Merge with 1f431b384d bcachefs: Refactor trans_(get|update)_key

1 files changed, 1082 insertions, 489 deletions

diff --git a/fs/bcachefs/btree_iter.c b/fs/bcachefs/btree_iter.c
index 70c3132eb538..8955555d6603 100644
--- a/fs/bcachefs/btree_iter.c
+++ b/fs/bcachefs/btree_iter.c
@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0
 
 #include "bcachefs.h"
 #include "bkey_methods.h"
@@ -14,11 +15,51 @@ static inline struct bkey_s_c __btree_iter_peek_all(struct btree_iter *,
 						    struct btree_iter_level *,
 						    struct bkey *);
 
-#define BTREE_ITER_NOT_END	((struct btree *) 1)
+#define BTREE_ITER_NO_NODE_GET_LOCKS	((struct btree *) 1)
+#define BTREE_ITER_NO_NODE_DROP		((struct btree *) 2)
+#define BTREE_ITER_NO_NODE_LOCK_ROOT	((struct btree *) 3)
+#define BTREE_ITER_NO_NODE_UP		((struct btree *) 4)
+#define BTREE_ITER_NO_NODE_DOWN		((struct btree *) 5)
+#define BTREE_ITER_NO_NODE_INIT		((struct btree *) 6)
+#define BTREE_ITER_NO_NODE_ERROR	((struct btree *) 7)
 
 static inline bool is_btree_node(struct btree_iter *iter, unsigned l)
 {
-	return iter->l[l].b && iter->l[l].b != BTREE_ITER_NOT_END;
+	return l < BTREE_MAX_DEPTH &&
+		(unsigned long) iter->l[l].b >= 128;
+}
+
+/* Returns < 0 if @k is before iter pos, > 0 if @k is after */
+static inline int __btree_iter_pos_cmp(struct btree_iter *iter,
+				       const struct btree *b,
+				       const struct bkey_packed *k,
+				       bool interior_node)
+{
+	int cmp = bkey_cmp_left_packed(b, k, &iter->pos);
+
+	if (cmp)
+		return cmp;
+	if (bkey_deleted(k))
+		return -1;
+
+	/*
+	 * Normally, for extents we want the first key strictly greater than
+	 * the iterator position - with the exception that for interior nodes,
+	 * we don't want to advance past the last key if the iterator position
+	 * is POS_MAX:
+	 */
+	if (iter->flags & BTREE_ITER_IS_EXTENTS &&
+	    (!interior_node ||
+	     bkey_cmp_left_packed_byval(b, k, POS_MAX)))
+		return -1;
+	return 1;
+}
+
+static inline int btree_iter_pos_cmp(struct btree_iter *iter,
+				     const struct btree *b,
+				     const struct bkey_packed *k)
+{
+	return __btree_iter_pos_cmp(iter, b, k, b->level != 0);
 }
 
 /* Btree node locking: */
@@ -32,23 +73,22 @@ void bch2_btree_node_unlock_write(struct btree *b, struct btree_iter *iter)
 	struct btree_iter *linked;
 
 	EBUG_ON(iter->l[b->level].b != b);
-	EBUG_ON(iter->lock_seq[b->level] + 1 != b->lock.state.seq);
+	EBUG_ON(iter->l[b->level].lock_seq + 1 != b->lock.state.seq);
 
-	for_each_linked_btree_node(iter, b, linked)
-		linked->lock_seq[b->level] += 2;
-
-	iter->lock_seq[b->level] += 2;
+	trans_for_each_iter_with_node(iter->trans, b, linked)
+		linked->l[b->level].lock_seq += 2;
 
 	six_unlock_write(&b->lock);
 }
 
 void __bch2_btree_node_lock_write(struct btree *b, struct btree_iter *iter)
 {
-	struct bch_fs *c = iter->c;
 	struct btree_iter *linked;
 	unsigned readers = 0;
 
-	for_each_linked_btree_iter(iter, linked)
+	EBUG_ON(btree_node_read_locked(iter, b->level));
+
+	trans_for_each_iter(iter->trans, linked)
 		if (linked->l[b->level].b == b &&
 		    btree_node_read_locked(linked, b->level))
 			readers++;
@@ -61,63 +101,112 @@ void __bch2_btree_node_lock_write(struct btree *b, struct btree_iter *iter)
 	 */
 	atomic64_sub(__SIX_VAL(read_lock, readers),
 		     &b->lock.state.counter);
-	btree_node_lock_type(c, b, SIX_LOCK_write);
+	btree_node_lock_type(iter->trans->c, b, SIX_LOCK_write);
 	atomic64_add(__SIX_VAL(read_lock, readers),
 		     &b->lock.state.counter);
 }
 
 bool __bch2_btree_node_relock(struct btree_iter *iter, unsigned level)
 {
-	struct btree_iter *linked;
-	struct btree *b = iter->l[level].b;
-	int want = btree_lock_want(iter, level);
-	int have = btree_node_locked_type(iter, level);
+	struct btree *b = btree_iter_node(iter, level);
+	int want = __btree_lock_want(iter, level);
+
+	if (!is_btree_node(iter, level))
+		return false;
+
+	if (race_fault())
+		return false;
 
-	if (want == have)
+	if (six_relock_type(&b->lock, want, iter->l[level].lock_seq) ||
+	    (btree_node_lock_seq_matches(iter, b, level) &&
+	     btree_node_lock_increment(iter, b, level, want))) {
+		mark_btree_node_locked(iter, level, want);
 		return true;
+	} else {
+		return false;
+	}
+}
+
+static bool bch2_btree_node_upgrade(struct btree_iter *iter, unsigned level)
+{
+	struct btree *b = iter->l[level].b;
+
+	EBUG_ON(btree_lock_want(iter, level) != BTREE_NODE_INTENT_LOCKED);
 
 	if (!is_btree_node(iter, level))
 		return false;
 
+	if (btree_node_intent_locked(iter, level))
+		return true;
+
 	if (race_fault())
 		return false;
 
-	if (have != BTREE_NODE_UNLOCKED
-	    ? six_trylock_convert(&b->lock, have, want)
-	    : six_relock_type(&b->lock, want, iter->lock_seq[level]))
+	if (btree_node_locked(iter, level)
+	    ? six_lock_tryupgrade(&b->lock)
+	    : six_relock_type(&b->lock, SIX_LOCK_intent, iter->l[level].lock_seq))
 		goto success;
 
-	for_each_linked_btree_iter(iter, linked)
-		if (linked->l[level].b == b &&
-		    btree_node_locked_type(linked, level) == want &&
-		    iter->lock_seq[level] == b->lock.state.seq) {
-			btree_node_unlock(iter, level);
-			six_lock_increment(&b->lock, want);
-			goto success;
-		}
+	if (btree_node_lock_seq_matches(iter, b, level) &&
+	    btree_node_lock_increment(iter, b, level, BTREE_NODE_INTENT_LOCKED)) {
+		btree_node_unlock(iter, level);
+		goto success;
+	}
 
 	return false;
 success:
-	mark_btree_node_unlocked(iter, level);
-	mark_btree_node_locked(iter, level, want);
+	mark_btree_node_intent_locked(iter, level);
 	return true;
 }
 
-bool bch2_btree_iter_relock(struct btree_iter *iter)
+static inline bool btree_iter_get_locks(struct btree_iter *iter,
+					bool upgrade, bool trace)
 {
-	unsigned l;
+	unsigned l = iter->level;
+	int fail_idx = -1;
 
-	for (l = iter->level;
-	     l < max_t(unsigned, iter->locks_want, 1) && iter->l[l].b;
-	     l++)
-		if (!bch2_btree_node_relock(iter, l)) {
+	do {
+		if (!btree_iter_node(iter, l))
+			break;
+
+		if (!(upgrade
+		      ? bch2_btree_node_upgrade(iter, l)
+		      : bch2_btree_node_relock(iter, l))) {
+			if (trace)
+				(upgrade
+				 ? trace_node_upgrade_fail
+				 : trace_node_relock_fail)(l, iter->l[l].lock_seq,
+						is_btree_node(iter, l)
+						? 0
+						: (unsigned long) iter->l[l].b,
+						is_btree_node(iter, l)
+						? iter->l[l].b->lock.state.seq
+						: 0);
+
+			fail_idx = l;
 			btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
-			return false;
 		}
 
+		l++;
+	} while (l < iter->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
+	 * 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;
+		--fail_idx;
+	}
+
 	if (iter->uptodate == BTREE_ITER_NEED_RELOCK)
 		iter->uptodate = BTREE_ITER_NEED_PEEK;
-	return true;
+
+	bch2_btree_trans_verify_locks(iter->trans);
+
+	return iter->uptodate < BTREE_ITER_NEED_RELOCK;
 }
 
 /* Slowpath: */
@@ -126,40 +215,18 @@ bool __bch2_btree_node_lock(struct btree *b, struct bpos pos,
 			   struct btree_iter *iter,
 			   enum six_lock_type type)
 {
-	struct bch_fs *c = iter->c;
 	struct btree_iter *linked;
+	bool ret = true;
 
-	/* Can't have children locked before ancestors: */
-	EBUG_ON(iter->nodes_locked && level > __ffs(iter->nodes_locked));
-
-	/*
-	 * Can't hold any read locks while we block taking an intent lock - see
-	 * below for reasoning, and we should have already dropped any read
-	 * locks in the current iterator
-	 */
-	EBUG_ON(type == SIX_LOCK_intent &&
-		iter->nodes_locked != iter->nodes_intent_locked);
-
-	for_each_linked_btree_iter(iter, linked)
-		if (linked->l[level].b == b &&
-		    btree_node_locked_type(linked, level) == type) {
-			six_lock_increment(&b->lock, type);
-			return true;
-		}
-
-	/*
-	 * Must lock btree nodes in key order - this case hapens when locking
-	 * the prev sibling in btree node merging:
-	 */
-	if (iter->nodes_locked &&
-	    __ffs(iter->nodes_locked) == level &&
-	    __btree_iter_cmp(iter->btree_id, pos, iter))
-		return false;
-
-	for_each_linked_btree_iter(iter, linked) {
+	/* Check if it's safe to block: */
+	trans_for_each_iter(iter->trans, linked) {
 		if (!linked->nodes_locked)
 			continue;
 
