_SIZE(path->l); i++)
+			path->l[i].b		= BTREE_ITER_NO_NODE_INIT;
+#ifdef CONFIG_BCACHEFS_DEBUG
+		path->ip_allocated		= _RET_IP_;
+#endif
+		trans->paths_sorted		= false;
+	}
+
+	if (path->intent_ref)
+		locks_want = max(locks_want, level + 1);
+
+	/*
+	 * If the path has locks_want greater than requested, we don't downgrade
+	 * it here - on transaction restart because btree node split needs to
+	 * upgrade locks, we might be putting/getting the iterator again.
+	 * Downgrading iterators only happens via bch2_trans_downgrade(), after
+	 * a successful transaction commit.
+	 */
+
+	locks_want = min(locks_want, BTREE_MAX_DEPTH);
+	if (locks_want > path->locks_want) {
+		path->locks_want = locks_want;
+		btree_path_get_locks(trans, path, true, _THIS_IP_);
+	}
+
+	trace_trans_get_path(_RET_IP_, trans->ip, btree_id,
+			     &pos, locks_want, path->uptodate,
+			     best ? &best->pos		: &pos_min,
+			     best ? best->locks_want	: U8_MAX,
+			     best ? best->uptodate	: U8_MAX);
+
+	return path;
+}
+
+inline struct bkey_s_c bch2_btree_path_peek_slot(struct btree_path *path, struct bkey *u)
+{
+
+	struct bkey_s_c k;
+
+	BUG_ON(path->uptodate != BTREE_ITER_UPTODATE);
+
+	if (!path->cached) {
+		struct btree_path_level *l = path_l(path);
+		struct bkey_packed *_k =
+			bch2_btree_node_iter_peek_all(&l->iter, l->b);
+
+		k = _k ? bkey_disassemble(l->b, _k, u) : bkey_s_c_null;
+
+		EBUG_ON(k.k && bkey_deleted(k.k) && bpos_cmp(k.k->p, path->pos) == 0);
+
+		if (!k.k || bpos_cmp(path->pos, k.k->p))
+			goto hole;
+	} else {
+		struct bkey_cached *ck = (void *) path->l[0].b;
+
+		EBUG_ON(path->btree_id != ck->key.btree_id ||
+			bkey_cmp(path->pos, ck->key.pos));
+
+		/* BTREE_ITER_CACHED_NOFILL? */
+		if (unlikely(!ck->valid))
+			goto hole;
+
+		k = bkey_i_to_s_c(ck->k);
+	}
+
+	return k;
+hole:
+	bkey_init(u);
+	u->p = path->pos;
+	return (struct bkey_s_c) { u, NULL };
+}
+
+/* Btree iterators: */
+
 int __must_check
 bch2_btree_iter_traverse(struct btree_iter *iter)
 {
 	int ret;
 
-	btree_iter_set_search_pos(iter, btree_iter_search_key(iter));
+	iter->path = btree_path_set_pos(iter->trans, iter->path,
+					btree_iter_search_key(iter),
+					iter->flags & BTREE_ITER_INTENT);
 
-	ret = btree_iter_traverse(iter);
+	ret = bch2_btree_path_traverse(iter->trans, iter->path, iter->flags);
 	if (ret)
 		return ret;
 
-	iter->should_be_locked = true;
+	iter->path->should_be_locked = true;
 	return 0;
 }
 
@@ -1518,23 +1879,22 @@ struct btree *bch2_btree_iter_peek_node(struct btree_iter *iter)
 	struct btree *b = NULL;
 	int ret;
 
-	EBUG_ON(iter->cached);
+	EBUG_ON(iter->path->cached);
 	bch2_btree_iter_verify(iter);
 
-	ret = btree_iter_traverse(iter);
+	ret = bch2_btree_path_traverse(iter->trans, iter->path, iter->flags);
 	if (ret)
 		goto out;
 
-	b = btree_iter_node(iter, iter->level);
+	b = btree_path_node(iter->path, iter->path->level);
 	if (!b)
 		goto out;
 
 	BUG_ON(bpos_cmp(b->key.k.p, iter->pos) < 0);
 
 	bkey_init(&iter->k);
-	iter->k.p = iter->pos = iter->real_pos = b->key.k.p;
-	iter->trans->iters_sorted = false;
-	iter->should_be_locked = true;
+	iter->k.p = iter->pos = b->key.k.p;
+	iter->path->should_be_locked = true;
 out:
 	bch2_btree_iter_verify_entry_exit(iter);
 	bch2_btree_iter_verify(iter);
@@ -1544,29 +1904,31 @@ out:
 
 struct btree *bch2_btree_iter_next_node(struct btree_iter *iter)
 {
+	struct btree_trans *trans = iter->trans;
+	struct btree_path *path = iter->path;
 	struct btree *b = NULL;
 	int ret;
 
-	EBUG_ON(iter->cached);
+	EBUG_ON(iter->path->cached);
 	bch2_btree_iter_verify(iter);
 
 	/* already got to end? */
-	if (!btree_iter_node(iter, iter->level))
+	if (!btree_path_node(path, path->level))
 		goto out;
 
-	bch2_trans_cond_resched(iter->trans);
+	bch2_trans_cond_resched(trans);
 
-	btree_node_unlock(iter, iter->level);
-	iter->l[iter->level].b = BTREE_ITER_NO_NODE_UP;
-	iter->level++;
+	btree_node_unlock(path, path->level);
+	path->l[path->level].b = BTREE_ITER_NO_NODE_UP;
+	path->level++;
 
-	btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
-	ret = btree_iter_traverse(iter);
+	btree_path_set_dirty(path, BTREE_ITER_NEED_TRAVERSE);
+	ret = bch2_btree_path_traverse(trans, path, iter->flags);
 	if (ret)
 		goto out;
 
 	/* got to end? */
-	b = btree_iter_node(iter, iter->level);
+	b = btree_path_node(path, path->level);
 	if (!b)
 		goto out;
 
@@ -1575,28 +1937,29 @@ struct btree *bch2_btree_iter_next_node(struct btree_iter *iter)
 		 * Haven't gotten to the end of the parent node: go back down to
 		 * the next child node
 		 */
-		btree_iter_set_search_pos(iter, bpos_successor(iter->pos));
+		path = iter->path =
+			btree_path_set_pos(trans, path, bpos_successor(iter->pos),
+					   iter->flags & BTREE_ITER_INTENT);
 
 		/* Unlock to avoid screwing up our lock invariants: */
-		btree_node_unlock(iter, iter->level);
+		btree_node_unlock(path, path->level);
 
-		iter->level = iter->min_depth;
-		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+		path->level = iter->min_depth;
+		btree_path_set_dirty(path, BTREE_ITER_NEED_TRAVERSE);
 		bch2_btree_iter_verify(iter);
 
-		ret = btree_iter_traverse(iter);
+		ret = bch2_btree_path_traverse(trans, path, iter->flags);
 		if (ret) {
 			b = NULL;
 			goto out;
 		}
 
