Merge tag 'nvme-6.10-2024-05-29' of git://git.infradead.org/nvme into block-6.10

Pull NVMe fixes from Keith:

"nvme fixes for Linux 6.10

 - Removing unused fields (Kanchan)
 - Large folio offsets support (Kundan)
 - Multipath NUMA node initialiazation fix (Nilay)
 - Multipath IO stats accounting fixes (Keith)
 - Circular lockdep fix (Keith)
 - Target race condition fix (Sagi)
 - Target memory leak fix (Sagi)"

* tag 'nvme-6.10-2024-05-29' of git://git.infradead.org/nvme:
  nvmet: fix a possible leak when destroy a ctrl during qp establishment
  nvme: use srcu for iterating namespace list
  nvme: adjust multiples of NVME_CTRL_PAGE_SIZE in offset
  nvme: remove sgs and sws
  nvmet: fix ns enable/disable possible hang
  nvme-multipath: fix io accounting on failover
  nvme: fix multipath batched completion accounting
  nvme-multipath: find NUMA path only for online numa-node

drivers/nvme/host/core.c

@@ -414,7 +414,15 @@ static inline void nvme_end_req_zoned(struct request *req)
 	}
 }
 
-static inline void nvme_end_req(struct request *req)
+static inline void __nvme_end_req(struct request *req)
+{
+	nvme_end_req_zoned(req);
+	nvme_trace_bio_complete(req);
+	if (req->cmd_flags & REQ_NVME_MPATH)
+		nvme_mpath_end_request(req);
+}
+
+void nvme_end_req(struct request *req)
 {
 	blk_status_t status = nvme_error_status(nvme_req(req)->status);
 
@@ -424,10 +432,7 @@ static inline void nvme_end_req(struct request *req)
 		else
 			nvme_log_error(req);
 	}
-	nvme_end_req_zoned(req);
-	nvme_trace_bio_complete(req);
-	if (req->cmd_flags & REQ_NVME_MPATH)
-		nvme_mpath_end_request(req);
+	__nvme_end_req(req);
 	blk_mq_end_request(req, status);
 }
 
@@ -476,7 +481,7 @@ void nvme_complete_batch_req(struct request *req)
 {
 	trace_nvme_complete_rq(req);
 	nvme_cleanup_cmd(req);
-	nvme_end_req_zoned(req);
+	__nvme_end_req(req);
 }
 EXPORT_SYMBOL_GPL(nvme_complete_batch_req);
 
@@ -673,7 +678,7 @@ static void nvme_free_ns(struct kref *kref)
 	kfree(ns);
 }
 
-static inline bool nvme_get_ns(struct nvme_ns *ns)
+bool nvme_get_ns(struct nvme_ns *ns)
 {
 	return kref_get_unless_zero(&ns->kref);
 }
@@ -3679,9 +3684,10 @@ static int nvme_init_ns_head(struct nvme_ns *ns, struct nvme_ns_info *info)
 struct nvme_ns *nvme_find_get_ns(struct nvme_ctrl *ctrl, unsigned nsid)
 {
 	struct nvme_ns *ns, *ret = NULL;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list) {
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list) {
 		if (ns->head->ns_id == nsid) {
 			if (!nvme_get_ns(ns))
 				continue;
@@ -3691,7 +3697,7 @@ struct nvme_ns *nvme_find_get_ns(struct nvme_ctrl *ctrl, unsigned nsid)
 		if (ns->head->ns_id > nsid)
 			break;
 	}
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 	return ret;
 }
 EXPORT_SYMBOL_NS_GPL(nvme_find_get_ns, NVME_TARGET_PASSTHRU);
@@ -3705,7 +3711,7 @@ static void nvme_ns_add_to_ctrl_list(struct nvme_ns *ns)
 
 	list_for_each_entry_reverse(tmp, &ns->ctrl->namespaces, list) {
 		if (tmp->head->ns_id < ns->head->ns_id) {
-			list_add(&ns->list, &tmp->list);
+			list_add_rcu(&ns->list, &tmp->list);
 			return;
 		}
 	}
@@ -3771,17 +3777,18 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 	if (nvme_update_ns_info(ns, info))
 		goto out_unlink_ns;
 