+		/* * Must lock btree nodes in key order: */
+		if (__btree_iter_cmp(iter->btree_id, pos, linked) < 0)
+			ret = false;
+
 		/*
 		 * Can't block taking an intent lock if we have _any_ nodes read
 		 * locked:
@@ -174,16 +241,15 @@ bool __bch2_btree_node_lock(struct btree *b, struct bpos pos,
 		 */
 		if (type == SIX_LOCK_intent &&
 		    linked->nodes_locked != linked->nodes_intent_locked) {
-			linked->locks_want = max_t(unsigned,
-						   linked->locks_want,
-						   iter->locks_want);
-			return false;
+			if (!(iter->trans->nounlock)) {
+				linked->locks_want = max_t(unsigned,
+						linked->locks_want,
+						__fls(linked->nodes_locked) + 1);
+				btree_iter_get_locks(linked, true, false);
+			}
+			ret = false;
 		}
 
-		/* We have to lock btree nodes in key order: */
-		if (__btree_iter_cmp(iter->btree_id, pos, linked) < 0)
-			return false;
-
 		/*
 		 * Interior nodes must be locked before their descendants: if
 		 * another iterator has possible descendants locked of the node
@@ -191,87 +257,169 @@ bool __bch2_btree_node_lock(struct btree *b, struct bpos pos,
 		 */
 		if (linked->btree_id == iter->btree_id &&
 		    level > __fls(linked->nodes_locked)) {
-			linked->locks_want = max_t(unsigned,
-						   linked->locks_want,
-						   iter->locks_want);
-			return false;
+			if (!(iter->trans->nounlock)) {
+				linked->locks_want =
+					max(level + 1, max_t(unsigned,
+					    linked->locks_want,
+					    iter->locks_want));
+				btree_iter_get_locks(linked, true, false);
+			}
+			ret = false;
 		}
 	}
 
-	__btree_node_lock_type(c, b, type);
+	if (unlikely(!ret)) {
+		trace_trans_restart_would_deadlock(iter->trans->ip);
+		return false;
+	}
+
+	__btree_node_lock_type(iter->trans->c, b, type);
 	return true;
 }
 
 /* Btree iterator locking: */
 
-static void btree_iter_drop_extra_locks(struct btree_iter *iter)
+#ifdef CONFIG_BCACHEFS_DEBUG
+void bch2_btree_iter_verify_locks(struct btree_iter *iter)
 {
 	unsigned l;
 
-	while (iter->nodes_locked &&
-	       (l = __fls(iter->nodes_locked)) > iter->locks_want) {
-		if (l > iter->level) {
-			btree_node_unlock(iter, l);
-		} else {
-			if (btree_node_intent_locked(iter, l)) {
-				six_lock_downgrade(&iter->l[l].b->lock);
-				iter->nodes_intent_locked ^= 1 << l;
-			}
-			break;
-		}
+	for (l = 0; btree_iter_node(iter, l); l++) {
+		if (iter->uptodate >= BTREE_ITER_NEED_RELOCK &&
+		    !btree_node_locked(iter, l))
+			continue;
+
+		BUG_ON(btree_lock_want(iter, l) !=
+		       btree_node_locked_type(iter, l));
 	}
 }
 
-bool __bch2_btree_iter_set_locks_want(struct btree_iter *iter,
-				     unsigned new_locks_want)
+void bch2_btree_trans_verify_locks(struct btree_trans *trans)
+{
+	struct btree_iter *iter;
+
+	trans_for_each_iter(trans, iter)
+		bch2_btree_iter_verify_locks(iter);
+}
+#endif
+
+__flatten
+static bool bch2_btree_iter_relock(struct btree_iter *iter, bool trace)
+{
+	return iter->uptodate >= BTREE_ITER_NEED_RELOCK
+		? btree_iter_get_locks(iter, false, trace)
+		: true;
+}
+
+bool __bch2_btree_iter_upgrade(struct btree_iter *iter,
+			       unsigned new_locks_want)
 {
 	struct btree_iter *linked;
 
-	/* Drop locks we don't want anymore: */
-	if (new_locks_want < iter->locks_want)
-		for_each_linked_btree_iter(iter, linked)
-			if (linked->locks_want > new_locks_want) {
-				linked->locks_want = max_t(unsigned, 1,
-							   new_locks_want);
-				btree_iter_drop_extra_locks(linked);
-			}
+	EBUG_ON(iter->locks_want >= new_locks_want);
 
 	iter->locks_want = new_locks_want;
-	btree_iter_drop_extra_locks(iter);
 
-	if (bch2_btree_iter_relock(iter))
+	if (btree_iter_get_locks(iter, true, true))
 		return true;
 
 	/*
-	 * Just an optimization: ancestor nodes must be locked before child
-	 * nodes, so set locks_want on iterators that might lock ancestors
-	 * before us to avoid getting -EINTR later:
+	 * Ancestor nodes must be locked before child nodes, so set locks_want
+	 * on iterators that might lock ancestors before us to avoid getting
+	 * -EINTR later:
 	 */
-	for_each_linked_btree_iter(iter, linked)
-		if (linked->btree_id == iter->btree_id &&
-		    btree_iter_cmp(linked, iter) <= 0)
-			linked->locks_want = max_t(unsigned, linked->locks_want,
-						   new_locks_want);
+	trans_for_each_iter(iter->trans, linked)
+		if (linked != iter &&
+		    linked->btree_id == iter->btree_id &&
+		    linked->locks_want < new_locks_want) {
+			linked->locks_want = new_locks_want;
+			btree_iter_get_locks(linked, true, false);
+		}
+
 	return false;
 }
 
-static void __bch2_btree_iter_unlock(struct btree_iter *iter)
+bool __bch2_btree_iter_upgrade_nounlock(struct btree_iter *iter,
+					unsigned new_locks_want)
 {
-	btree_iter_set_dirty(iter, BTREE_ITER_NEED_RELOCK);
+	unsigned l = iter->level;
+
+	EBUG_ON(iter->locks_want >= new_locks_want);
+
+	iter->locks_want = new_locks_want;
+
+	do {
+		if (!btree_iter_node(iter, l))
+			break;
+
+		if (!bch2_btree_node_upgrade(iter, l)) {
+			iter->locks_want = l;
+			return false;
+		}
 
-	while (iter->nodes_locked)
-		btree_node_unlock(iter, __ffs(iter->nodes_locked));
+		l++;
+	} while (l < iter->locks_want);
+
+	return true;
 }
 
-int bch2_btree_iter_unlock(struct btree_iter *iter)
+void __bch2_btree_iter_downgrade(struct btree_iter *iter,
+				 unsigned downgrade_to)
 {
 	struct btree_iter *linked;
+	unsigned l;
+
+	/*
+	 * We downgrade linked iterators as well because btree_iter_upgrade
+	 * might have had to modify locks_want on linked iterators due to lock
+	 * ordering:
+	 */
+	trans_for_each_iter(iter->trans, linked) {
+		unsigned new_locks_want = downgrade_to ?:
+			(linked->flags & BTREE_ITER_INTENT ? 1 : 0);
+
+		if (linked->locks_want <= new_locks_want)
+			continue;
 
-	for_each_linked_btree_iter(iter, linked)
-		__bch2_btree_iter_unlock(linked);
-	__bch2_btree_iter_unlock(iter);
+		linked->locks_want = new_locks_want;
+
+		while (linked->nodes_locked &&
+		       (l = __fls(linked->nodes_locked)) >= linked->locks_want) {
+			if (l > linked->level) {
+				btree_node_unlock(linked, l);
+			} else {
+				if (btree_node_intent_locked(linked, l)) {
+					six_lock_downgrade(&linked->l[l].b->lock);
+					linked->nodes_intent_locked ^= 1 << l;
+				}
+				break;
+			}
+		}
+	}
 
-	return iter->flags & BTREE_ITER_ERROR ? -EIO : 0;
+	bch2_btree_trans_verify_locks(iter->trans);
+}
+
+/* Btree transaction locking: */
+
+bool bch2_trans_relock(struct btree_trans *trans)
+{
+	struct btree_iter *iter;
+	bool ret = true;
+
+	trans_for_each_iter(trans, iter)
+		if (iter->uptodate == BTREE_ITER_NEED_RELOCK)
+			ret &= bch2_btree_iter_relock(iter, true);
+
+	return ret;
+}
+
+void bch2_trans_unlock(struct btree_trans *trans)
+{
+	struct btree_iter *iter;
+
+	trans_for_each_iter(trans, iter)
+		__bch2_btree_iter_unlock(iter);
 }
 
 /* Btree iterator: */
@@ -285,48 +433,67 @@ static void __bch2_btree_iter_verify(struct btree_iter *iter,
 	struct btree_node_iter tmp = l->iter;
 	struct bkey_packed *k;
 
+	if (!debug_check_iterators(iter->trans->c))
+		return;
+
+	if (iter->uptodate > BTREE_ITER_NEED_PEEK)
+		return;
+
 	bch2_btree_node_iter_verify(&l->iter, b);
 
 	/*
 	 * For interior nodes, the iterator will have skipped past
 	 * deleted keys:
+	 *
+	 * For extents, the iterator may have skipped past deleted keys (but not
+	 * whiteouts)
 	 */
-	k = b->level
-		? bch2_btree_node_iter_prev(&tmp, b)
+	k = b->level || iter->flags & BTREE_ITER_IS_EXTENTS
+		? bch2_btree_node_iter_prev_filter(&tmp, b, KEY_TYPE_discard)
 		: bch2_btree_node_iter_prev_all(&tmp, b);
-	if (k && btree_iter_pos_cmp_packed(b, &iter->pos, k,
-				iter->flags & BTREE_ITER_IS_EXTENTS)) {
+	if (k && btree_iter_pos_cmp(iter, b, k) > 0) {
 		char buf[100];
 		struct bkey uk = bkey_unpack_key(b, k);
 
-		bch2_bkey_to_text(buf, sizeof(buf), &uk);
-		panic("prev key should be before after pos:\n%s\n%llu:%llu\n",
+		bch2_bkey_to_text(&PBUF(buf), &uk);
+		panic("prev key should be before iter pos:\n%s\n%llu:%llu\n",
 		      buf, iter->pos.inode, iter->pos.offset);
 	}
 
 	k = bch2_btree_node_iter_peek_all(&l->iter, b);
-	if (k && !btree_iter_pos_cmp_packed(b, &iter->pos, k,
-				iter->flags & BTREE_ITER_IS_EXTENTS)) {
+	if (k && btree_iter_pos_cmp(iter, b, k) < 0) {
 		char buf[100];
 		struct bkey uk = bkey_unpack_key(b, k);
 