-		b = iter->l[iter->level].b;
+		b = path->l[path->level].b;
 	}
 
 	bkey_init(&iter->k);
-	iter->k.p = iter->pos = iter->real_pos = b->key.k.p;
-	iter->trans->iters_sorted = false;
-	iter->should_be_locked = true;
+	iter->k.p = iter->pos = b->key.k.p;
+	iter->path->should_be_locked = true;
 out:
 	bch2_btree_iter_verify_entry_exit(iter);
 	bch2_btree_iter_verify(iter);
@@ -1606,60 +1969,6 @@ out:
 
 /* Iterate across keys (in leaf nodes only) */
 
-static void btree_iter_set_search_pos(struct btree_iter *iter, struct bpos new_pos)
-{
-	struct btree_trans *trans = iter->trans;
-#ifdef CONFIG_BCACHEFS_DEBUG
-	struct bpos old_pos = iter->real_pos;
-#endif
-	int cmp = bpos_cmp(new_pos, iter->real_pos);
-	unsigned l = iter->level;
-
-	EBUG_ON(trans->restarted);
-
-	if (!cmp)
-		goto out;
-
-	iter->real_pos = new_pos;
-	iter->should_be_locked = false;
-	trans->iters_sorted = false;
-
-	if (unlikely(iter->cached)) {
-		btree_node_unlock(iter, 0);
-		iter->l[0].b = BTREE_ITER_NO_NODE_CACHED;
-		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
-		return;
-	}
-
-	l = btree_iter_up_until_good_node(trans, iter, cmp);
-
-	if (btree_iter_node(iter, l)) {
-		/*
-		 * We might have to skip over many keys, or just a few: try
-		 * advancing the node iterator, and if we have to skip over too
-		 * many keys just reinit it (or if we're rewinding, since that
-		 * is expensive).
-		 */
-		if (cmp < 0 ||
-		    !btree_iter_advance_to_pos(iter, &iter->l[l], 8))
-			__btree_iter_level_init(iter, l);
-
-		/* Don't leave it locked if we're not supposed to: */
-		if (btree_lock_want(iter, l) == BTREE_NODE_UNLOCKED)
-			btree_node_unlock(iter, l);
-	}
-out:
-	if (l != iter->level)
-		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
-
-	bch2_btree_iter_verify(iter);
-#ifdef CONFIG_BCACHEFS_DEBUG
-	trace_iter_set_search_pos(trans->ip, _RET_IP_,
-				  iter->btree_id,
-				  &old_pos, &new_pos, l);
-#endif
-}
-
 inline bool bch2_btree_iter_advance(struct btree_iter *iter)
 {
 	struct bpos pos = iter->k.p;
@@ -1684,7 +1993,7 @@ inline bool bch2_btree_iter_rewind(struct btree_iter *iter)
 	return ret;
 }
 
-static noinline struct bkey_i *__btree_trans_peek_updates(struct btree_iter *iter)
+struct bkey_i *__bch2_btree_trans_peek_updates(struct btree_iter *iter)
 {
 	struct btree_insert_entry *i;
 	struct bkey_i *ret = NULL;
@@ -1694,7 +2003,7 @@ static noinline struct bkey_i *__btree_trans_peek_updates(struct btree_iter *ite
 			continue;
 		if (i->btree_id > iter->btree_id)
 			break;
-		if (bpos_cmp(i->k->k.p, iter->real_pos) < 0)
+		if (bpos_cmp(i->k->k.p, iter->path->pos) < 0)
 			continue;
 		if (!ret || bpos_cmp(i->k->k.p, ret->k.p) < 0)
 			ret = i->k;
@@ -1703,33 +2012,27 @@ static noinline struct bkey_i *__btree_trans_peek_updates(struct btree_iter *ite
 	return ret;
 }
 
-static inline struct bkey_i *btree_trans_peek_updates(struct btree_iter *iter)
-{
-	return iter->flags & BTREE_ITER_WITH_UPDATES
-		? __btree_trans_peek_updates(iter)
-		: NULL;
-}
-
 /**
  * bch2_btree_iter_peek: returns first key greater than or equal to iterator's
  * current position
  */
 struct bkey_s_c bch2_btree_iter_peek(struct btree_iter *iter)
 {
-	struct btree_iter_level *l = &iter->l[0];
+	struct btree_trans *trans = iter->trans;
 	struct bpos search_key = btree_iter_search_key(iter);
 	struct bkey_i *next_update;
 	struct bkey_s_c k;
-	int ret;
+	int ret, cmp;
 
-	EBUG_ON(iter->cached || iter->level);
+	EBUG_ON(iter->path->cached || iter->path->level);
 	bch2_btree_iter_verify(iter);
 	bch2_btree_iter_verify_entry_exit(iter);
 
 	while (1) {
-		btree_iter_set_search_pos(iter, search_key);
+		iter->path = btree_path_set_pos(trans, iter->path, search_key,
+				   iter->flags & BTREE_ITER_INTENT);
 
-		ret = btree_iter_traverse(iter);
+		ret = bch2_btree_path_traverse(trans, iter->path, iter->flags);
 		if (unlikely(ret)) {
 			/* ensure that iter->k is consistent with iter->pos: */
 			bch2_btree_iter_set_pos(iter, iter->pos);
@@ -1738,7 +2041,7 @@ struct bkey_s_c bch2_btree_iter_peek(struct btree_iter *iter)
 		}
 
 		next_update = btree_trans_peek_updates(iter);
-		k = btree_iter_level_peek_all(iter, l);
+		k = btree_path_level_peek_all(trans->c, &iter->path->l[0], &iter->k);
 
 		/* * In the btree, deleted keys sort before non deleted: */
 		if (k.k && bkey_deleted(k.k) &&
@@ -1750,7 +2053,7 @@ struct bkey_s_c bch2_btree_iter_peek(struct btree_iter *iter)
 
 		if (next_update &&
 		    bpos_cmp(next_update->k.p,
-			     k.k ? k.k->p : l->b->key.k.p) <= 0) {
+			     k.k ? k.k->p : iter->path->l[0].b->key.k.p) <= 0) {
 			iter->k = next_update->k;
 			k = bkey_i_to_s_c(next_update);
 		}
@@ -1761,13 +2064,12 @@ struct bkey_s_c bch2_btree_iter_peek(struct btree_iter *iter)
 