-	down_write(&ctrl->namespaces_rwsem);
+	mutex_lock(&ctrl->namespaces_lock);
 	/*
 	 * Ensure that no namespaces are added to the ctrl list after the queues
 	 * are frozen, thereby avoiding a deadlock between scan and reset.
 	 */
 	if (test_bit(NVME_CTRL_FROZEN, &ctrl->flags)) {
-		up_write(&ctrl->namespaces_rwsem);
+		mutex_unlock(&ctrl->namespaces_lock);
 		goto out_unlink_ns;
 	}
 	nvme_ns_add_to_ctrl_list(ns);
-	up_write(&ctrl->namespaces_rwsem);
+	mutex_unlock(&ctrl->namespaces_lock);
+	synchronize_srcu(&ctrl->srcu);
 	nvme_get_ctrl(ctrl);
 
 	if (device_add_disk(ctrl->device, ns->disk, nvme_ns_attr_groups))
@@ -3804,9 +3811,10 @@ static void nvme_alloc_ns(struct nvme_ctrl *ctrl, struct nvme_ns_info *info)
 
  out_cleanup_ns_from_list:
 	nvme_put_ctrl(ctrl);
-	down_write(&ctrl->namespaces_rwsem);
-	list_del_init(&ns->list);
-	up_write(&ctrl->namespaces_rwsem);
+	mutex_lock(&ctrl->namespaces_lock);
+	list_del_rcu(&ns->list);
+	mutex_unlock(&ctrl->namespaces_lock);
+	synchronize_srcu(&ctrl->srcu);
  out_unlink_ns:
 	mutex_lock(&ctrl->subsys->lock);
 	list_del_rcu(&ns->siblings);
@@ -3856,9 +3864,10 @@ static void nvme_ns_remove(struct nvme_ns *ns)
 		nvme_cdev_del(&ns->cdev, &ns->cdev_device);
 	del_gendisk(ns->disk);
 
-	down_write(&ns->ctrl->namespaces_rwsem);
-	list_del_init(&ns->list);
-	up_write(&ns->ctrl->namespaces_rwsem);
+	mutex_lock(&ns->ctrl->namespaces_lock);
+	list_del_rcu(&ns->list);
+	mutex_unlock(&ns->ctrl->namespaces_lock);
+	synchronize_srcu(&ns->ctrl->srcu);
 
 	if (last_path)
 		nvme_mpath_shutdown_disk(ns->head);
@@ -3948,16 +3957,17 @@ static void nvme_remove_invalid_namespaces(struct nvme_ctrl *ctrl,
 	struct nvme_ns *ns, *next;
 	LIST_HEAD(rm_list);
 
-	down_write(&ctrl->namespaces_rwsem);
+	mutex_lock(&ctrl->namespaces_lock);
 	list_for_each_entry_safe(ns, next, &ctrl->namespaces, list) {
 		if (ns->head->ns_id > nsid)
-			list_move_tail(&ns->list, &rm_list);
+			list_splice_init_rcu(&ns->list, &rm_list,
+					     synchronize_rcu);
 	}
-	up_write(&ctrl->namespaces_rwsem);
+	mutex_unlock(&ctrl->namespaces_lock);
+	synchronize_srcu(&ctrl->srcu);
 
 	list_for_each_entry_safe(ns, next, &rm_list, list)
 		nvme_ns_remove(ns);
-
 }
 
 static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
@@ -4127,9 +4137,10 @@ void nvme_remove_namespaces(struct nvme_ctrl *ctrl)
 	/* this is a no-op when called from the controller reset handler */
 	nvme_change_ctrl_state(ctrl, NVME_CTRL_DELETING_NOIO);
 