-		bch2_bkey_to_text(buf, sizeof(buf), &uk);
-		panic("next key should be before iter pos:\n%llu:%llu\n%s\n",
+		bch2_bkey_to_text(&PBUF(buf), &uk);
+		panic("iter should be after current key:\n"
+		      "iter pos %llu:%llu\n"
+		      "cur key  %s\n",
 		      iter->pos.inode, iter->pos.offset, buf);
 	}
+
+	BUG_ON(iter->uptodate == BTREE_ITER_UPTODATE &&
+	       (iter->flags & BTREE_ITER_TYPE) == BTREE_ITER_KEYS &&
+	       !bkey_whiteout(&iter->k) &&
+	       bch2_btree_node_iter_end(&l->iter));
 }
 
 void bch2_btree_iter_verify(struct btree_iter *iter, struct btree *b)
 {
 	struct btree_iter *linked;
 
-	if (iter->l[b->level].b == b)
-		__bch2_btree_iter_verify(iter, b);
+	if (!debug_check_iterators(iter->trans->c))
+		return;
 
-	for_each_linked_btree_node(iter, b, linked)
-		__bch2_btree_iter_verify(iter, b);
+	trans_for_each_iter_with_node(iter->trans, b, linked)
+		__bch2_btree_iter_verify(linked, b);
 }
 
+#else
+
+static inline void __bch2_btree_iter_verify(struct btree_iter *iter,
+					    struct btree *b) {}
+
 #endif
 
 static void __bch2_btree_node_iter_fix(struct btree_iter *iter,
@@ -341,7 +508,7 @@ static void __bch2_btree_node_iter_fix(struct btree_iter *iter,
 	struct btree_node_iter_set *set;
 	unsigned offset = __btree_node_key_to_offset(b, where);
 	int shift = new_u64s - clobber_u64s;
-	unsigned old_end = (int) __btree_node_key_to_offset(b, end) - shift;
+	unsigned old_end = t->end_offset - shift;
 
 	btree_node_iter_for_each(node_iter, set)
 		if (set->end == old_end)
@@ -349,8 +516,9 @@ 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 &&
-	    btree_iter_pos_cmp_packed(b, &iter->pos, where,
-				      iter->flags & BTREE_ITER_IS_EXTENTS)) {
+	    btree_iter_pos_cmp(iter, b, where) > 0) {
+		btree_iter_set_dirty(iter, BTREE_ITER_NEED_PEEK);
+
 		bch2_btree_node_iter_push(node_iter, b, where, end);
 
 		if (!b->level &&
@@ -361,15 +529,14 @@ static void __bch2_btree_node_iter_fix(struct btree_iter *iter,
 	}
 	return;
 found:
-	set->end = (int) set->end + shift;
+	set->end = t->end_offset;
 
 	/* Iterator hasn't gotten to the key that changed yet: */
 	if (set->k < offset)
 		return;
 
 	if (new_u64s &&
-	    btree_iter_pos_cmp_packed(b, &iter->pos, where,
-				iter->flags & BTREE_ITER_IS_EXTENTS)) {
+	    btree_iter_pos_cmp(iter, b, where) > 0) {
 		set->k = offset;
 	} else if (set->k < offset + clobber_u64s) {
 		set->k = offset + new_u64s;
@@ -380,9 +547,27 @@ found:
 		goto iter_current_key_not_modified;
 	}
 
+	btree_iter_set_dirty(iter, BTREE_ITER_NEED_PEEK);
+
 	bch2_btree_node_iter_sort(node_iter, b);
-	if (!b->level && node_iter == &iter->l[0].iter)
+	if (!b->level && node_iter == &iter->l[0].iter) {
+		/*
+		 * not legal to call bkey_debugcheck() here, because we're
+		 * called midway through the update path after update has been
+		 * marked but before deletes have actually happened:
+		 */
+#if 0
 		__btree_iter_peek_all(iter, &iter->l[0], &iter->k);
+#endif
+		struct btree_iter_level *l = &iter->l[0];
+		struct bkey_packed *k =
+			bch2_btree_node_iter_peek_all(&l->iter, l->b);
+
+		if (unlikely(!k))
+			iter->k.type = KEY_TYPE_deleted;
+		else
+			bkey_disassemble(l->b, k, &iter->k);
+	}
 iter_current_key_not_modified:
 
 	/*
@@ -407,9 +592,8 @@ iter_current_key_not_modified:
 	 * always point to the key for the child node the btree iterator points
 	 * to.
 	 */
-	if (b->level && new_u64s && !bkey_deleted(where) &&
-	    btree_iter_pos_cmp_packed(b, &iter->pos, where,
-				iter->flags & BTREE_ITER_IS_EXTENTS)) {
+	if (b->level && new_u64s &&
+	    btree_iter_pos_cmp(iter, b, where) > 0) {
 		struct bset_tree *t;
 		struct bkey_packed *k;
 
@@ -420,8 +604,7 @@ iter_current_key_not_modified:
 			k = bch2_bkey_prev_all(b, t,
 				bch2_btree_node_iter_bset_pos(node_iter, b, t));
 			if (k &&
-			    __btree_node_iter_cmp(node_iter, b,
-						  k, where) > 0) {
+			    bkey_iter_cmp(b, k, where) > 0) {
 				struct btree_node_iter_set *set;
 				unsigned offset =
 					__btree_node_key_to_offset(b, bkey_next(k));
@@ -443,32 +626,23 @@ next_bset:
 }
 
 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)
+			      struct btree *b,
+			      struct btree_node_iter *node_iter,
+			      struct bkey_packed *where,
+			      unsigned clobber_u64s,
+			      unsigned new_u64s)
 {
+	struct bset_tree *t = bch2_bkey_to_bset(b, where);
 	struct btree_iter *linked;
 
 	if (node_iter != &iter->l[b->level].iter)
 		__bch2_btree_node_iter_fix(iter, b, node_iter, t,
 					  where, clobber_u64s, new_u64s);
 
-	if (iter->l[b->level].b == b)
-		__bch2_btree_node_iter_fix(iter, b,
-					  &iter->l[b->level].iter, t,
-					  where, clobber_u64s, new_u64s);
-
-	for_each_linked_btree_node(iter, b, linked)
+	trans_for_each_iter_with_node(iter->trans, b, linked)
 		__bch2_btree_node_iter_fix(linked, b,
 					  &linked->l[b->level].iter, t,
 					  where, clobber_u64s, new_u64s);
-
-	/* interior node iterators are... special... */
-	if (!b->level)
-		bch2_btree_iter_verify(iter, b);
 }
 
 static inline struct bkey_s_c __btree_iter_unpack(struct btree_iter *iter,
@@ -483,14 +657,14 @@ static inline struct bkey_s_c __btree_iter_unpack(struct btree_iter *iter,
 		 * signal to bch2_btree_iter_peek_slot() that we're currently at
 		 * a hole
 		 */
-		u->type = KEY_TYPE_DELETED;
+		u->type = KEY_TYPE_deleted;
 		return bkey_s_c_null;
 	}
 
 	ret = bkey_disassemble(l->b, k, u);
 
-	if (debug_check_bkeys(iter->c))
-		bch2_bkey_debugcheck(iter->c, l->b, ret);
+	if (debug_check_bkeys(iter->trans->c))
+		bch2_bkey_debugcheck(iter->trans->c, l->b, ret);
 
 	return ret;
 }
@@ -511,9 +685,23 @@ static inline struct bkey_s_c __btree_iter_peek(struct btree_iter *iter,
 			bch2_btree_node_iter_peek(&l->iter, l->b));
 }
 
-static inline void __btree_iter_advance(struct btree_iter_level *l)
+static inline bool btree_iter_advance_to_pos(struct btree_iter *iter,
+					     struct btree_iter_level *l,
+					     int max_advance)
 {
-	bch2_btree_node_iter_advance(&l->iter, l->b);
+	struct bkey_packed *k;
+	int nr_advanced = 0;
+
+	while ((k = bch2_btree_node_iter_peek_all(&l->iter, l->b)) &&
+	       btree_iter_pos_cmp(iter, l->b, k) < 0) {
+		if (max_advance > 0 && nr_advanced >= max_advance)
+			return false;
+
+		bch2_btree_node_iter_advance(&l->iter, l->b);
+		nr_advanced++;
+	}
+
+	return true;
 }
 
 /*
@@ -546,7 +734,7 @@ static void btree_iter_verify_new_node(struct btree_iter *iter, struct btree *b)
 		char buf[100];
 		struct bkey uk = bkey_unpack_key(b, k);
 
-		bch2_bkey_to_text(buf, sizeof(buf), &uk);
+		bch2_bkey_to_text(&PBUF(buf), &uk);
 		panic("parent iter doesn't point to new node:\n%s\n%llu:%llu\n",
 		      buf, b->key.k.p.inode, b->key.k.p.offset);
 	}
@@ -555,21 +743,11 @@ static void btree_iter_verify_new_node(struct btree_iter *iter, struct btree *b)
 		btree_node_unlock(iter, b->level + 1);
 }
 
-/* Returns true if @k is after iterator position @pos */
-static inline bool btree_iter_pos_cmp(struct btree_iter *iter,
-				      const struct bkey *k)
-{
-	int cmp = bkey_cmp(k->p, iter->pos);
-
-	return cmp > 0 ||
-		(cmp == 0 &&
-		 !(iter->flags & BTREE_ITER_IS_EXTENTS) && !bkey_deleted(k));
-}
-
 static inline bool btree_iter_pos_after_node(struct btree_iter *iter,
 					     struct btree *b)
 {
-	return !btree_iter_pos_cmp(iter, &b->key.k);
+	return __btree_iter_pos_cmp(iter, NULL,
+			bkey_to_packed(&b->key), true) < 0;
 }
 
 static inline bool btree_iter_pos_in_node(struct btree_iter *iter,
@@ -581,17 +759,20 @@ static inline bool btree_iter_pos_in_node(struct btree_iter *iter,
 }
 
 static inline void __btree_iter_init(struct btree_iter *iter,
-				     struct btree *b)
+				     unsigned level)
 {
-	struct btree_iter_level *l = &iter->l[b->level];
+	struct btree_iter_level *l = &iter->l[level];
+
+	bch2_btree_node_iter_init(&l->iter, l->b, &iter->pos);
 
-	bch2_btree_node_iter_init(&l->iter, b, iter->pos,
-				  iter->flags & BTREE_ITER_IS_EXTENTS,
-				  btree_node_is_extents(b));
+	if (iter->flags & BTREE_ITER_IS_EXTENTS)
+		btree_iter_advance_to_pos(iter, l, -1);
 
