bcachefs: backpointers fsck no longer uses BTREE_ITER_ALL_LEVELS

It appears that BTREE_ITER_ALL_LEVELS is racy with concurrent interior
node btree updates; unfortunate but not terribly surprising it's a
difficult problem - that was the original reason for gc_lock.

BTREE_ITER_ALL_LEVELS will probably be deleted in a subsequent patch,
this changes backpointers fsck to instead walk keys at one level of the
btree at a time.

This fixes the tiering_drop_alloc test, which stopped working with the
patch to not flush the journal after journal replay.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/backpointers.c

@@ -426,11 +426,13 @@ static int check_bp_exists(struct btree_trans *trans,
 	    memcmp(bkey_s_c_to_backpointer(bp_k).v, &bp, sizeof(bp))) {
 		if (last_flushed->level != bp.level ||
 		    !bpos_eq(last_flushed->pos, orig_k.k->p)) {
+			ret = bch2_btree_write_buffer_flush_sync(trans);
+			if (ret)
+				goto err;
+
 			last_flushed->level = bp.level;
 			last_flushed->pos = orig_k.k->p;
-
-			ret = bch2_btree_write_buffer_flush_sync(trans) ?:
-				-BCH_ERR_transaction_restart_write_buffer_flush;
+			ret = -BCH_ERR_transaction_restart_write_buffer_flush;
 			goto out;
 		}
 		goto missing;
@@ -457,25 +459,18 @@ static int check_bp_exists(struct btree_trans *trans,
 }
 
 static int check_extent_to_backpointers(struct btree_trans *trans,
-					struct btree_iter *iter,
+					enum btree_id btree, unsigned level,
 					struct bpos bucket_start,
 					struct bpos bucket_end,
-					struct bpos_level *last_flushed)
+					struct bpos_level *last_flushed,
+					struct bkey_s_c k)
 {
 	struct bch_fs *c = trans->c;
 	struct bkey_ptrs_c ptrs;
 	const union bch_extent_entry *entry;
 	struct extent_ptr_decoded p;
-	struct bkey_s_c k;
 	int ret;
 
-	k = bch2_btree_iter_peek_all_levels(iter);
-	ret = bkey_err(k);
-	if (ret)
-		return ret;
-	if (!k.k)
-		return 0;
-
 	ptrs = bch2_bkey_ptrs_c(k);
 	bkey_for_each_ptr_decode(k.k, ptrs, p, entry) {
 		struct bpos bucket_pos;
@@ -484,7 +479,7 @@ static int check_extent_to_backpointers(struct btree_trans *trans,
 		if (p.ptr.cached)
 			continue;
 
-		bch2_extent_ptr_to_bp(c, iter->btree_id, iter->path->level,
+		bch2_extent_ptr_to_bp(c, btree, level,
 				      k, p, &bucket_pos, &bp);
 
 		ret = check_bp_exists(trans, bucket_pos, bp, k,
@@ -501,44 +496,33 @@ static int check_btree_root_to_backpointers(struct btree_trans *trans,
 					    enum btree_id btree_id,
 					    struct bpos bucket_start,
 					    struct bpos bucket_end,
-					    struct bpos_level *last_flushed)
+					    struct bpos_level *last_flushed,
+					    int *level)
 {
 	struct bch_fs *c = trans->c;
-	struct btree_root *r = bch2_btree_id_root(c, btree_id);
 	struct btree_iter iter;
 	struct btree *b;
 	struct bkey_s_c k;
-	struct bkey_ptrs_c ptrs;
-	struct extent_ptr_decoded p;
-	const union bch_extent_entry *entry;
 	int ret;
-
-	bch2_trans_node_iter_init(trans, &iter, btree_id, POS_MIN, 0, r->level, 0);
+retry:
+	bch2_trans_node_iter_init(trans, &iter, btree_id, POS_MIN,
+				  0, bch2_btree_id_root(c, btree_id)->b->c.level, 0);
 	b = bch2_btree_iter_peek_node(&iter);
 	ret = PTR_ERR_OR_ZERO(b);
 	if (ret)
 		goto err;
 