-	down_write(&ctrl->namespaces_rwsem);
-	list_splice_init(&ctrl->namespaces, &ns_list);
-	up_write(&ctrl->namespaces_rwsem);
+	mutex_lock(&ctrl->namespaces_lock);
+	list_splice_init_rcu(&ctrl->namespaces, &ns_list, synchronize_rcu);
+	mutex_unlock(&ctrl->namespaces_lock);
+	synchronize_srcu(&ctrl->srcu);
 
 	list_for_each_entry_safe(ns, next, &ns_list, list)
 		nvme_ns_remove(ns);
@@ -4577,6 +4588,7 @@ static void nvme_free_ctrl(struct device *dev)
 	key_put(ctrl->tls_key);
 	nvme_free_cels(ctrl);
 	nvme_mpath_uninit(ctrl);
+	cleanup_srcu_struct(&ctrl->srcu);
 	nvme_auth_stop(ctrl);
 	nvme_auth_free(ctrl);
 	__free_page(ctrl->discard_page);
@@ -4609,10 +4621,15 @@ int nvme_init_ctrl(struct nvme_ctrl *ctrl, struct device *dev,
 	ctrl->passthru_err_log_enabled = false;
 	clear_bit(NVME_CTRL_FAILFAST_EXPIRED, &ctrl->flags);
 	spin_lock_init(&ctrl->lock);
+	mutex_init(&ctrl->namespaces_lock);
+
+	ret = init_srcu_struct(&ctrl->srcu);
+	if (ret)
+		return ret;
+
 	mutex_init(&ctrl->scan_lock);
 	INIT_LIST_HEAD(&ctrl->namespaces);
 	xa_init(&ctrl->cels);
-	init_rwsem(&ctrl->namespaces_rwsem);
 	ctrl->dev = dev;
 	ctrl->ops = ops;
 	ctrl->quirks = quirks;
@@ -4692,6 +4709,7 @@ int nvme_init_ctrl(struct nvme_ctrl *ctrl, struct device *dev,
 out:
 	if (ctrl->discard_page)
 		__free_page(ctrl->discard_page);
+	cleanup_srcu_struct(&ctrl->srcu);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(nvme_init_ctrl);
@@ -4700,22 +4718,24 @@ EXPORT_SYMBOL_GPL(nvme_init_ctrl);
 void nvme_mark_namespaces_dead(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list)
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list)
 		blk_mark_disk_dead(ns->disk);
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 }
 EXPORT_SYMBOL_GPL(nvme_mark_namespaces_dead);
 
 void nvme_unfreeze(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list)
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list)
 		blk_mq_unfreeze_queue(ns->queue);
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 	clear_bit(NVME_CTRL_FROZEN, &ctrl->flags);
 }
 EXPORT_SYMBOL_GPL(nvme_unfreeze);
@@ -4723,14 +4743,15 @@ EXPORT_SYMBOL_GPL(nvme_unfreeze);
 int nvme_wait_freeze_timeout(struct nvme_ctrl *ctrl, long timeout)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list) {
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list) {
 		timeout = blk_mq_freeze_queue_wait_timeout(ns->queue, timeout);
 		if (timeout <= 0)
 			break;
 	}
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 	return timeout;
 }
 EXPORT_SYMBOL_GPL(nvme_wait_freeze_timeout);
@@ -4738,23 +4759,25 @@ EXPORT_SYMBOL_GPL(nvme_wait_freeze_timeout);
 void nvme_wait_freeze(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list)
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list)
 		blk_mq_freeze_queue_wait(ns->queue);
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 }
 EXPORT_SYMBOL_GPL(nvme_wait_freeze);
 
 void nvme_start_freeze(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
 	set_bit(NVME_CTRL_FROZEN, &ctrl->flags);
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list)
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list)
 		blk_freeze_queue_start(ns->queue);
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 }
 EXPORT_SYMBOL_GPL(nvme_start_freeze);
 
@@ -4797,11 +4820,12 @@ EXPORT_SYMBOL_GPL(nvme_unquiesce_admin_queue);
 void nvme_sync_io_queues(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list)
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list)
 		blk_sync_queue(ns->queue);
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 }
 EXPORT_SYMBOL_GPL(nvme_sync_io_queues);
 

drivers/nvme/host/ioctl.c

@@ -789,15 +789,15 @@ static int nvme_dev_user_cmd(struct nvme_ctrl *ctrl, void __user *argp,
 		bool open_for_write)
 {
 	struct nvme_ns *ns;
-	int ret;
+	int ret, srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
 	if (list_empty(&ctrl->namespaces)) {
 		ret = -ENOTTY;
 		goto out_unlock;
 	}
 