 	/* Skip to first non whiteout: */
-	if (b->level)
-		bch2_btree_node_iter_peek(&l->iter, b);
+	if (level)
+		bch2_btree_node_iter_peek(&l->iter, l->b);
+
+	btree_iter_set_dirty(iter, BTREE_ITER_NEED_PEEK);
 }
 
 static inline void btree_iter_node_set(struct btree_iter *iter,
@@ -602,20 +783,21 @@ static inline void btree_iter_node_set(struct btree_iter *iter,
 	EBUG_ON(!btree_iter_pos_in_node(iter, b));
 	EBUG_ON(b->lock.state.seq & 1);
 
-	iter->lock_seq[b->level] = b->lock.state.seq;
+	iter->l[b->level].lock_seq = b->lock.state.seq;
 	iter->l[b->level].b = b;
-	__btree_iter_init(iter, b);
+	__btree_iter_init(iter, b->level);
 }
 
 /*
  * A btree node is being replaced - update the iterator to point to the new
  * node:
  */
-bool bch2_btree_iter_node_replace(struct btree_iter *iter, struct btree *b)
+void bch2_btree_iter_node_replace(struct btree_iter *iter, struct btree *b)
 {
+	enum btree_node_locked_type t;
 	struct btree_iter *linked;
 
-	for_each_linked_btree_iter(iter, linked)
+	trans_for_each_iter(iter->trans, linked)
 		if (btree_iter_pos_in_node(linked, b)) {
 			/*
 			 * bch2_btree_iter_node_drop() has already been called -
@@ -624,52 +806,28 @@ bool bch2_btree_iter_node_replace(struct btree_iter *iter, struct btree *b)
 			 */
 			BUG_ON(btree_node_locked(linked, b->level));
 
-			/*
-			 * If @linked wants this node read locked, we don't want
-			 * to actually take the read lock now because it's not
-			 * legal to hold read locks on other nodes while we take
-			 * write locks, so the journal can make forward
-			 * progress...
-			 *
-			 * Instead, btree_iter_node_set() sets things up so
-			 * bch2_btree_node_relock() will succeed:
-			 */
-
-			if (btree_want_intent(linked, b->level)) {
-				six_lock_increment(&b->lock, SIX_LOCK_intent);
-				mark_btree_node_intent_locked(linked, b->level);
+			t = btree_lock_want(linked, b->level);
+			if (t != BTREE_NODE_UNLOCKED) {
+				six_lock_increment(&b->lock, t);
+				mark_btree_node_locked(linked, b->level, t);
 			}
 
 			btree_iter_node_set(linked, b);
 		}
 
-	if (!btree_iter_pos_in_node(iter, b)) {
-		six_unlock_intent(&b->lock);
-		return false;
-	}
-
-	mark_btree_node_intent_locked(iter, b->level);
-	btree_iter_node_set(iter, b);
-	return true;
-}
-
-void bch2_btree_iter_node_drop_linked(struct btree_iter *iter, struct btree *b)
-{
-	struct btree_iter *linked;
-
-	for_each_linked_btree_iter(iter, linked)
-		bch2_btree_iter_node_drop(linked, b);
+	six_unlock_intent(&b->lock);
 }
 
 void bch2_btree_iter_node_drop(struct btree_iter *iter, struct btree *b)
 {
+	struct btree_iter *linked;
 	unsigned level = b->level;
 
-	if (iter->l[level].b == b) {
-		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
-		btree_node_unlock(iter, level);
-		iter->l[level].b = BTREE_ITER_NOT_END;
-	}
+	trans_for_each_iter(iter->trans, linked)
+		if (linked->l[level].b == b) {
+			__btree_node_unlock(linked, level);
+			linked->l[level].b = BTREE_ITER_NO_NODE_DROP;
+		}
 }
 
 /*
@@ -680,15 +838,14 @@ void bch2_btree_iter_reinit_node(struct btree_iter *iter, struct btree *b)
 {
 	struct btree_iter *linked;
 
-	for_each_linked_btree_node(iter, b, linked)
-		__btree_iter_init(linked, b);
-	__btree_iter_init(iter, b);
+	trans_for_each_iter_with_node(iter->trans, b, linked)
+		__btree_iter_init(linked, b->level);
 }
 
 static inline int btree_iter_lock_root(struct btree_iter *iter,
 				       unsigned depth_want)
 {
-	struct bch_fs *c = iter->c;
+	struct bch_fs *c = iter->trans->c;
 	struct btree *b;
 	enum six_lock_type lock_type;
 	unsigned i;
@@ -707,11 +864,12 @@ static inline int btree_iter_lock_root(struct btree_iter *iter,
 			 * that depth
 			 */
 			iter->level = depth_want;
-			iter->l[iter->level].b = NULL;
-			return 0;
+			for (i = iter->level; i < BTREE_MAX_DEPTH; i++)
+				iter->l[i].b = NULL;
+			return 1;
 		}
 
-		lock_type = btree_lock_want(iter, iter->level);
+		lock_type = __btree_lock_want(iter, iter->level);
 		if (unlikely(!btree_node_lock(b, POS_MAX, iter->level,
 					      iter, lock_type)))
 			return -EINTR;
@@ -720,13 +878,14 @@ static inline int btree_iter_lock_root(struct btree_iter *iter,
 			   b->level == iter->level &&
 			   !race_fault())) {
 			for (i = 0; i < iter->level; i++)
-				iter->l[i].b = BTREE_ITER_NOT_END;
+				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_node_set(iter, b);
 			return 0;
-
 		}
 
 		six_unlock_type(&b->lock, lock_type);
@@ -736,11 +895,12 @@ static inline int btree_iter_lock_root(struct btree_iter *iter,
 noinline
 static void btree_iter_prefetch(struct btree_iter *iter)
 {
+	struct bch_fs *c = iter->trans->c;
 	struct btree_iter_level *l = &iter->l[iter->level];
 	struct btree_node_iter node_iter = l->iter;
 	struct bkey_packed *k;
 	BKEY_PADDED(k) tmp;
-	unsigned nr = test_bit(BCH_FS_STARTED, &iter->c->flags)
+	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);
@@ -755,9 +915,7 @@ static void btree_iter_prefetch(struct btree_iter *iter)
 			break;
 
 		bch2_bkey_unpack(l->b, &tmp.k, k);
-		bch2_btree_node_prefetch(iter->c, &tmp.k,
-					 iter->level - 1,
-					 iter->btree_id);
+		bch2_btree_node_prefetch(c, iter, &tmp.k, iter->level - 1);
 	}
 
 	if (!was_locked)
@@ -766,10 +924,11 @@ static void btree_iter_prefetch(struct btree_iter *iter)
 
 static inline int btree_iter_down(struct btree_iter *iter)
 {
+	struct bch_fs *c = iter->trans->c;
 	struct btree_iter_level *l = &iter->l[iter->level];
 	struct btree *b;
 	unsigned level = iter->level - 1;
-	enum six_lock_type lock_type = btree_lock_want(iter, level);
+	enum six_lock_type lock_type = __btree_lock_want(iter, level);
 	BKEY_PADDED(k) tmp;
 
 	BUG_ON(!btree_node_locked(iter, iter->level));
@@ -777,7 +936,7 @@ static inline int btree_iter_down(struct btree_iter *iter)
 	bch2_bkey_unpack(l->b, &tmp.k,
 			 bch2_btree_node_iter_peek(&l->iter, l->b));
 
-	b = bch2_btree_node_get(iter->c, iter, &tmp.k, level, lock_type);
+	b = bch2_btree_node_get(c, iter, &tmp.k, level, lock_type);
 	if (unlikely(IS_ERR(b)))
 		return PTR_ERR(b);
 
@@ -799,17 +958,27 @@ static void btree_iter_up(struct btree_iter *iter)
 
 int __must_check __bch2_btree_iter_traverse(struct btree_iter *);
 
-static int btree_iter_traverse_error(struct btree_iter *iter, int ret)
+static int __btree_iter_traverse_all(struct btree_trans *trans,
+				     struct btree_iter *orig_iter, int ret)
 {
-	struct bch_fs *c = iter->c;
-	struct btree_iter *linked, *sorted_iters, **i;
-retry_all:
-	bch2_btree_iter_unlock(iter);
+	struct bch_fs *c = trans->c;
+	struct btree_iter *iter;
+	u8 sorted[BTREE_ITER_MAX];
+	unsigned i, nr_sorted = 0;
+
+	trans_for_each_iter(trans, iter)
+		sorted[nr_sorted++] = iter - trans->iters;
 
-	if (ret != -ENOMEM && ret != -EINTR)
-		goto io_error;
+#define btree_iter_cmp_by_idx(_l, _r)				\
+		btree_iter_cmp(&trans->iters[_l], &trans->iters[_r])
 
-	if (ret == -ENOMEM) {
+	bubble_sort(sorted, nr_sorted, btree_iter_cmp_by_idx);
+#undef btree_iter_cmp_by_idx
+
+retry_all:
+	bch2_trans_unlock(trans);
+
+	if (unlikely(ret == -ENOMEM)) {
 		struct closure cl;
 
 		closure_init_stack(&cl);
@@ -820,57 +989,54 @@ retry_all:
 		} while (ret);
 	}
 
-	/*
-	 * Linked iters are normally a circular singly linked list - break cycle
-	 * while we sort them:
-	 */
-	linked = iter->next;
-	iter->next = NULL;
-	sorted_iters = NULL;
-
-	while (linked) {
-		iter = linked;
-		linked = linked->next;
-
-		i = &sorted_iters;
-		while (*i && btree_iter_cmp(iter, *i) > 0)
-			i = &(*i)->next;
-
-		iter->next = *i;
-		*i = iter;
+	if (unlikely(ret == -EIO)) {
+		trans->error = true;
+		orig_iter->flags |= BTREE_ITER_ERROR;
+		orig_iter->l[orig_iter->level].b = BTREE_ITER_NO_NODE_ERROR;
+		goto out;
 	}
 
-	/* Make list circular again: */
-	iter = sorted_iters;
-	while (iter->next)
-		iter = iter->next;
-	iter->next = sorted_iters;
+	BUG_ON(ret && ret != -EINTR);
 
 	/* Now, redo traversals in correct order: */
+	for (i = 0; i < nr_sorted; i++) {
+		iter = &trans->iters[sorted[i]];
 
-	iter = sorted_iters;
-	do {
-retry:
-		ret = __bch2_btree_iter_traverse(iter);
-		if (unlikely(ret)) {
-			if (ret == -EINTR)
-				goto retry;
-			goto retry_all;
-		}
+		do {
+			ret = __bch2_btree_iter_traverse(iter);
+		} while (ret == -EINTR);
 
-		iter = iter->next;
-	} while (iter != sorted_iters);
+		if (ret)
+			goto retry_all;
+	}
 