-	BUG_ON(b != btree_node_root(c, b));
-
-	k = bkey_i_to_s_c(&b->key);
-	ptrs = bch2_bkey_ptrs_c(k);
-	bkey_for_each_ptr_decode(k.k, ptrs, p, entry) {
-		struct bpos bucket_pos;
-		struct bch_backpointer bp;
-
-		if (p.ptr.cached)
-			continue;
+	if (b != btree_node_root(c, b)) {
+		bch2_trans_iter_exit(trans, &iter);
+		goto retry;
+	}
 
-		bch2_extent_ptr_to_bp(c, iter.btree_id, b->c.level + 1,
-				      k, p, &bucket_pos, &bp);
+	*level = b->c.level;
 
-		ret = check_bp_exists(trans, bucket_pos, bp, k,
+	k = bkey_i_to_s_c(&b->key);
+	ret = check_extent_to_backpointers(trans, btree_id, b->c.level + 1,
 				      bucket_start, bucket_end,
-				      last_flushed);
-		if (ret)
-			goto err;
-	}
+				      last_flushed, k);
 err:
 	bch2_trans_iter_exit(trans, &iter);
 	return ret;
@@ -616,43 +600,58 @@ static int bch2_check_extents_to_backpointers_pass(struct btree_trans *trans,
 	struct bch_fs *c = trans->c;
 	struct btree_iter iter;
 	enum btree_id btree_id;
-	struct bpos_level last_flushed = { UINT_MAX, POS_MIN };
+	struct bkey_s_c k;
 	int ret = 0;
 
 	for (btree_id = 0; btree_id < btree_id_nr_alive(c); btree_id++) {
-		unsigned depth = btree_type_has_ptrs(btree_id) ? 0 : 1;
-
-		bch2_trans_node_iter_init(trans, &iter, btree_id, POS_MIN, 0,
-					  depth,
-					  BTREE_ITER_ALL_LEVELS|
-					  BTREE_ITER_PREFETCH);
-
-		do {
-			ret = commit_do(trans, NULL, NULL,
-					BCH_TRANS_COMMIT_lazy_rw|
-					BCH_TRANS_COMMIT_no_enospc,
-					check_extent_to_backpointers(trans, &iter,
-								bucket_start, bucket_end,
-								&last_flushed));
-			if (ret)
-				break;
-		} while (!bch2_btree_iter_advance(&iter));
-
-		bch2_trans_iter_exit(trans, &iter);
-
-		if (ret)
-			break;
+		struct bpos_level last_flushed = { UINT_MAX, POS_MIN };
+		int level, depth = btree_type_has_ptrs(btree_id) ? 0 : 1;
 
 		ret = commit_do(trans, NULL, NULL,
 				BCH_TRANS_COMMIT_lazy_rw|
 				BCH_TRANS_COMMIT_no_enospc,
 				check_btree_root_to_backpointers(trans, btree_id,
 							bucket_start, bucket_end,
-							&last_flushed));
+							&last_flushed, &level));
 		if (ret)
-			break;
+			return ret;
+
+		while (level >= depth) {
+			bch2_trans_node_iter_init(trans, &iter, btree_id, POS_MIN, 0,
+						  level,
+						  BTREE_ITER_PREFETCH);
+			while (1) {
+				bch2_trans_begin(trans);
+				k = bch2_btree_iter_peek(&iter);
+				if (!k.k)
+					break;
+				ret = bkey_err(k) ?:
+					check_extent_to_backpointers(trans, btree_id, level,
+								     bucket_start, bucket_end,
+								     &last_flushed, k) ?:
+					bch2_trans_commit(trans, NULL, NULL,
+							  BCH_TRANS_COMMIT_lazy_rw|
+							  BCH_TRANS_COMMIT_no_enospc);
+				if (bch2_err_matches(ret, BCH_ERR_transaction_restart)) {
+					ret = 0;
+					continue;
+				}
+				if (ret)
+					break;
+				if (bpos_eq(iter.pos, SPOS_MAX))
+					break;
+				bch2_btree_iter_advance(&iter);
+			}
+			bch2_trans_iter_exit(trans, &iter);
+
+			if (ret)
+				return ret;
+
+			--level;
+		}
 	}
-	return ret;
+
+	return 0;
 }
 
 static struct bpos bucket_pos_to_bp_safe(const struct bch_fs *c,