 			/* Advance to next key: */
 			search_key = bkey_successor(iter, k.k->p);
-		} else if (likely(bpos_cmp(l->b->key.k.p, SPOS_MAX))) {
+		} else if (likely(bpos_cmp(iter->path->l[0].b->key.k.p, SPOS_MAX))) {
 			/* Advance to next leaf node: */
-			search_key = bpos_successor(l->b->key.k.p);
+			search_key = bpos_successor(iter->path->l[0].b->key.k.p);
 		} else {
 			/* End of btree: */
 			bch2_btree_iter_set_pos(iter, SPOS_MAX);
-			iter->real_pos = SPOS_MAX;
 			k = bkey_s_c_null;
 			goto out;
 		}
@@ -1781,9 +2083,15 @@ struct bkey_s_c bch2_btree_iter_peek(struct btree_iter *iter)
 		iter->pos = k.k->p;
 	else if (bkey_cmp(bkey_start_pos(k.k), iter->pos) > 0)
 		iter->pos = bkey_start_pos(k.k);
-	iter->real_pos = k.k->p;
+
+	cmp = bpos_cmp(k.k->p, iter->path->pos);
+	if (cmp) {
+		iter->path->pos = k.k->p;
+		trans->paths_sorted = false;
+	}
 out:
-	iter->should_be_locked = true;
+	iter->path->should_be_locked = true;
+
 	bch2_btree_iter_verify_entry_exit(iter);
 	bch2_btree_iter_verify(iter);
 	return k;
@@ -1807,20 +2115,21 @@ struct bkey_s_c bch2_btree_iter_next(struct btree_iter *iter)
  */
 struct bkey_s_c bch2_btree_iter_peek_prev(struct btree_iter *iter)
 {
+	struct btree_trans *trans = iter->trans;
 	struct bpos search_key = iter->pos;
-	struct btree_iter_level *l = &iter->l[0];
 	struct bkey_s_c k;
 	int ret;
 
-	EBUG_ON(iter->cached || iter->level);
+	EBUG_ON(iter->path->cached || iter->path->level);
 	EBUG_ON(iter->flags & BTREE_ITER_WITH_UPDATES);
 	bch2_btree_iter_verify(iter);
 	bch2_btree_iter_verify_entry_exit(iter);
 
 	while (1) {
-		btree_iter_set_search_pos(iter, search_key);
+		iter->path = btree_path_set_pos(trans, iter->path, search_key,
+						iter->flags & BTREE_ITER_INTENT);
 
-		ret = btree_iter_traverse(iter);
+		ret = bch2_btree_path_traverse(trans, iter->path, iter->flags);
 		if (unlikely(ret)) {
 			/* ensure that iter->k is consistent with iter->pos: */
 			bch2_btree_iter_set_pos(iter, iter->pos);
@@ -1828,18 +2137,20 @@ struct bkey_s_c bch2_btree_iter_peek_prev(struct btree_iter *iter)
 			goto out;
 		}
 
-		k = btree_iter_level_peek(iter, l);
+		k = btree_path_level_peek(trans, iter->path,
+					  &iter->path->l[0], &iter->k);
 		if (!k.k ||
 		    ((iter->flags & BTREE_ITER_IS_EXTENTS)
 		     ? bkey_cmp(bkey_start_pos(k.k), iter->pos) >= 0
 		     : bkey_cmp(k.k->p, iter->pos) > 0))
-			k = btree_iter_level_prev(iter, l);
+			k = btree_path_level_prev(trans, iter->path,
+						  &iter->path->l[0], &iter->k);
 
 		if (likely(k.k)) {
 			break;
-		} else if (likely(bpos_cmp(l->b->data->min_key, POS_MIN))) {
+		} else if (likely(bpos_cmp(iter->path->l[0].b->data->min_key, POS_MIN))) {
 			/* Advance to previous leaf node: */
-			search_key = bpos_predecessor(l->b->data->min_key);
+			search_key = bpos_predecessor(iter->path->l[0].b->data->min_key);
 		} else {
 			/* Start of btree: */
 			bch2_btree_iter_set_pos(iter, POS_MIN);
@@ -1854,9 +2165,11 @@ struct bkey_s_c bch2_btree_iter_peek_prev(struct btree_iter *iter)
 	if (bkey_cmp(k.k->p, iter->pos) < 0)
 		iter->pos = k.k->p;
 out:
-	iter->should_be_locked = true;
+	iter->path->should_be_locked = true;
+
 	bch2_btree_iter_verify_entry_exit(iter);
 	bch2_btree_iter_verify(iter);
+
 	return k;
 }
 
@@ -1879,7 +2192,7 @@ struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
 	struct bkey_s_c k;
 	int ret;
 
-	EBUG_ON(iter->level);
+	EBUG_ON(iter->path->level);
 	bch2_btree_iter_verify(iter);
 	bch2_btree_iter_verify_entry_exit(iter);
 
@@ -1893,9 +2206,10 @@ struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
 	}
 
 	search_key = btree_iter_search_key(iter);
-	btree_iter_set_search_pos(iter, search_key);
+	iter->path = btree_path_set_pos(trans, iter->path, search_key,
+					iter->flags & BTREE_ITER_INTENT);
 
-	ret = btree_iter_traverse(iter);
+	ret = bch2_btree_path_traverse(trans, iter->path, iter->flags);
 	if (unlikely(ret))
 		return bkey_s_c_err(ret);
 
@@ -1903,23 +2217,12 @@ struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
 		struct bkey_i *next_update;
 
 		next_update = btree_trans_peek_updates(iter);
-
-		if (!iter->cached) {
-			k = btree_iter_level_peek_all(iter, &iter->l[0]);
-			EBUG_ON(k.k && bkey_deleted(k.k) && bpos_cmp(k.k->p, iter->pos) == 0);
-		} else {
-			struct bkey_cached *ck = (void *) iter->l[0].b;
-			EBUG_ON(iter->btree_id != ck->key.btree_id ||
-				bkey_cmp(iter->pos, ck->key.pos));
-			BUG_ON(!ck->valid);
-
-			k = bkey_i_to_s_c(ck->k);
-		}
-
 		if (next_update &&
-		    (!k.k || bpos_cmp(next_update->k.p, k.k->p) <= 0)) {
+		    !bpos_cmp(next_update->k.p, iter->pos)) {
 			iter->k = next_update->k;
 			k = bkey_i_to_s_c(next_update);
+		} else {
+			k = bch2_btree_path_peek_slot(iter->path, &iter->k);
 		}
 
 		if (!k.k ||
@@ -1934,14 +2237,16 @@ struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
 		struct bpos next;
 
 		if (iter->flags & BTREE_ITER_INTENT) {
-			struct btree_iter *child =
-				btree_iter_child_alloc(trans, iter, _THIS_IP_);
+			struct btree_iter iter2;
 
-			btree_iter_copy(trans, child, iter);
-			k = bch2_btree_iter_peek(child);
+			bch2_trans_copy_iter(&iter2, iter);
+			k = bch2_btree_iter_peek(&iter2);
 
-			if (k.k && !bkey_err(k))
-				iter->k = child->k;
+			if (k.k && !bkey_err(k)) {
+				iter->k = iter2.k;
+				k.k = &iter->k;
+			}
+			bch2_trans_iter_exit(trans, &iter2);
 		} else {
 			struct bpos pos = iter->pos;
 
@@ -1969,9 +2274,10 @@ struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
 		}
 	}
 
+	iter->path->should_be_locked = true;
+
 	bch2_btree_iter_verify_entry_exit(iter);
 	bch2_btree_iter_verify(iter);
-	iter->should_be_locked = true;
 
 	return k;
 }
@@ -1992,47 +2298,26 @@ struct bkey_s_c bch2_btree_iter_prev_slot(struct btree_iter *iter)
 	return bch2_btree_iter_peek_slot(iter);
 }
 