-	ret = btree_iter_linked(iter) ? -EINTR : 0;
+	ret = hweight64(trans->iters_live) > 1 ? -EINTR : 0;
 out:
 	bch2_btree_cache_cannibalize_unlock(c);
 	return ret;
-io_error:
-	BUG_ON(ret != -EIO);
+}
+
+int bch2_btree_iter_traverse_all(struct btree_trans *trans)
+{
+	return __btree_iter_traverse_all(trans, NULL, 0);
+}
 
-	iter->flags |= BTREE_ITER_ERROR;
-	iter->l[iter->level].b = NULL;
-	goto out;
+static unsigned btree_iter_up_until_locked(struct btree_iter *iter,
+					   bool check_pos)
+{
+	unsigned l = iter->level;
+
+	while (btree_iter_node(iter, l) &&
+	       (!is_btree_node(iter, l) ||
+		!bch2_btree_node_relock(iter, l) ||
+		 (check_pos &&
+		  !btree_iter_pos_in_node(iter, iter->l[l].b)))) {
+		btree_node_unlock(iter, l);
+		iter->l[l].b = BTREE_ITER_NO_NODE_UP;
+		l++;
+	}
+
+	return l;
 }
 
 /*
@@ -880,48 +1046,23 @@ io_error:
  * Returns 0 on success, -EIO on error (error reading in a btree node).
  *
  * On error, caller (peek_node()/peek_key()) must return NULL; the error is
- * stashed in the iterator and returned from bch2_btree_iter_unlock().
+ * stashed in the iterator and returned from bch2_trans_exit().
  */
 int __must_check __bch2_btree_iter_traverse(struct btree_iter *iter)
 {
 	unsigned depth_want = iter->level;
 
-	if (unlikely(!iter->l[iter->level].b))
+	if (unlikely(iter->level >= BTREE_MAX_DEPTH))
 		return 0;
 
-	iter->flags &= ~BTREE_ITER_AT_END_OF_LEAF;
-
-	/* make sure we have all the intent locks we need - ugh */
-	if (unlikely(iter->l[iter->level].b &&
-		     iter->level + 1 < iter->locks_want)) {
-		unsigned i;
-
-		for (i = iter->level + 1;
-		     i < iter->locks_want && iter->l[i].b;
-		     i++)
-			if (!bch2_btree_node_relock(iter, i)) {
-				while (iter->level < BTREE_MAX_DEPTH &&
-				       iter->l[iter->level].b &&
-				       iter->level + 1 < iter->locks_want)
-					btree_iter_up(iter);
-				break;
-			}
-	}
+	if (bch2_btree_iter_relock(iter, false))
+		return 0;
 
 	/*
-	 * If the current node isn't locked, go up until we have a locked node
-	 * or run out of nodes:
+	 * XXX: correctly using BTREE_ITER_UPTODATE should make using check_pos
+	 * here unnecessary
 	 */
-	while (btree_iter_node(iter, iter->level) &&
-	       !(is_btree_node(iter, iter->level) &&
-		 bch2_btree_node_relock(iter, iter->level) &&
-
-		 /*
-		  * XXX: correctly using BTREE_ITER_UPTODATE should make
-		  * comparing iter->pos against node's key unnecessary
-		  */
-		 btree_iter_pos_in_node(iter, iter->l[iter->level].b)))
-		btree_iter_up(iter);
+	iter->level = btree_iter_up_until_locked(iter, true);
 
 	/*
 	 * If we've got a btree node locked (i.e. we aren't about to relock the
@@ -929,15 +1070,8 @@ int __must_check __bch2_btree_iter_traverse(struct btree_iter *iter)
 	 *
 	 * XXX correctly using BTREE_ITER_UPTODATE should make this unnecessary
 	 */
-	if (btree_iter_node(iter, iter->level)) {
-		struct btree_iter_level *l = &iter->l[iter->level];
-		struct bkey_s_c k;
-		struct bkey u;
-
-		while ((k = __btree_iter_peek_all(iter, l, &u)).k &&
-		       !btree_iter_pos_cmp(iter, k.k))
-			__btree_iter_advance(l);
-	}
+	if (btree_iter_node(iter, iter->level))
+		btree_iter_advance_to_pos(iter, &iter->l[iter->level], -1);
 
 	/*
 	 * Note: iter->nodes[iter->level] may be temporarily NULL here - that
@@ -950,13 +1084,19 @@ int __must_check __bch2_btree_iter_traverse(struct btree_iter *iter)
 			? btree_iter_down(iter)
 			: btree_iter_lock_root(iter, depth_want);
 		if (unlikely(ret)) {
+			if (ret == 1)
+				return 0;
+
 			iter->level = depth_want;
-			iter->l[iter->level].b = BTREE_ITER_NOT_END;
+			iter->l[iter->level].b = BTREE_ITER_NO_NODE_DOWN;
 			return ret;
 		}
 	}
 
 	iter->uptodate = BTREE_ITER_NEED_PEEK;
+
+	bch2_btree_trans_verify_locks(iter->trans);
+	__bch2_btree_iter_verify(iter, iter->l[iter->level].b);
 	return 0;
 }
 
@@ -964,16 +1104,25 @@ int __must_check bch2_btree_iter_traverse(struct btree_iter *iter)
 {
 	int ret;
 
-	if (iter->uptodate < BTREE_ITER_NEED_RELOCK)
-		return 0;
-
-	ret = __bch2_btree_iter_traverse(iter);
+	ret =   bch2_trans_cond_resched(iter->trans) ?:
+		__bch2_btree_iter_traverse(iter);
 	if (unlikely(ret))
-		ret = btree_iter_traverse_error(iter, ret);
+		ret = __btree_iter_traverse_all(iter->trans, iter, ret);
 
 	return ret;
 }
 
+static inline void bch2_btree_iter_checks(struct btree_iter *iter,
+					  enum btree_iter_type type)
+{
+	EBUG_ON(iter->btree_id >= BTREE_ID_NR);
+	EBUG_ON(!!(iter->flags & BTREE_ITER_IS_EXTENTS) !=
+		(btree_node_type_is_extents(iter->btree_id) &&
+		 type != BTREE_ITER_NODES));
+
+	bch2_btree_trans_verify_locks(iter->trans);
+}
+
 /* Iterate across nodes (leaf and interior nodes) */
 
 struct btree *bch2_btree_iter_peek_node(struct btree_iter *iter)
@@ -981,18 +1130,23 @@ struct btree *bch2_btree_iter_peek_node(struct btree_iter *iter)
 	struct btree *b;
 	int ret;
 
-	EBUG_ON(iter->flags & BTREE_ITER_IS_EXTENTS);
+	bch2_btree_iter_checks(iter, BTREE_ITER_NODES);
+
+	if (iter->uptodate == BTREE_ITER_UPTODATE)
+		return iter->l[iter->level].b;
 
 	ret = bch2_btree_iter_traverse(iter);
 	if (ret)
-		return ERR_PTR(ret);
+		return NULL;
 
-	b = iter->l[iter->level].b;
+	b = btree_iter_node(iter, iter->level);
+	if (!b)
+		return NULL;
 
-	if (b) {
-		EBUG_ON(bkey_cmp(b->key.k.p, iter->pos) < 0);
-		iter->pos = b->key.k.p;
-	}
+	BUG_ON(bkey_cmp(b->key.k.p, iter->pos) < 0);
+
+	iter->pos = b->key.k.p;
+	iter->uptodate = BTREE_ITER_UPTODATE;
 
 	return b;
 }
@@ -1002,25 +1156,42 @@ struct btree *bch2_btree_iter_next_node(struct btree_iter *iter, unsigned depth)
 	struct btree *b;
 	int ret;
 
-	EBUG_ON(iter->flags & BTREE_ITER_IS_EXTENTS);
-
-	btree_iter_up(iter);
+	bch2_btree_iter_checks(iter, BTREE_ITER_NODES);
 
+	/* already got to end? */
 	if (!btree_iter_node(iter, iter->level))
 		return NULL;
 
-	/* parent node usually won't be locked: redo traversal if necessary */
-	btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+	bch2_trans_cond_resched(iter->trans);
+
+	btree_iter_up(iter);
+
+	if (!bch2_btree_node_relock(iter, iter->level))
+		btree_iter_set_dirty(iter, BTREE_ITER_NEED_RELOCK);
+
 	ret = bch2_btree_iter_traverse(iter);
 	if (ret)
 		return NULL;
 
-	b = iter->l[iter->level].b;
+	/* got to end? */
+	b = btree_iter_node(iter, iter->level);
 	if (!b)
-		return b;
+		return NULL;
 
 	if (bkey_cmp(iter->pos, b->key.k.p) < 0) {
-		/* Haven't gotten to the end of the parent node: */
+		/*
+		 * Haven't gotten to the end of the parent node: go back down to
+		 * the next child node
+		 */
+
+		/*
+		 * We don't really want to be unlocking here except we can't
+		 * directly tell btree_iter_traverse() "traverse to this level"
+		 * except by setting iter->level, so we have to unlock so we
+		 * don't screw up our lock invariants:
+		 */
+		if (btree_node_read_locked(iter, iter->level))
+			btree_node_unlock(iter, iter->level);
 
 		/* ick: */
 		iter->pos	= iter->btree_id == BTREE_ID_INODES
@@ -1037,6 +1208,7 @@ struct btree *bch2_btree_iter_next_node(struct btree_iter *iter, unsigned depth)
 	}
 
 	iter->pos = b->key.k.p;
+	iter->uptodate = BTREE_ITER_UPTODATE;
 
 	return b;
 }
@@ -1046,7 +1218,6 @@ struct btree *bch2_btree_iter_next_node(struct btree_iter *iter, unsigned depth)
 void bch2_btree_iter_set_pos_same_leaf(struct btree_iter *iter, struct bpos new_pos)
 {
 	struct btree_iter_level *l = &iter->l[0];
-	struct bkey_packed *k;
 
 	EBUG_ON(iter->level != 0);
 	EBUG_ON(bkey_cmp(new_pos, iter->pos) < 0);
@@ -1056,71 +1227,94 @@ void bch2_btree_iter_set_pos_same_leaf(struct btree_iter *iter, struct bpos new_
 	iter->pos = new_pos;
 	btree_iter_set_dirty(iter, BTREE_ITER_NEED_PEEK);
 
-	while ((k = bch2_btree_node_iter_peek_all(&l->iter, l->b)) &&
-	       !btree_iter_pos_cmp_packed(l->b, &iter->pos, k,
-					  iter->flags & BTREE_ITER_IS_EXTENTS))
-		__btree_iter_advance(l);
+	btree_iter_advance_to_pos(iter, l, -1);
 