-	ns = list_first_entry(&ctrl->namespaces, struct nvme_ns, list);
+	ns = list_first_or_null_rcu(&ctrl->namespaces, struct nvme_ns, list);
 	if (ns != list_last_entry(&ctrl->namespaces, struct nvme_ns, list)) {
 		dev_warn(ctrl->device,
 			"NVME_IOCTL_IO_CMD not supported when multiple namespaces present!\n");
@@ -807,15 +807,18 @@ static int nvme_dev_user_cmd(struct nvme_ctrl *ctrl, void __user *argp,
 
 	dev_warn(ctrl->device,
 		"using deprecated NVME_IOCTL_IO_CMD ioctl on the char device!\n");
-	kref_get(&ns->kref);
-	up_read(&ctrl->namespaces_rwsem);
+	if (!nvme_get_ns(ns)) {
+		ret = -ENXIO;
+		goto out_unlock;
+	}
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 
 	ret = nvme_user_cmd(ctrl, ns, argp, 0, open_for_write);
 	nvme_put_ns(ns);
 	return ret;
 
 out_unlock:
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 	return ret;
 }
 

drivers/nvme/host/multipath.c

@@ -118,7 +118,8 @@ void nvme_failover_req(struct request *req)
 	blk_steal_bios(&ns->head->requeue_list, req);
 	spin_unlock_irqrestore(&ns->head->requeue_lock, flags);
 
-	blk_mq_end_request(req, 0);
+	nvme_req(req)->status = 0;
+	nvme_end_req(req);
 	kblockd_schedule_work(&ns->head->requeue_work);
 }
 
@@ -150,16 +151,17 @@ void nvme_mpath_end_request(struct request *rq)
 void nvme_kick_requeue_lists(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list) {
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list) {
 		if (!ns->head->disk)
 			continue;
 		kblockd_schedule_work(&ns->head->requeue_work);
 		if (nvme_ctrl_state(ns->ctrl) == NVME_CTRL_LIVE)
 			disk_uevent(ns->head->disk, KOBJ_CHANGE);
 	}
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 }
 
 static const char *nvme_ana_state_names[] = {
@@ -193,13 +195,14 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
 void nvme_mpath_clear_ctrl_paths(struct nvme_ctrl *ctrl)
 {
 	struct nvme_ns *ns;
+	int srcu_idx;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list) {
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list) {
 		nvme_mpath_clear_current_path(ns);
 		kblockd_schedule_work(&ns->head->requeue_work);
 	}
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 }
 
 void nvme_mpath_revalidate_paths(struct nvme_ns *ns)
@@ -595,7 +598,7 @@ static void nvme_mpath_set_live(struct nvme_ns *ns)
 		int node, srcu_idx;
 
 		srcu_idx = srcu_read_lock(&head->srcu);
-		for_each_node(node)
+		for_each_online_node(node)
 			__nvme_find_path(head, node);
 		srcu_read_unlock(&head->srcu, srcu_idx);
 	}
@@ -680,6 +683,7 @@ static int nvme_update_ana_state(struct nvme_ctrl *ctrl,
 	u32 nr_nsids = le32_to_cpu(desc->nnsids), n = 0;
 	unsigned *nr_change_groups = data;
 	struct nvme_ns *ns;
+	int srcu_idx;
 
 	dev_dbg(ctrl->device, "ANA group %d: %s.\n",
 			le32_to_cpu(desc->grpid),
@@ -691,8 +695,8 @@ static int nvme_update_ana_state(struct nvme_ctrl *ctrl,
 	if (!nr_nsids)
 		return 0;
 