-static inline void bch2_btree_iter_init(struct btree_trans *trans,
-			struct btree_iter *iter, enum btree_id btree_id)
-{
-	struct bch_fs *c = trans->c;
-	unsigned i;
-
-	iter->trans			= trans;
-	iter->uptodate			= BTREE_ITER_NEED_TRAVERSE;
-	iter->btree_id			= btree_id;
-	iter->real_pos			= POS_MIN;
-	iter->level			= 0;
-	iter->min_depth			= 0;
-	iter->locks_want		= 0;
-	iter->nodes_locked		= 0;
-	iter->nodes_intent_locked	= 0;
-	for (i = 0; i < ARRAY_SIZE(iter->l); i++)
-		iter->l[i].b		= BTREE_ITER_NO_NODE_INIT;
-
-	prefetch(c->btree_roots[btree_id].b);
-}
-
 /* new transactional stuff: */
 
 #ifdef CONFIG_BCACHEFS_DEBUG
 static void btree_trans_verify_sorted_refs(struct btree_trans *trans)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 	unsigned i;
 
-	BUG_ON(trans->nr_sorted != hweight64(trans->iters_linked));
+	BUG_ON(trans->nr_sorted != hweight64(trans->paths_allocated));
 
-	trans_for_each_iter(trans, iter) {
-		BUG_ON(iter->sorted_idx >= trans->nr_sorted);
-		BUG_ON(trans->sorted[iter->sorted_idx] != iter->idx);
+	trans_for_each_path(trans, path) {
+		BUG_ON(path->sorted_idx >= trans->nr_sorted);
+		BUG_ON(trans->sorted[path->sorted_idx] != path->idx);
 	}
 
 	for (i = 0; i < trans->nr_sorted; i++) {
 		unsigned idx = trans->sorted[i];
 
-		EBUG_ON(!(trans->iters_linked & (1ULL << idx)));
-		BUG_ON(trans->iters[idx].sorted_idx != i);
+		EBUG_ON(!(trans->paths_allocated & (1ULL << idx)));
+		BUG_ON(trans->paths[idx].sorted_idx != i);
 	}
 }
 #else
@@ -2042,17 +2327,17 @@ static inline void btree_trans_verify_sorted_refs(struct btree_trans *trans) {}
 static void btree_trans_verify_sorted(struct btree_trans *trans)
 {
 #ifdef CONFIG_BCACHEFS_DEBUG
-	struct btree_iter *iter, *prev = NULL;
+	struct btree_path *path, *prev = NULL;
 	unsigned i;
 
-	trans_for_each_iter_inorder(trans, iter, i) {
-		BUG_ON(prev && btree_iter_cmp(prev, iter) > 0);
-		prev = iter;
+	trans_for_each_path_inorder(trans, path, i) {
+		BUG_ON(prev && btree_path_cmp(prev, path) > 0);
+		prev = path;
 	}
 #endif
 }
 
-static noinline void __btree_trans_sort_iters(struct btree_trans *trans)
+static noinline void __btree_trans_sort_paths(struct btree_trans *trans)
 {
 	int i, l = 0, r = trans->nr_sorted, inc = 1;
 	bool swapped;
@@ -2067,11 +2352,11 @@ static noinline void __btree_trans_sort_iters(struct btree_trans *trans)
 		for (i = inc > 0 ? l : r - 2;
 		     i + 1 < r && i >= l;
 		     i += inc) {
-			if (btree_iter_cmp(trans->iters + trans->sorted[i],
-					   trans->iters + trans->sorted[i + 1]) > 0) {
+			if (btree_path_cmp(trans->paths + trans->sorted[i],
+					   trans->paths + trans->sorted[i + 1]) > 0) {
 				swap(trans->sorted[i], trans->sorted[i + 1]);
-				trans->iters[trans->sorted[i]].sorted_idx = i;
-				trans->iters[trans->sorted[i + 1]].sorted_idx = i + 1;
+				trans->paths[trans->sorted[i]].sorted_idx = i;
+				trans->paths[trans->sorted[i + 1]].sorted_idx = i + 1;
 				swapped = true;
 			}
 		}
@@ -2083,246 +2368,82 @@ static noinline void __btree_trans_sort_iters(struct btree_trans *trans)
 		inc = -inc;
 	} while (swapped);
 
-	trans->iters_sorted = true;
+	trans->paths_sorted = true;
 
 	btree_trans_verify_sorted(trans);
 }
 
-static inline void btree_trans_sort_iters(struct btree_trans *trans)
+static inline void btree_trans_sort_paths(struct btree_trans *trans)
 {
 	btree_trans_verify_sorted_refs(trans);
 
-	if (trans->iters_sorted) {
+	if (trans->paths_sorted) {
 		if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG))
 			btree_trans_verify_sorted(trans);
 		return;
 	}
-	__btree_trans_sort_iters(trans);
+	__btree_trans_sort_paths(trans);
 }
 
-static inline void btree_iter_list_remove(struct btree_trans *trans,
-					  struct btree_iter *iter)
+static inline void btree_path_list_remove(struct btree_trans *trans,
+					  struct btree_path *path)
 {
 	unsigned i;
 
-	EBUG_ON(iter->sorted_idx >= trans->nr_sorted);
+	EBUG_ON(path->sorted_idx >= trans->nr_sorted);
 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
 	trans->nr_sorted--;
-	memmove_u64s_down_small(trans->sorted + iter->sorted_idx,
-				trans->sorted + iter->sorted_idx + 1,
-				DIV_ROUND_UP(trans->nr_sorted - iter->sorted_idx, 8));
+	memmove_u64s_down_small(trans->sorted + path->sorted_idx,
+				trans->sorted + path->sorted_idx + 1,
+				DIV_ROUND_UP(trans->nr_sorted - path->sorted_idx, 8));
 #else
-	array_remove_item(trans->sorted, trans->nr_sorted, iter->sorted_idx);
+	array_remove_item(trans->sorted, trans->nr_sorted, path->sorted_idx);
 #endif
-	for (i = iter->sorted_idx; i < trans->nr_sorted; i++)
-		trans->iters[trans->sorted[i]].sorted_idx = i;
+	for (i = path->sorted_idx; i < trans->nr_sorted; i++)
+		trans->paths[trans->sorted[i]].sorted_idx = i;
 
-	iter->sorted_idx = U8_MAX;
+	path->sorted_idx = U8_MAX;
 }
 
-static inline void btree_iter_list_add(struct btree_trans *trans,
-				       struct btree_iter *pos,
-				       struct btree_iter *iter)
+static inline void btree_path_list_add(struct btree_trans *trans,
+				       struct btree_path *pos,
+				       struct btree_path *path)
 {
 	unsigned i;
 
-	iter->sorted_idx = pos ? pos->sorted_idx + 1 : 0;
+	path->sorted_idx = pos ? pos->sorted_idx + 1 : 0;
 