-	if (!k && btree_iter_pos_after_node(iter, l->b)) {
+	if (bch2_btree_node_iter_end(&l->iter) &&
+	    btree_iter_pos_after_node(iter, l->b))
 		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
-		iter->flags |= BTREE_ITER_AT_END_OF_LEAF;
-	}
 }
 
 void bch2_btree_iter_set_pos(struct btree_iter *iter, struct bpos new_pos)
 {
-	EBUG_ON(bkey_cmp(new_pos, iter->pos) < 0); /* XXX handle this */
+	int cmp = bkey_cmp(new_pos, iter->pos);
+	unsigned level;
+
+	if (!cmp)
+		return;
+
 	iter->pos = new_pos;
 
-	btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+	level = btree_iter_up_until_locked(iter, true);
+
+	if (btree_iter_node(iter, level)) {
+		/*
+		 * 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[level], 8))
+			__btree_iter_init(iter, level);
+
+		/* Don't leave it locked if we're not supposed to: */
+		if (btree_lock_want(iter, level) == BTREE_NODE_UNLOCKED)
+			btree_node_unlock(iter, level);
+	}
+
+	if (level != iter->level)
+		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+	else
+		btree_iter_set_dirty(iter, BTREE_ITER_NEED_PEEK);
 }
 
-struct bkey_s_c bch2_btree_iter_peek(struct btree_iter *iter)
+static inline struct bkey_s_c btree_iter_peek_uptodate(struct btree_iter *iter)
 {
 	struct btree_iter_level *l = &iter->l[0];
-	struct bkey_s_c k;
-	int ret;
+	struct bkey_s_c ret = { .k = &iter->k };
 
-	EBUG_ON(!!(iter->flags & BTREE_ITER_IS_EXTENTS) !=
-		(iter->btree_id == BTREE_ID_EXTENTS));
-	EBUG_ON(iter->flags & BTREE_ITER_SLOTS);
+	if (!bkey_deleted(&iter->k)) {
+		EBUG_ON(bch2_btree_node_iter_end(&l->iter));
+		ret.v = bkeyp_val(&l->b->format,
+			__bch2_btree_node_iter_peek_all(&l->iter, l->b));
+	}
 
-	if (iter->uptodate == BTREE_ITER_UPTODATE) {
-		struct bkey_packed *k =
-			__bch2_btree_node_iter_peek_all(&l->iter, l->b);
-		struct bkey_s_c ret = {
-			.k = &iter->k,
-			.v = bkeyp_val(&l->b->format, k)
-		};
+	if (debug_check_bkeys(iter->trans->c) &&
+	    !bkey_deleted(ret.k))
+		bch2_bkey_debugcheck(iter->trans->c, l->b, ret);
+	return ret;
+}
 
-		EBUG_ON(!btree_node_locked(iter, 0));
+struct bkey_s_c bch2_btree_iter_peek(struct btree_iter *iter)
+{
+	struct btree_iter_level *l = &iter->l[0];
+	struct bkey_s_c k;
+	int ret;
 
-		if (debug_check_bkeys(iter->c))
-			bch2_bkey_debugcheck(iter->c, l->b, ret);
-		return ret;
-	}
+	bch2_btree_iter_checks(iter, BTREE_ITER_KEYS);
 
-	if (iter->uptodate == BTREE_ITER_END)
-		return bkey_s_c_null;
+	if (iter->uptodate == BTREE_ITER_UPTODATE)
+		return btree_iter_peek_uptodate(iter);
 
 	while (1) {
-		ret = bch2_btree_iter_traverse(iter);
-		if (unlikely(ret))
-			return bkey_s_c_err(ret);
+		if (iter->uptodate >= BTREE_ITER_NEED_RELOCK) {
+			ret = bch2_btree_iter_traverse(iter);
+			if (unlikely(ret))
+				return bkey_s_c_err(ret);
+		}
 
 		k = __btree_iter_peek(iter, l);
 		if (likely(k.k))
 			break;
 
 		/* got to the end of the leaf, iterator needs to be traversed: */
-		iter->pos = l->b->key.k.p;
-		if (!bkey_cmp(iter->pos, POS_MAX)) {
-			iter->uptodate = BTREE_ITER_END;
+		iter->pos	= l->b->key.k.p;
+		iter->uptodate	= BTREE_ITER_NEED_TRAVERSE;
+
+		if (!bkey_cmp(iter->pos, POS_MAX))
 			return bkey_s_c_null;
-		}
 
 		iter->pos = btree_type_successor(iter->btree_id, iter->pos);
-		iter->uptodate = BTREE_ITER_NEED_TRAVERSE;
 	}
 
 	/*
@@ -1140,14 +1334,13 @@ struct bkey_s_c bch2_btree_iter_peek_next_leaf(struct btree_iter *iter)
 {
 	struct btree_iter_level *l = &iter->l[0];
 
-	iter->pos = l->b->key.k.p;
-	if (!bkey_cmp(iter->pos, POS_MAX)) {
-		iter->uptodate = BTREE_ITER_END;
+	iter->pos	= l->b->key.k.p;
+	iter->uptodate	= BTREE_ITER_NEED_TRAVERSE;
+
+	if (!bkey_cmp(iter->pos, POS_MAX))
 		return bkey_s_c_null;
-	}
 
 	iter->pos = btree_type_successor(iter->btree_id, iter->pos);
-	iter->uptodate = BTREE_ITER_NEED_TRAVERSE;
 
 	return bch2_btree_iter_peek(iter);
 }
@@ -1158,22 +1351,27 @@ struct bkey_s_c bch2_btree_iter_next(struct btree_iter *iter)
 	struct bkey_packed *p;
 	struct bkey_s_c k;
 
-	EBUG_ON(!!(iter->flags & BTREE_ITER_IS_EXTENTS) !=
-		(iter->btree_id == BTREE_ID_EXTENTS));
-	EBUG_ON(iter->flags & BTREE_ITER_SLOTS);
+	bch2_btree_iter_checks(iter, BTREE_ITER_KEYS);
+
+	iter->pos = btree_type_successor(iter->btree_id, iter->k.p);
 
 	if (unlikely(iter->uptodate != BTREE_ITER_UPTODATE)) {
-		k = bch2_btree_iter_peek(iter);
-		if (IS_ERR_OR_NULL(k.k))
-			return k;
+		/*
+		 * XXX: when we just need to relock we should be able to avoid
+		 * calling traverse, but we need to kill BTREE_ITER_NEED_PEEK
+		 * for that to work
+		 */
+		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+
+		return bch2_btree_iter_peek(iter);
 	}
 
 	do {
-		__btree_iter_advance(l);
+		bch2_btree_node_iter_advance(&l->iter, l->b);
 		p = bch2_btree_node_iter_peek_all(&l->iter, l->b);
 		if (unlikely(!p))
 			return bch2_btree_iter_peek_next_leaf(iter);
-	} while (bkey_deleted(p));
+	} while (bkey_whiteout(p));
 
 	k = __btree_iter_unpack(iter, l, &iter->k, p);
 
@@ -1182,10 +1380,56 @@ struct bkey_s_c bch2_btree_iter_next(struct btree_iter *iter)
 	return k;
 }
 
+struct bkey_s_c bch2_btree_iter_prev(struct btree_iter *iter)
+{
+	struct btree_iter_level *l = &iter->l[0];
+	struct bkey_packed *p;
+	struct bkey_s_c k;
+	int ret;
+
+	bch2_btree_iter_checks(iter, BTREE_ITER_KEYS);
+
+	if (unlikely(iter->uptodate != BTREE_ITER_UPTODATE)) {
+		k = bch2_btree_iter_peek(iter);
+		if (IS_ERR(k.k))
+			return k;
+	}
+
+	while (1) {
+		p = bch2_btree_node_iter_prev(&l->iter, l->b);
+		if (likely(p))
+			break;
+
+		iter->pos = l->b->data->min_key;
+		if (!bkey_cmp(iter->pos, POS_MIN))
+			return bkey_s_c_null;
+
+		bch2_btree_iter_set_pos(iter,
+			btree_type_predecessor(iter->btree_id, iter->pos));
+
+		ret = bch2_btree_iter_traverse(iter);
+		if (unlikely(ret))
+			return bkey_s_c_err(ret);
+
+		p = bch2_btree_node_iter_peek(&l->iter, l->b);
+		if (p)
+			break;
+	}
+
+	k = __btree_iter_unpack(iter, l, &iter->k, p);
+
+	EBUG_ON(bkey_cmp(bkey_start_pos(k.k), iter->pos) > 0);
+
+	iter->pos	= bkey_start_pos(k.k);
+	iter->uptodate	= BTREE_ITER_UPTODATE;
+	return k;
+}
+
 static inline struct bkey_s_c
-__bch2_btree_iter_peek_slot(struct btree_iter *iter)
+__bch2_btree_iter_peek_slot_extents(struct btree_iter *iter)
 {
 	struct btree_iter_level *l = &iter->l[0];
+	struct btree_node_iter node_iter;
 	struct bkey_s_c k;
 	struct bkey n;
 	int ret;
@@ -1194,14 +1438,18 @@ recheck:
 	while ((k = __btree_iter_peek_all(iter, l, &iter->k)).k &&
 	       bkey_deleted(k.k) &&
 	       bkey_cmp(bkey_start_pos(k.k), iter->pos) == 0)
-		__btree_iter_advance(l);
+		bch2_btree_node_iter_advance(&l->iter, l->b);
 
-	if (k.k && bkey_cmp(bkey_start_pos(k.k), iter->pos) <= 0) {
-		EBUG_ON(bkey_cmp(k.k->p, iter->pos) < 0);
-		EBUG_ON(bkey_deleted(k.k));
-		iter->uptodate = BTREE_ITER_UPTODATE;
-		return k;
-	}
+	/*
+	 * iterator is now at the correct position for inserting at iter->pos,
+	 * but we need to keep iterating until we find the first non whiteout so
+	 * we know how big a hole we have, if any:
+	 */
+
+	node_iter = l->iter;
+	if (k.k && bkey_whiteout(k.k))
+		k = __btree_iter_unpack(iter, l, &iter->k,
+			bch2_btree_node_iter_peek(&node_iter, l->b));
 
 	/*
 	 * If we got to the end of the node, check if we need to traverse to the
@@ -1216,74 +1464,118 @@ recheck:
 		goto recheck;
 	}
 