-	down_read(&ctrl->namespaces_rwsem);
-	list_for_each_entry(ns, &ctrl->namespaces, list) {
+	srcu_idx = srcu_read_lock(&ctrl->srcu);
+	list_for_each_entry_rcu(ns, &ctrl->namespaces, list) {
 		unsigned nsid;
 again:
 		nsid = le32_to_cpu(desc->nsids[n]);
@@ -705,7 +709,7 @@ static int nvme_update_ana_state(struct nvme_ctrl *ctrl,
 		if (ns->head->ns_id > nsid)
 			goto again;
 	}
-	up_read(&ctrl->namespaces_rwsem);
+	srcu_read_unlock(&ctrl->srcu, srcu_idx);
 	return 0;
 }
 

drivers/nvme/host/nvme.h

@@ -282,7 +282,8 @@ struct nvme_ctrl {
 	struct blk_mq_tag_set *tagset;
 	struct blk_mq_tag_set *admin_tagset;
 	struct list_head namespaces;
-	struct rw_semaphore namespaces_rwsem;
+	struct mutex namespaces_lock;
+	struct srcu_struct srcu;
 	struct device ctrl_device;
 	struct device *device;	/* char device */
 #ifdef CONFIG_NVME_HWMON
@@ -471,8 +472,6 @@ struct nvme_ns_head {
 	u8			pi_type;
 	u8			pi_offset;
 	u8			guard_type;
-	u16			sgs;
-	u32			sws;
 #ifdef CONFIG_BLK_DEV_ZONED
 	u64			zsze;
 #endif
@@ -767,6 +766,7 @@ static inline bool nvme_state_terminal(struct nvme_ctrl *ctrl)
 	}
 }
 
+void nvme_end_req(struct request *req);
 void nvme_complete_rq(struct request *req);
 void nvme_complete_batch_req(struct request *req);
 
@@ -1161,6 +1161,7 @@ void nvme_passthru_end(struct nvme_ctrl *ctrl, struct nvme_ns *ns, u32 effects,
 		       struct nvme_command *cmd, int status);
 struct nvme_ctrl *nvme_ctrl_from_file(struct file *file);
 struct nvme_ns *nvme_find_get_ns(struct nvme_ctrl *ctrl, unsigned nsid);
+bool nvme_get_ns(struct nvme_ns *ns);
 void nvme_put_ns(struct nvme_ns *ns);
 
 static inline bool nvme_multi_css(struct nvme_ctrl *ctrl)

drivers/nvme/host/pci.c

@@ -778,7 +778,8 @@ static blk_status_t nvme_map_data(struct nvme_dev *dev, struct request *req,
 		struct bio_vec bv = req_bvec(req);
 
 		if (!is_pci_p2pdma_page(bv.bv_page)) {
-			if (bv.bv_offset + bv.bv_len <= NVME_CTRL_PAGE_SIZE * 2)
+			if ((bv.bv_offset & (NVME_CTRL_PAGE_SIZE - 1)) +
+			     bv.bv_len <= NVME_CTRL_PAGE_SIZE * 2)
 				return nvme_setup_prp_simple(dev, req,
 							     &cmnd->rw, &bv);
 

drivers/nvme/target/configfs.c

@@ -676,10 +676,18 @@ static ssize_t nvmet_ns_enable_store(struct config_item *item,
 	if (kstrtobool(page, &enable))
 		return -EINVAL;
 
+	/*
+	 * take a global nvmet_config_sem because the disable routine has a
+	 * window where it releases the subsys-lock, giving a chance to
+	 * a parallel enable to concurrently execute causing the disable to
+	 * have a misaccounting of the ns percpu_ref.
+	 */
+	down_write(&nvmet_config_sem);
 	if (enable)
 		ret = nvmet_ns_enable(ns);
 	else
 		nvmet_ns_disable(ns);
+	up_write(&nvmet_config_sem);
 
 	return ret ? ret : count;
 }

drivers/nvme/target/core.c

@@ -818,6 +818,15 @@ void nvmet_sq_destroy(struct nvmet_sq *sq)
 	percpu_ref_exit(&sq->ref);
 	nvmet_auth_sq_free(sq);
 
+	/*
+	 * we must reference the ctrl again after waiting for inflight IO
+	 * to complete. Because admin connect may have sneaked in after we
+	 * store sq->ctrl locally, but before we killed the percpu_ref. the
+	 * admin connect allocates and assigns sq->ctrl, which now needs a
+	 * final ref put, as this ctrl is going away.
+	 */
+	ctrl = sq->ctrl;
+
 	if (ctrl) {
 		/*
 		 * The teardown flow may take some time, and the host may not