 #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
-	memmove_u64s_up_small(trans->sorted + iter->sorted_idx + 1,
-			      trans->sorted + iter->sorted_idx,
-			      DIV_ROUND_UP(trans->nr_sorted - iter->sorted_idx, 8));
+	memmove_u64s_up_small(trans->sorted + path->sorted_idx + 1,
+			      trans->sorted + path->sorted_idx,
+			      DIV_ROUND_UP(trans->nr_sorted - path->sorted_idx, 8));
 	trans->nr_sorted++;
-	trans->sorted[iter->sorted_idx] = iter->idx;
+	trans->sorted[path->sorted_idx] = path->idx;
 #else
-	array_insert_item(trans->sorted, trans->nr_sorted, iter->sorted_idx, iter->idx);
+	array_insert_item(trans->sorted, trans->nr_sorted, path->sorted_idx, path->idx);
 #endif
 
-	for (i = iter->sorted_idx; i < trans->nr_sorted; i++)
-		trans->iters[trans->sorted[i]].sorted_idx = i;
-
-	btree_trans_verify_sorted_refs(trans);
-}
-
-static void btree_iter_child_free(struct btree_trans *trans, struct btree_iter *iter)
-{
-	struct btree_iter *child = btree_iter_child(trans, iter);
-
-	if (child) {
-		bch2_trans_iter_free(trans, child);
-		iter->child_idx = U8_MAX;
-	}
-}
-
-static struct btree_iter *btree_iter_child_alloc(struct btree_trans *trans,
-						 struct btree_iter *iter,
-						 unsigned long ip)
-{
-	struct btree_iter *child = btree_iter_child(trans, iter);
-
-	if (!child) {
-		child = btree_trans_iter_alloc(trans, iter);
-		child->ip_allocated	= ip;
-		iter->child_idx		= child->idx;
-
-		trans->iters_live	|= 1ULL << child->idx;
-		trans->iters_touched	|= 1ULL << child->idx;
-	}
-
-	return child;
-}
-
-static inline void __bch2_trans_iter_free(struct btree_trans *trans,
-					  unsigned idx)
-{
-	btree_iter_child_free(trans, &trans->iters[idx]);
-
-	btree_iter_list_remove(trans, &trans->iters[idx]);
-
-	__bch2_btree_iter_unlock(&trans->iters[idx]);
-	trans->iters_linked		&= ~(1ULL << idx);
-	trans->iters_live		&= ~(1ULL << idx);
-	trans->iters_touched		&= ~(1ULL << idx);
-
-	btree_trans_verify_sorted_refs(trans);
-}
-
-static bool have_iter_at_pos(struct btree_trans *trans,
-			     struct btree_iter *iter)
-{
-	struct btree_iter *n;
-
-	n = prev_btree_iter(trans, iter);
-	if (n && !btree_iter_cmp(n, iter))
-		return true;
-
-	n = next_btree_iter(trans, iter);
-	if (n && !btree_iter_cmp(n, iter))
-		return true;
-
-	return false;
-}
-
-int bch2_trans_iter_put(struct btree_trans *trans,
-			struct btree_iter *iter)
-{
-	int ret;
-
-	if (IS_ERR_OR_NULL(iter))
-		return 0;
-
-	BUG_ON(trans->iters + iter->idx != iter);
-	BUG_ON(!btree_iter_live(trans, iter));
-
-	ret = btree_iter_err(iter);
-
-	if (!(iter->flags & BTREE_ITER_KEEP_UNTIL_COMMIT) &&
-	    (!(trans->iters_touched & (1ULL << iter->idx)) ||
-	     have_iter_at_pos(trans, iter)))
-		__bch2_trans_iter_free(trans, iter->idx);
-
-	trans->iters_live	&= ~(1ULL << iter->idx);
-	return ret;
-}
-
-int bch2_trans_iter_free(struct btree_trans *trans,
-			 struct btree_iter *iter)
-{
-	if (IS_ERR_OR_NULL(iter))
-		return 0;
-
-	set_btree_iter_dontneed(trans, iter);
-
-	return bch2_trans_iter_put(trans, iter);
-}
-
-noinline __cold
-void bch2_dump_trans_iters_updates(struct btree_trans *trans)
-{
-	struct btree_iter *iter;
-	struct btree_insert_entry *i;
-	unsigned idx;
-	char buf1[300], buf2[100];
-
-	btree_trans_sort_iters(trans);
-
-	trans_for_each_iter_inorder(trans, iter, idx)
-		printk(KERN_ERR "iter: btree %s pos %s real_pos %s%s%s%s %pS\n",
-		       bch2_btree_ids[iter->btree_id],
-		       (bch2_bpos_to_text(&PBUF(buf1), iter->pos), buf1),
-		       (bch2_bpos_to_text(&PBUF(buf2), iter->real_pos), buf2),
-		       btree_iter_live(trans, iter) ? " live" : "",
-		       (trans->iters_touched & (1ULL << iter->idx)) ? " touched" : "",
-		       iter->flags & BTREE_ITER_KEEP_UNTIL_COMMIT ? " keep" : "",
-		       (void *) iter->ip_allocated);
-
-	trans_for_each_update(trans, i)
-		printk(KERN_ERR "update: btree %s %s %pS\n",
-		       bch2_btree_ids[i->btree_id],
-		       (bch2_bkey_val_to_text(&PBUF(buf1), trans->c, bkey_i_to_s_c(i->k)), buf1),
-		       (void *) i->ip_allocated);
-}
-
-static struct btree_iter *btree_trans_iter_alloc(struct btree_trans *trans,
-						 struct btree_iter *pos)
-{
-	struct btree_iter *iter;
-	unsigned idx;
+	for (i = path->sorted_idx; i < trans->nr_sorted; i++)
+		trans->paths[trans->sorted[i]].sorted_idx = i;
 
 	btree_trans_verify_sorted_refs(trans);
-
-	if (unlikely(trans->iters_linked ==
-		     ~((~0ULL << 1) << (BTREE_ITER_MAX - 1)))) {
-		bch2_dump_trans_iters_updates(trans);
-		panic("trans iter oveflow\n");
-	}
-
-	idx = __ffs64(~trans->iters_linked);
-	iter = &trans->iters[idx];
-
-	iter->trans		= trans;
-	iter->idx		= idx;
-	iter->child_idx		= U8_MAX;
-	iter->sorted_idx	= U8_MAX;
-	iter->flags		= 0;
-	iter->nodes_locked	= 0;
-	iter->nodes_intent_locked = 0;
-	trans->iters_linked	|= 1ULL << idx;
-
-	btree_iter_list_add(trans, pos, iter);
-	return iter;
 }
 