+	if (k.k &&
+	    !bkey_whiteout(k.k) &&
+	    bkey_cmp(bkey_start_pos(k.k), iter->pos) <= 0) {
+		/*
+		 * if we skipped forward to find the first non whiteout and
+		 * there _wasn't_ actually a hole, we want the iterator to be
+		 * pointed at the key we found:
+		 */
+		l->iter = node_iter;
+
+		EBUG_ON(bkey_cmp(k.k->p, iter->pos) < 0);
+		EBUG_ON(bkey_deleted(k.k));
+		iter->uptodate = BTREE_ITER_UPTODATE;
+		return k;
+	}
+
 	/* hole */
-	bkey_init(&n);
-	n.p = iter->pos;
 
-	if (iter->flags & BTREE_ITER_IS_EXTENTS) {
-		if (n.p.offset == KEY_OFFSET_MAX) {
-			if (n.p.inode == KEY_INODE_MAX) {
-				iter->uptodate = BTREE_ITER_END;
-				return bkey_s_c_null;
-			}
+	/* holes can't span inode numbers: */
+	if (iter->pos.offset == KEY_OFFSET_MAX) {
+		if (iter->pos.inode == KEY_INODE_MAX)
+			return bkey_s_c_null;
 
-			iter->pos = bkey_successor(iter->pos);
-			goto recheck;
-		}
+		iter->pos = bkey_successor(iter->pos);
+		goto recheck;
+	}
 
-		if (!k.k)
-			k.k = &l->b->key.k;
+	if (!k.k)
+		k.k = &l->b->key.k;
 
-		bch2_key_resize(&n,
-				min_t(u64, KEY_SIZE_MAX,
-				      (k.k->p.inode == n.p.inode
-				       ? bkey_start_offset(k.k)
-				       : KEY_OFFSET_MAX) -
-				      n.p.offset));
+	bkey_init(&n);
+	n.p = iter->pos;
+	bch2_key_resize(&n,
+			min_t(u64, KEY_SIZE_MAX,
+			      (k.k->p.inode == n.p.inode
+			       ? bkey_start_offset(k.k)
+			       : KEY_OFFSET_MAX) -
+			      n.p.offset));
 
-		EBUG_ON(!n.size);
-	}
+	EBUG_ON(!n.size);
 
-	iter->k = n;
+	iter->k	= n;
 	iter->uptodate = BTREE_ITER_UPTODATE;
 	return (struct bkey_s_c) { &iter->k, NULL };
 }
 
-struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
+static inline struct bkey_s_c
+__bch2_btree_iter_peek_slot(struct btree_iter *iter)
 {
 	struct btree_iter_level *l = &iter->l[0];
+	struct bkey_s_c k;
 	int ret;
 
-	EBUG_ON(!!(iter->flags & BTREE_ITER_IS_EXTENTS) !=
-		(iter->btree_id == BTREE_ID_EXTENTS));
-	EBUG_ON(!(iter->flags & BTREE_ITER_SLOTS));
+	if (iter->flags & BTREE_ITER_IS_EXTENTS)
+		return __bch2_btree_iter_peek_slot_extents(iter);
 
-	if (iter->uptodate == BTREE_ITER_UPTODATE) {
-		struct bkey_s_c ret = { .k = &iter->k };;
+recheck:
+	while ((k = __btree_iter_peek_all(iter, l, &iter->k)).k &&
+	       bkey_deleted(k.k) &&
+	       bkey_cmp(k.k->p, iter->pos) == 0)
+		bch2_btree_node_iter_advance(&l->iter, l->b);
 
-		if (!bkey_deleted(&iter->k))
-			ret.v = bkeyp_val(&l->b->format,
-				__bch2_btree_node_iter_peek_all(&l->iter, l->b));
+	/*
+	 * If we got to the end of the node, check if we need to traverse to the
+	 * next node:
+	 */
+	if (unlikely(!k.k && btree_iter_pos_after_node(iter, l->b))) {
+		btree_iter_set_dirty(iter, BTREE_ITER_NEED_TRAVERSE);
+		ret = bch2_btree_iter_traverse(iter);
+		if (unlikely(ret))
+			return bkey_s_c_err(ret);
 
-		EBUG_ON(!btree_node_locked(iter, 0));
+		goto recheck;
+	}
 
-		if (debug_check_bkeys(iter->c))
-			bch2_bkey_debugcheck(iter->c, l->b, ret);
-		return ret;
+	if (k.k &&
+	    !bkey_deleted(k.k) &&
+	    !bkey_cmp(iter->pos, k.k->p)) {
+		iter->uptodate = BTREE_ITER_UPTODATE;
+		return k;
+	} else {
+		/* hole */
+		bkey_init(&iter->k);
+		iter->k.p = iter->pos;
+
+		iter->uptodate = BTREE_ITER_UPTODATE;
+		return (struct bkey_s_c) { &iter->k, NULL };
 	}
+}
 
-	if (iter->uptodate == BTREE_ITER_END)
-		return bkey_s_c_null;
+struct bkey_s_c bch2_btree_iter_peek_slot(struct btree_iter *iter)
+{
+	int ret;
 
-	ret = bch2_btree_iter_traverse(iter);
-	if (unlikely(ret))
-		return bkey_s_c_err(ret);
+	bch2_btree_iter_checks(iter, BTREE_ITER_SLOTS);
+
+	if (iter->uptodate == BTREE_ITER_UPTODATE)
+		return btree_iter_peek_uptodate(iter);
+
+	if (iter->uptodate >= BTREE_ITER_NEED_RELOCK) {
+		ret = bch2_btree_iter_traverse(iter);
+		if (unlikely(ret))
+			return bkey_s_c_err(ret);
+	}
 
 	return __bch2_btree_iter_peek_slot(iter);
 }
 
 struct bkey_s_c bch2_btree_iter_next_slot(struct btree_iter *iter)
 {
+	bch2_btree_iter_checks(iter, BTREE_ITER_SLOTS);
+
 	iter->pos = btree_type_successor(iter->btree_id, iter->k.p);
 
 	if (unlikely(iter->uptodate != BTREE_ITER_UPTODATE)) {
@@ -1298,85 +1590,386 @@ struct bkey_s_c bch2_btree_iter_next_slot(struct btree_iter *iter)
 	}
 
 	if (!bkey_deleted(&iter->k))
-		__btree_iter_advance(&iter->l[0]);
+		bch2_btree_node_iter_advance(&iter->l[0].iter, iter->l[0].b);
+
+	btree_iter_set_dirty(iter, BTREE_ITER_NEED_PEEK);
 
 	return __bch2_btree_iter_peek_slot(iter);
 }
 
-void __bch2_btree_iter_init(struct btree_iter *iter, struct bch_fs *c,
-			    enum btree_id btree_id, struct bpos pos,
-			    unsigned locks_want, unsigned depth,
-			    unsigned flags)
+static inline void bch2_btree_iter_init(struct btree_trans *trans,
+			struct btree_iter *iter, enum btree_id btree_id,
+			struct bpos pos, unsigned flags)
 {
+	struct bch_fs *c = trans->c;
 	unsigned i;
 
-	EBUG_ON(depth >= BTREE_MAX_DEPTH);
-	EBUG_ON(locks_want > BTREE_MAX_DEPTH);
+	if (btree_node_type_is_extents(btree_id) &&
+	    !(flags & BTREE_ITER_NODES))
+		flags |= BTREE_ITER_IS_EXTENTS;
 
-	iter->c				= c;
+	iter->trans			= trans;
 	iter->pos			= pos;
 	bkey_init(&iter->k);
 	iter->k.p			= pos;
 	iter->flags			= flags;
 	iter->uptodate			= BTREE_ITER_NEED_TRAVERSE;
 	iter->btree_id			= btree_id;
-	iter->level			= depth;
-	iter->locks_want		= locks_want;
+	iter->level			= 0;
+	iter->locks_want		= flags & BTREE_ITER_INTENT ? 1 : 0;
 	iter->nodes_locked		= 0;
 	iter->nodes_intent_locked	= 0;
 	for (i = 0; i < ARRAY_SIZE(iter->l); i++)
 		iter->l[i].b		= NULL;
-	iter->l[iter->level].b		= BTREE_ITER_NOT_END;
-	iter->next			= iter;
-
-	if (unlikely((flags & BTREE_ITER_IS_EXTENTS) &&
-		     !bkey_cmp(pos, POS_MAX)))
-		iter->uptodate = BTREE_ITER_END;
+	iter->l[iter->level].b		= BTREE_ITER_NO_NODE_INIT;
 
 	prefetch(c->btree_roots[btree_id].b);
 }
 
-void bch2_btree_iter_unlink(struct btree_iter *iter)
+/* new transactional stuff: */
+
+int bch2_trans_iter_put(struct btree_trans *trans,
+			struct btree_iter *iter)
 {
-	struct btree_iter *linked;
+	int ret = btree_iter_err(iter);
 
-	__bch2_btree_iter_unlock(iter);
+	trans->iters_live	&= ~(1ULL << iter->idx);
+	return ret;
+}
 
-	if (!btree_iter_linked(iter))
-		return;
+static inline void __bch2_trans_iter_free(struct btree_trans *trans,
+					  unsigned idx)
+{
+	__bch2_btree_iter_unlock(&trans->iters[idx]);
+	trans->iters_linked		&= ~(1ULL << idx);
+	trans->iters_live		&= ~(1ULL << idx);
+	trans->iters_touched		&= ~(1ULL << idx);
+	trans->iters_unlink_on_restart	&= ~(1ULL << idx);
+	trans->iters_unlink_on_commit	&= ~(1ULL << idx);
+}
 
-	for_each_linked_btree_iter(iter, linked) {
+int bch2_trans_iter_free(struct btree_trans *trans,
+			 struct btree_iter *iter)
+{
+	int ret = btree_iter_err(iter);
 