-static void btree_iter_copy(struct btree_trans *trans, struct btree_iter *dst,
-			    struct btree_iter *src)
+void bch2_trans_iter_exit(struct btree_trans *trans, struct btree_iter *iter)
 {
-	unsigned i, offset = offsetof(struct btree_iter, flags);
-
-	__bch2_btree_iter_unlock(dst);
-	btree_iter_child_free(trans, dst);
-
-	memcpy((void *) dst + offset,
-	       (void *) src + offset,
-	       sizeof(struct btree_iter) - offset);
-
-	for (i = 0; i < BTREE_MAX_DEPTH; i++)
-		if (btree_node_locked(dst, i))
-			six_lock_increment(&dst->l[i].b->c.lock,
-					   __btree_lock_want(dst, i));
-
-	dst->flags &= ~BTREE_ITER_KEEP_UNTIL_COMMIT;
-	trans->iters_sorted = false;
+	if (iter->path)
+		bch2_path_put(trans, iter->path,
+			      iter->flags & BTREE_ITER_INTENT);
+	iter->path = NULL;
 }
 
-struct btree_iter *__bch2_trans_get_iter(struct btree_trans *trans,
-					 enum btree_id btree_id, struct bpos pos,
-					 unsigned locks_want,
-					 unsigned depth,
-					 unsigned flags)
+static void __bch2_trans_iter_init(struct btree_trans *trans,
+				   struct btree_iter *iter,
+				   enum btree_id btree_id, struct bpos pos,
+				   unsigned locks_want,
+				   unsigned depth,
+				   unsigned flags)
 {
-	struct btree_iter *iter, *best = NULL;
-	struct bpos real_pos, pos_min = POS_MIN;
-
 	EBUG_ON(trans->restarted);
 
 	if (!(flags & (BTREE_ITER_ALL_SNAPSHOTS|BTREE_ITER_NOT_EXTENTS)) &&
@@ -2337,127 +2458,58 @@ struct btree_iter *__bch2_trans_get_iter(struct btree_trans *trans,
 		pos.snapshot = btree_type_has_snapshots(btree_id)
 			? U32_MAX : 0;
 
-	real_pos = pos;
-
-	if ((flags & BTREE_ITER_IS_EXTENTS) &&
-	    bkey_cmp(pos, POS_MAX))
-		real_pos = bpos_nosnap_successor(pos);
-
-	trans_for_each_iter(trans, iter) {
-		if (iter->cached != (flags & BTREE_ITER_CACHED))
-			continue;
-
-		if (iter->btree_id != btree_id)
-			continue;
-
-		if (best) {
-			int cmp = bkey_cmp(bpos_diff(best->real_pos, real_pos),
-					   bpos_diff(iter->real_pos, real_pos));
-
-			if (cmp < 0 ||
-			    ((cmp == 0 && btree_iter_keep(trans, iter))))
-				continue;
-		}
-
-		best = iter;
-	}
-
-	if (!best) {
-		iter = btree_trans_iter_alloc(trans, best);
-		bch2_btree_iter_init(trans, iter, btree_id);
-	} else if (btree_iter_keep(trans, best)) {
-		iter = btree_trans_iter_alloc(trans, best);
-		btree_iter_copy(trans, iter, best);
-	} else {
-		iter = best;
-	}
-
-	trans->iters_live	|= 1ULL << iter->idx;
-	trans->iters_touched	|= 1ULL << iter->idx;
-
-	iter->cached	= flags & BTREE_ITER_CACHED;
+	iter->trans	= trans;
+	iter->path	= NULL;
+	iter->btree_id	= btree_id;
+	iter->min_depth	= depth;
 	iter->flags	= flags;
 	iter->snapshot	= pos.snapshot;
+	iter->pos	= pos;
+	iter->k.type	= KEY_TYPE_deleted;
+	iter->k.p	= pos;
+	iter->k.size	= 0;
 
-	/*
-	 * If the iterator has locks_want greater than requested, we explicitly
-	 * do not downgrade it here - on transaction restart because btree node
-	 * split needs to upgrade locks, we might be putting/getting the
-	 * iterator again. Downgrading iterators only happens via an explicit
-	 * bch2_trans_downgrade().
-	 */
-
-	locks_want = min(locks_want, BTREE_MAX_DEPTH);
-	if (locks_want > iter->locks_want) {
-		iter->locks_want = locks_want;
-		btree_iter_get_locks(trans, iter, true, _THIS_IP_);
-	}
-
-	while (iter->level != depth) {
-		btree_node_unlock(iter, iter->level);
-		iter->l[iter->level].b = BTREE_ITER_NO_NODE_INIT;
-		iter->uptodate = BTREE_ITER_NEED_TRAVERSE;
-		if (iter->level < depth)
-			iter->level++;
-		else
-			iter->level--;
-	}
-
-	iter->min_depth	= depth;
-
-	bch2_btree_iter_set_pos(iter, pos);
-	btree_iter_set_search_pos(iter, real_pos);
-
-	trace_trans_get_iter(_RET_IP_, trans->ip,
-			     btree_id,
-			     &real_pos, locks_want, iter->uptodate,
-			     best ? &best->real_pos	: &pos_min,
-			     best ? best->locks_want	: U8_MAX,
-			     best ? best->uptodate	: U8_MAX);
-
-	return iter;
+	iter->path = bch2_path_get(trans,
+				   flags & BTREE_ITER_CACHED,
+				   btree_id,
+				   btree_iter_search_key(iter),
+				   locks_want,
+				   depth,
+				   flags & BTREE_ITER_INTENT);
 }
 
-struct btree_iter *bch2_trans_get_node_iter(struct btree_trans *trans,
-					    enum btree_id btree_id,
-					    struct bpos pos,
-					    unsigned locks_want,
-					    unsigned depth,
-					    unsigned flags)
+void bch2_trans_iter_init(struct btree_trans *trans,
+			  struct btree_iter *iter,
+			  unsigned btree_id, struct bpos pos,
+			  unsigned flags)
 {
-	struct btree_iter *iter =
-		__bch2_trans_get_iter(trans, btree_id, pos,
-				      locks_want, depth,
-				      BTREE_ITER_NOT_EXTENTS|
-				      __BTREE_ITER_ALL_SNAPSHOTS|
-				      BTREE_ITER_ALL_SNAPSHOTS|
-				      flags);
-
-	BUG_ON(bkey_cmp(iter->pos, pos));
-	BUG_ON(iter->locks_want != min(locks_want, BTREE_MAX_DEPTH));
-	BUG_ON(iter->level	!= depth);
-	BUG_ON(iter->min_depth	!= depth);
-	iter->ip_allocated = _RET_IP_;
-
-	return iter;
+	__bch2_trans_iter_init(trans, iter, btree_id, pos,
+			       0, 0, flags);
 }
 
-struct btree_iter *__bch2_trans_copy_iter(struct btree_trans *trans,
-					  struct btree_iter *src)
+void bch2_trans_node_iter_init(struct btree_trans *trans,
+			       struct btree_iter *iter,
+			       enum btree_id btree_id,
+			       struct bpos pos,
+			       unsigned locks_want,
+			       unsigned depth,
+			       unsigned flags)
 {
-	struct btree_iter *iter;
-
-	iter = btree_trans_iter_alloc(trans, src);
-	btree_iter_copy(trans, iter, src);
-
-	trans->iters_live |= 1ULL << iter->idx;
-	/*
-	 * We don't need to preserve this iter since it's cheap to copy it
-	 * again - this will cause trans_iter_put() to free it right away:
-	 */
-	set_btree_iter_dontneed(trans, iter);
+	__bch2_trans_iter_init(trans, iter, btree_id, pos, locks_want, depth,
+			       BTREE_ITER_NOT_EXTENTS|
+			       __BTREE_ITER_ALL_SNAPSHOTS|
+			       BTREE_ITER_ALL_SNAPSHOTS|
+			       flags);
+	BUG_ON(iter->path->locks_want	 < min(locks_want, BTREE_MAX_DEPTH));
+	BUG_ON(iter->path->level	!= depth);
+	BUG_ON(iter->min_depth		!= depth);
+}
 
-	return iter;
+void bch2_trans_copy_iter(struct btree_iter *dst, struct btree_iter *src)
+{
+	*dst = *src;
+	if (src->path)
+		__btree_path_get(src->path, src->flags & BTREE_ITER_INTENT);
 }
 
 void *bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
@@ -2498,20 +2550,6 @@ void *bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
 	return p;
 }
 
-inline void bch2_trans_unlink_iters(struct btree_trans *trans)
-{
-	u64 iters = trans->iters_linked &
-		~trans->iters_touched &
-		~trans->iters_live;
-
-	while (iters) {
-		unsigned idx = __ffs64(iters);
-
-		iters &= ~(1ULL << idx);
-		__bch2_trans_iter_free(trans, idx);
-	}
-}
-
 /**
  * bch2_trans_begin() - reset a transaction after a interrupted attempt
  * @trans: transaction to reset
@@ -2522,17 +2560,11 @@ inline void bch2_trans_unlink_iters(struct btree_trans *trans)
  */
 void bch2_trans_begin(struct btree_trans *trans)
 {
-	struct btree_iter *iter;
-
-	trans_for_each_iter(trans, iter)
-		iter->flags &= ~BTREE_ITER_KEEP_UNTIL_COMMIT;
+	struct btree_insert_entry *i;
+	struct btree_path *path;
 
-	/*
-	 * XXX: we shouldn't be doing this if the transaction was restarted, but
-	 * currently we still overflow transaction iterators if we do that
-	 * */
-	bch2_trans_unlink_iters(trans);
-	trans->iters_touched &= trans->iters_live;
+	trans_for_each_update(trans, i)
+		__btree_path_put(i->path, true);
 
 	trans->extra_journal_res	= 0;
 	trans->nr_updates		= 0;
@@ -2550,29 +2582,41 @@ void bch2_trans_begin(struct btree_trans *trans)
 		       (void *) &trans->fs_usage_deltas->memset_start);
 	}
 
+	trans_for_each_path(trans, path) {
+		/*
+		 * XXX: we probably shouldn't be doing this if the transaction
+		 * was restarted, but currently we still overflow transaction
+		 * iterators if we do that
+		 */
+		if (!path->ref && !path->preserve)
+			__bch2_path_free(trans, path);
+		else
+			path->preserve = path->should_be_locked = false;
+	}
+
 	bch2_trans_cond_resched(trans);
 
 	if (trans->restarted)
-		bch2_btree_iter_traverse_all(trans);
+		bch2_btree_path_traverse_all(trans);
 
 	trans->restarted = false;
 }
 
-static void bch2_trans_alloc_iters(struct btree_trans *trans, struct bch_fs *c)
+static void bch2_trans_alloc_paths(struct btree_trans *trans, struct bch_fs *c)
 {
-	size_t iters_bytes	= sizeof(struct btree_iter) * BTREE_ITER_MAX;
+	size_t paths_bytes	= sizeof(struct btree_path) * BTREE_ITER_MAX;
 	size_t updates_bytes	= sizeof(struct btree_insert_entry) * BTREE_ITER_MAX;
 	void *p = NULL;
 
 	BUG_ON(trans->used_mempool);
 
 #ifdef __KERNEL__
-	p = this_cpu_xchg(c->btree_iters_bufs->iter, NULL);
+	p = this_cpu_xchg(c->btree_paths_bufs->path , NULL);
 #endif
 	if (!p)
-		p = mempool_alloc(&trans->c->btree_iters_pool, GFP_NOFS);
+		p = mempool_alloc(&trans->c->btree_paths_pool, GFP_NOFS);
 
-	trans->iters		= p; p += iters_bytes;
+	trans->paths		= p; p += paths_bytes;
 	trans->updates		= p; p += updates_bytes;
 }
 
@@ -2585,11 +2629,7 @@ void bch2_trans_init(struct btree_trans *trans, struct bch_fs *c,
 	trans->c		= c;
 	trans->ip		= _RET_IP_;
 
-	/*
-	 * reallocating iterators currently completely breaks
-	 * bch2_trans_iter_put(), we always allocate the max:
-	 */
-	bch2_trans_alloc_iters(trans, c);
+	bch2_trans_alloc_paths(trans, c);
 
 	if (expected_mem_bytes) {
 		expected_mem_bytes = roundup_pow_of_two(expected_mem_bytes);
@@ -2613,54 +2653,63 @@ void bch2_trans_init(struct btree_trans *trans, struct bch_fs *c,
 #endif
 }
 
+static void check_btree_paths_leaked(struct btree_trans *trans)
+{
+#ifdef CONFIG_BCACHEFS_DEBUG
+	struct bch_fs *c = trans->c;
+	struct btree_path *path;
+
+	trans_for_each_path(trans, path)
+		if (path->ref)
+			goto leaked;
+	return;
+leaked:
+	bch_err(c, "btree paths leaked from %pS!", (void *) trans->ip);
+	trans_for_each_path(trans, path)
+		if (path->ref)
+			printk(KERN_ERR "  btree %s %pS\n",
+			       bch2_btree_ids[path->btree_id],
+			       (void *) path->ip_allocated);
+	/* Be noisy about this: */
+	bch2_fatal_error(c);
+#endif
+}
+
 int bch2_trans_exit(struct btree_trans *trans)
 	__releases(&c->btree_trans_barrier)
 {
+	struct btree_insert_entry *i;
 	struct bch_fs *c = trans->c;
 
 	bch2_trans_unlock(trans);
 
-#ifdef CONFIG_BCACHEFS_DEBUG
-	if (trans->iters_live) {
-		struct btree_iter *iter;
-
-		trans_for_each_iter(trans, iter)
-			btree_iter_child_free(trans, iter);
-	}
+	trans_for_each_update(trans, i)
+		__btree_path_put(i->path, true);
+	trans->nr_updates		= 0;
 
-	if (trans->iters_live) {
-		struct btree_iter *iter;
-
-		bch_err(c, "btree iterators leaked!");
-		trans_for_each_iter(trans, iter)
-			if (btree_iter_live(trans, iter))
-				printk(KERN_ERR "  btree %s allocated at %pS\n",
-				       bch2_btree_ids[iter->btree_id],
-				       (void *) iter->ip_allocated);
-		/* Be noisy about this: */
-		bch2_fatal_error(c);
-	}
+	check_btree_paths_leaked(trans);
 