-		if (linked->next == iter) {
-			linked->next = iter->next;
-			return;
+	__bch2_trans_iter_free(trans, iter->idx);
+	return ret;
+}
+
+int bch2_trans_iter_free_on_commit(struct btree_trans *trans,
+				   struct btree_iter *iter)
+{
+	int ret = btree_iter_err(iter);
+
+	trans->iters_unlink_on_commit |= 1ULL << iter->idx;
+	return ret;
+}
+
+static int bch2_trans_realloc_iters(struct btree_trans *trans,
+				    unsigned new_size)
+{
+	void *new_iters, *new_updates;
+
+	new_size = roundup_pow_of_two(new_size);
+
+	BUG_ON(new_size > BTREE_ITER_MAX);
+
+	if (new_size <= trans->size)
+		return 0;
+
+	BUG_ON(trans->used_mempool);
+
+	bch2_trans_unlock(trans);
+
+	new_iters = kmalloc(sizeof(struct btree_iter) * new_size +
+			    sizeof(struct btree_insert_entry) * (new_size + 4),
+			    GFP_NOFS);
+	if (new_iters)
+		goto success;
+
+	new_iters = mempool_alloc(&trans->c->btree_iters_pool, GFP_NOFS);
+	new_size = BTREE_ITER_MAX;
+
+	trans->used_mempool = true;
+success:
+	new_updates = new_iters + sizeof(struct btree_iter) * new_size;
+
+	memcpy(new_iters, trans->iters,
+	       sizeof(struct btree_iter) * trans->nr_iters);
+	memcpy(new_updates, trans->updates,
+	       sizeof(struct btree_insert_entry) * trans->nr_updates);
+
+	if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG))
+		memset(trans->iters, POISON_FREE,
+		       sizeof(struct btree_iter) * trans->nr_iters +
+		       sizeof(struct btree_insert_entry) * trans->nr_iters);
+
+	if (trans->iters != trans->iters_onstack)
+		kfree(trans->iters);
+
+	trans->iters	= new_iters;
+	trans->updates	= new_updates;
+	trans->size	= new_size;
+
+	if (trans->iters_live) {
+		trace_trans_restart_iters_realloced(trans->ip, trans->size);
+		return -EINTR;
+	}
+
+	return 0;
+}
+
+static int btree_trans_iter_alloc(struct btree_trans *trans)
+{
+	unsigned idx = __ffs64(~trans->iters_linked);
+
+	if (idx < trans->nr_iters)
+		goto got_slot;
+
+	if (trans->nr_iters == trans->size) {
+		int ret = bch2_trans_realloc_iters(trans, trans->size * 2);
+		if (ret)
+			return ret;
+	}
+
+	idx = trans->nr_iters++;
+	BUG_ON(trans->nr_iters > trans->size);
+
+	trans->iters[idx].idx = idx;
+got_slot:
+	BUG_ON(trans->iters_linked & (1ULL << idx));
+	trans->iters_linked |= 1ULL << idx;
+	return idx;
+}
+
+static struct btree_iter *__btree_trans_get_iter(struct btree_trans *trans,
+						 unsigned btree_id, struct bpos pos,
+						 unsigned flags, u64 iter_id)
+{
+	struct btree_iter *iter;
+	int idx;
+
+	BUG_ON(trans->nr_iters > BTREE_ITER_MAX);
+
+	for (idx = 0; idx < trans->nr_iters; idx++) {
+		if (!(trans->iters_linked & (1ULL << idx)))
+			continue;
+
+		iter = &trans->iters[idx];
+		if (iter_id
+		    ? iter->id == iter_id
+		    : (iter->btree_id == btree_id &&
+		       !bkey_cmp(iter->pos, pos)))
+			goto found;
+	}
+	idx = -1;
+found:
+	if (idx < 0) {
+		idx = btree_trans_iter_alloc(trans);
+		if (idx < 0)
+			return ERR_PTR(idx);
+
+		iter = &trans->iters[idx];
+		iter->id = iter_id;
+
+		bch2_btree_iter_init(trans, iter, btree_id, pos, flags);
+	} else {
+		iter = &trans->iters[idx];
+
+		iter->flags &= ~(BTREE_ITER_INTENT|BTREE_ITER_PREFETCH);
+		iter->flags |= flags & (BTREE_ITER_INTENT|BTREE_ITER_PREFETCH);
+	}
+
+	BUG_ON(iter->btree_id != btree_id);
+	BUG_ON(trans->iters_live & (1ULL << idx));
+	trans->iters_live	|= 1ULL << idx;
+	trans->iters_touched	|= 1ULL << idx;
+
+	BUG_ON(iter->btree_id != btree_id);
+	BUG_ON((iter->flags ^ flags) & BTREE_ITER_TYPE);
+
+	return iter;
+}
+
+struct btree_iter *__bch2_trans_get_iter(struct btree_trans *trans,
+					 enum btree_id btree_id,
+					 struct bpos pos, unsigned flags,
+					 u64 iter_id)
+{
+	struct btree_iter *iter =
+		__btree_trans_get_iter(trans, btree_id, pos, flags, iter_id);
+
+	if (!IS_ERR(iter))
+		bch2_btree_iter_set_pos(iter, pos);
+	return iter;
+}
+
+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)
+{
+	struct btree_iter *iter =
+		__btree_trans_get_iter(trans, btree_id, pos,
+				       flags|BTREE_ITER_NODES, 0);
+	unsigned i;
+
+	BUG_ON(IS_ERR(iter));
+	BUG_ON(bkey_cmp(iter->pos, pos));
+
+	iter->locks_want = locks_want;
+	iter->level	= depth;
+
+	for (i = 0; i < ARRAY_SIZE(iter->l); i++)
+		iter->l[i].b		= NULL;
+	iter->l[iter->level].b		= BTREE_ITER_NO_NODE_INIT;
+
+	return iter;
+}
+
+struct btree_iter *bch2_trans_copy_iter(struct btree_trans *trans,
+					struct btree_iter *src)
+{
+	struct btree_iter *iter;
+	int i, idx;
+
+	idx = btree_trans_iter_alloc(trans);
+	if (idx < 0)
+		return ERR_PTR(idx);
+
+	trans->iters_live		|= 1ULL << idx;
+	trans->iters_touched		|= 1ULL << idx;
+	trans->iters_unlink_on_restart	|= 1ULL << idx;
+
+	iter = &trans->iters[idx];
+
+	memcpy(&iter->trans,
+	       &src->trans,
+	       (void *) &iter[1] - (void *) &iter->trans);
+
+	for (i = 0; i < BTREE_MAX_DEPTH; i++)
+		if (btree_node_locked(iter, i))
+			six_lock_increment(&iter->l[i].b->lock,
+					   __btree_lock_want(iter, i));
+
+	return &trans->iters[idx];
+}
+
+static int bch2_trans_preload_mem(struct btree_trans *trans, size_t size)
+{
+	if (size > trans->mem_bytes) {
+		size_t old_bytes = trans->mem_bytes;
+		size_t new_bytes = roundup_pow_of_two(size);
+		void *new_mem = krealloc(trans->mem, new_bytes, GFP_NOFS);
+
+		if (!new_mem)
+			return -ENOMEM;
+
+		trans->mem = new_mem;
+		trans->mem_bytes = new_bytes;
+
+		if (old_bytes) {
+			trace_trans_restart_mem_realloced(trans->ip, new_bytes);
+			return -EINTR;
 		}
 	}
 
-	BUG();
+	return 0;
 }
 
-void bch2_btree_iter_link(struct btree_iter *iter, struct btree_iter *new)
+void *bch2_trans_kmalloc(struct btree_trans *trans, size_t size)
 {
-	BUG_ON(btree_iter_linked(new));
+	void *p;
+	int ret;
 
-	new->next = iter->next;
-	iter->next = new;
+	ret = bch2_trans_preload_mem(trans, trans->mem_top + size);
+	if (ret)
+		return ERR_PTR(ret);
+
+	p = trans->mem + trans->mem_top;
+	trans->mem_top += size;
+	return p;
+}
 
-	if (IS_ENABLED(CONFIG_BCACHEFS_DEBUG)) {
-		unsigned nr_iters = 1;
+inline void bch2_trans_unlink_iters(struct btree_trans *trans, u64 iters)
+{
+	iters &= trans->iters_linked;
+	iters &= ~trans->iters_live;
 
-		for_each_linked_btree_iter(iter, new)
-			nr_iters++;
+	while (iters) {
+		unsigned idx = __ffs64(iters);
 
-		BUG_ON(nr_iters > SIX_LOCK_MAX_RECURSE);
+		iters &= ~(1ULL << idx);
+		__bch2_trans_iter_free(trans, idx);
 	}
 }
 
-void bch2_btree_iter_copy(struct btree_iter *dst, struct btree_iter *src)
+void bch2_trans_begin(struct btree_trans *trans)
+{
+	u64 iters_to_unlink;
+
+	/*
+	 * On transaction restart, the transaction isn't required to allocate
+	 * all the same iterators it on the last iteration:
+	 *
+	 * Unlink any iterators it didn't use this iteration, assuming it got
+	 * further (allocated an iter with a higher idx) than where the iter
+	 * was originally allocated:
+	 */
+	iters_to_unlink = ~trans->iters_live &
+		((1ULL << fls64(trans->iters_live)) - 1);
+
+	iters_to_unlink |= trans->iters_unlink_on_restart;
+	iters_to_unlink |= trans->iters_unlink_on_commit;
+
+	trans->iters_live		= 0;
+
+	bch2_trans_unlink_iters(trans, iters_to_unlink);
+
+	trans->iters_touched		= 0;
+	trans->iters_unlink_on_restart	= 0;
+	trans->iters_unlink_on_commit	= 0;
+	trans->nr_updates		= 0;
+	trans->mem_top			= 0;
+
+	bch2_btree_iter_traverse_all(trans);
+}
+
+void bch2_trans_init(struct btree_trans *trans, struct bch_fs *c,
+		     unsigned expected_nr_iters,
+		     size_t expected_mem_bytes)
+{
+	memset(trans, 0, offsetof(struct btree_trans, iters_onstack));
+
+	trans->c		= c;
+	trans->ip		= _RET_IP_;
+	trans->size		= ARRAY_SIZE(trans->iters_onstack);
+	trans->iters		= trans->iters_onstack;
+	trans->updates		= trans->updates_onstack;
+	trans->fs_usage_deltas	= NULL;
+
+	if (expected_nr_iters > trans->size)
+		bch2_trans_realloc_iters(trans, expected_nr_iters);
+
+	if (expected_mem_bytes)
+		bch2_trans_preload_mem(trans, expected_mem_bytes);
+}
+
+int bch2_trans_exit(struct btree_trans *trans)
 {
-	__bch2_btree_iter_unlock(dst);
-	memcpy(dst, src, offsetof(struct btree_iter, next));
-	dst->nodes_locked = dst->nodes_intent_locked = 0;
-	dst->uptodate = BTREE_ITER_NEED_RELOCK;
+	bch2_trans_unlock(trans);
+
+	kfree(trans->fs_usage_deltas);
+	kfree(trans->mem);
+	if (trans->used_mempool)
+		mempool_free(trans->iters, &trans->c->btree_iters_pool);
+	else if (trans->iters != trans->iters_onstack)
+		kfree(trans->iters);
+	trans->mem	= (void *) 0x1;
+	trans->iters	= (void *) 0x1;
+
+	return trans->error ? -EIO : 0;
 }