-	mutex_lock(&trans->c->btree_trans_lock);
+#ifdef CONFIG_BCACHEFS_DEBUG
+	mutex_lock(&c->btree_trans_lock);
 	list_del(&trans->list);
-	mutex_unlock(&trans->c->btree_trans_lock);
+	mutex_unlock(&c->btree_trans_lock);
 #endif
 
 	srcu_read_unlock(&c->btree_trans_barrier, trans->srcu_idx);
 
-	bch2_journal_preres_put(&trans->c->journal, &trans->journal_preres);
+	bch2_journal_preres_put(&c->journal, &trans->journal_preres);
 
 	if (trans->fs_usage_deltas) {
 		if (trans->fs_usage_deltas->size + sizeof(trans->fs_usage_deltas) ==
 		    REPLICAS_DELTA_LIST_MAX)
 			mempool_free(trans->fs_usage_deltas,
-				     &trans->c->replicas_delta_pool);
+				     &c->replicas_delta_pool);
 		else
 			kfree(trans->fs_usage_deltas);
 	}
 
 	if (trans->mem_bytes == BTREE_TRANS_MEM_MAX)
-		mempool_free(trans->mem, &trans->c->btree_trans_mem_pool);
+		mempool_free(trans->mem, &c->btree_trans_mem_pool);
 	else
 		kfree(trans->mem);
 
@@ -2668,20 +2717,20 @@ int bch2_trans_exit(struct btree_trans *trans)
 	/*
 	 * Userspace doesn't have a real percpu implementation:
 	 */
-	trans->iters = this_cpu_xchg(c->btree_iters_bufs->iter, trans->iters);
+	trans->paths = this_cpu_xchg(c->btree_paths_bufs->path, trans->paths);
 #endif
 
-	if (trans->iters)
-		mempool_free(trans->iters, &trans->c->btree_iters_pool);
+	if (trans->paths)
+		mempool_free(trans->paths, &c->btree_paths_pool);
 
 	trans->mem	= (void *) 0x1;
-	trans->iters	= (void *) 0x1;
+	trans->paths	= (void *) 0x1;
 
 	return trans->error ? -EIO : 0;
 }
 
 static void __maybe_unused
-bch2_btree_iter_node_to_text(struct printbuf *out,
+bch2_btree_path_node_to_text(struct printbuf *out,
 			     struct btree_bkey_cached_common *_b,
 			     bool cached)
 {
@@ -2693,10 +2742,10 @@ bch2_btree_iter_node_to_text(struct printbuf *out,
 #ifdef CONFIG_BCACHEFS_DEBUG
 static bool trans_has_locks(struct btree_trans *trans)
 {
-	struct btree_iter *iter;
+	struct btree_path *path;
 
-	trans_for_each_iter(trans, iter)
-		if (iter->nodes_locked)
+	trans_for_each_path(trans, path)
+		if (path->nodes_locked)
 			return true;
 	return false;
 }
@@ -2706,7 +2755,7 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct bch_fs *c)
 {
 #ifdef CONFIG_BCACHEFS_DEBUG
 	struct btree_trans *trans;
-	struct btree_iter *iter;
+	struct btree_path *path;
 	struct btree *b;
 	unsigned l;
 
@@ -2717,24 +2766,24 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct bch_fs *c)
 
 		pr_buf(out, "%i %ps\n", trans->pid, (void *) trans->ip);
 
-		trans_for_each_iter(trans, iter) {
-			if (!iter->nodes_locked)
+		trans_for_each_path(trans, path) {
+			if (!path->nodes_locked)
 				continue;
 
-			pr_buf(out, "  iter %u %c %s:",
-			       iter->idx,
-			       iter->cached ? 'c' : 'b',
-			       bch2_btree_ids[iter->btree_id]);
-			bch2_bpos_to_text(out, iter->pos);
+			pr_buf(out, "  path %u %c %s:",
+			       path->idx,
+			       path->cached ? 'c' : 'b',
+			       bch2_btree_ids[path->btree_id]);
+			bch2_bpos_to_text(out, path->pos);
 			pr_buf(out, "\n");
 
 			for (l = 0; l < BTREE_MAX_DEPTH; l++) {
-				if (btree_node_locked(iter, l)) {
+				if (btree_node_locked(path, l)) {
 					pr_buf(out, "    %s l=%u ",
-					       btree_node_intent_locked(iter, l) ? "i" : "r", l);
-					bch2_btree_iter_node_to_text(out,
-							(void *) iter->l[l].b,
-							iter->cached);
+					       btree_node_intent_locked(path, l) ? "i" : "r", l);
+					bch2_btree_path_node_to_text(out,
+							(void *) path->l[l].b,
+							path->cached);
 					pr_buf(out, "\n");
 				}
 			}
@@ -2742,18 +2791,17 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct bch_fs *c)
 
 		b = READ_ONCE(trans->locking);
 		if (b) {
-			iter = &trans->iters[trans->locking_iter_idx];
-			pr_buf(out, "  locking iter %u %c l=%u %s:",
-			       trans->locking_iter_idx,
-			       iter->cached ? 'c' : 'b',
+			path = &trans->paths[trans->locking_path_idx];
+			pr_buf(out, "  locking path %u %c l=%u %s:",
+			       trans->locking_path_idx,
+			       path->cached ? 'c' : 'b',
 			       trans->locking_level,
 			       bch2_btree_ids[trans->locking_btree_id]);
 			bch2_bpos_to_text(out, trans->locking_pos);
 
 			pr_buf(out, " node ");
-			bch2_btree_iter_node_to_text(out,
-					(void *) b,
-					iter->cached);
+			bch2_btree_path_node_to_text(out,
+					(void *) b, path->cached);
 			pr_buf(out, "\n");
 		}
 	}
@@ -2764,7 +2812,7 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct bch_fs *c)
 void bch2_fs_btree_iter_exit(struct bch_fs *c)
 {
 	mempool_exit(&c->btree_trans_mem_pool);
-	mempool_exit(&c->btree_iters_pool);
+	mempool_exit(&c->btree_paths_pool);
 	cleanup_srcu_struct(&c->btree_trans_barrier);
 }
 
@@ -2776,8 +2824,8 @@ int bch2_fs_btree_iter_init(struct bch_fs *c)
 	mutex_init(&c->btree_trans_lock);
 
 	return  init_srcu_struct(&c->btree_trans_barrier) ?:
-		mempool_init_kmalloc_pool(&c->btree_iters_pool, 1,
-			sizeof(struct btree_iter) * nr +
+		mempool_init_kmalloc_pool(&c->btree_paths_pool, 1,
+			sizeof(struct btree_path) * nr +
 			sizeof(struct btree_insert_entry) * nr) ?:
 		mempool_init_kmalloc_pool(&c->btree_trans_mem_pool, 1,
 					  BTREE_TRANS_MEM_MAX);