[RFC PATCH V2 6/6] ublk_drv: add START_USER_RECOVERY and END_USER_RECOVERY support

[ZiyangZhang <ZiyangZhang@linux.alibaba.com>]

START_USER_RECOVERY and END_USER_RECOVERY are two new control commands
to support user recovery feature.

After a crash, user should send START_USER_RECOVERY, it will:
(1) check if (a)current ublk_device is UBLK_S_DEV_RECOVERING which was
    set by monitor_work and (b)the file struct is released. We always
	expect crash of the ublksrv 'process', not exit of a single
	ubq_daemon 'pthread'.
(2) reinit all ubqs, including:
    (a) put the dying task(thread) and reset ->ubq_daemon to NULL.
    (b) reset all ublk_io.
(3) reset ub->mm to NULL.

Then, user should start a new 'process' and send FETCH_REQ on each
ubq_daemon.

Finally, user should send END_USER_RECOVERY, it will:
(1) wait for all new ubq_daemons getting ready.
(2) unquiesce the request queue and expect incoming ublk_queue_rq()
(3) convert state to UBLK_S_DEV_LIVE
(4) schedule monitor_work again

Signed-off-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
---
 drivers/block/ublk_drv.c | 130 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 130 insertions(+)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 0e185d1fa2c4..2d1f3e032606 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -1964,6 +1964,130 @@ static int ublk_ctrl_set_params(struct io_uring_cmd *cmd)
 	return ret;
 }
 
+static void ublk_queue_start_recovery(struct ublk_device *ub, struct ublk_queue *ubq)
+{
+	int i;
+
+	/* All old ioucmds have to be completed/canceled by io_uring_cmd_done(). */
+	WARN_ON_ONCE(ubq->nr_io_ready);
+
+	/* old daemon is PF_EXITING, put it now */
+	put_task_struct(ubq->ubq_daemon);
+	/* have to set it to NULL, otherwise ub won't accept new FETCH_REQ */
+	ubq->ubq_daemon = NULL;
+
+	for (i = 0; i < ubq->q_depth; i++) {
+		struct ublk_io *io = &ubq->ios[i];
+
+		/* forget everything now and be ready for new FETCH_REQ */
+		io->flags = 0;
+		io->cmd = NULL;
+		io->addr = 0;
+	}
+}
+
+static int ublk_ctrl_start_recovery(struct io_uring_cmd *cmd)
+{
+	struct ublksrv_ctrl_cmd *header = (struct ublksrv_ctrl_cmd *)cmd->cmd;
+	struct ublk_device *ub;
+	int ret = -EINVAL;
+	int i;
+
+	ub = ublk_get_device_from_id(header->dev_id);
+	if (!ub)
+		return ret;
+
+	mutex_lock(&ub->mutex);
+
+	if (!ublk_can_use_recovery(ub))
+		goto out_unlock;
+
+	/*
+	 * START_RECOVERY is only allowd after:
+	 *
+	 * (1) UB_STATE_OPEN is not set, which means the dying process is exited
+	 *     and related io_uring ctx is freed so file struct of /dev/ublkcX is
+	 *     released.
+	 *
+	 * (2) UBLK_S_DEV_RECOVERING is set, which means the monitor_work:
+	 *     (a)has requeued all inflight rqs whose io_flags is ACTIVE
+	 *     (b)has requeued/aborted all inflight rqs whose io_flags is NOT ACTIVE
+	 *     (c)has completed/camceled all ioucmds owned by ther dying process
+	 */
+	if (test_bit(UB_STATE_OPEN, &ub->state) ||
+			ub->dev_info.state != UBLK_S_DEV_RECOVERING) {
+		ret = -EBUSY;
+		goto out_unlock;
+	}
+
+	pr_devel("%s: start recovery for dev id %d.\n", __func__, header->dev_id);
+
+	for (i = 0; i < ub->dev_info.nr_hw_queues; i++) {
+		struct ublk_queue *ubq = ublk_get_queue(ub, i);
+
+		WARN_ON_ONCE(!(ubq->ubq_daemon && ubq_daemon_is_dying(ubq)));
+		pr_devel("%s: prepare for recovering qid %d\n", __func__, ubq->q_id);
+		ublk_queue_start_recovery(ub, ubq);
+	}
+
+	/* set to NULL, otherwise new ubq_daemon cannot mmap the io_cmd_buf */
+	ub->mm = NULL;
+	ub->nr_queues_ready = 0;
+	init_completion(&ub->completion);
+	ret = 0;
+
+ out_unlock:
+	mutex_unlock(&ub->mutex);
+	ublk_put_device(ub);
+	return ret;
+}
+
+static int ublk_ctrl_end_recovery(struct io_uring_cmd *cmd)
+{
+	struct ublksrv_ctrl_cmd *header = (struct ublksrv_ctrl_cmd *)cmd->cmd;
+	int ublksrv_pid = (int)header->data[0];
+	struct ublk_device *ub;
+	int ret = -EINVAL;
+
+	ub = ublk_get_device_from_id(header->dev_id);
+	if (!ub)
+		return ret;
+
+	pr_devel("%s: Waiting for new ubq_daemon is ready, dev id %d...\n",
+			__func__, header->dev_id);
+	/* wait until new ubq_daemon sending all FETCH_REQ */
+	wait_for_completion_interruptible(&ub->completion);
+	pr_devel("%s: All new ubq_daemon is ready, dev id %d\n",
+			__func__, header->dev_id);
+
+	mutex_lock(&ub->mutex);
+
+	if (!ublk_can_use_recovery(ub))
+		goto out_unlock;
+
+	/* monitor_work should set UBLK_S_DEV_RECOVERING */
+	if (ub->dev_info.state != UBLK_S_DEV_RECOVERING) {
+		ret = -EBUSY;
+		goto out_unlock;
+	}
+	ub->dev_info.ublksrv_pid = ublksrv_pid;
+	pr_devel("%s: new ublksrv_pid %d, dev id %d\n",
+			__func__, ublksrv_pid, header->dev_id);
+	blk_mq_unquiesce_queue(ub->ub_disk->queue);
+	pr_devel("%s: queue unquiesced, dev id %d.\n",
+			__func__, header->dev_id);
+
+	ub->dev_info.state = UBLK_S_DEV_LIVE;
+	schedule_delayed_work(&ub->monitor_work, UBLK_DAEMON_MONITOR_PERIOD);
+	/* We are good to redo requests now */
+	blk_mq_kick_requeue_list(ub->ub_disk->queue);
+	ret = 0;
+ out_unlock:
+	mutex_unlock(&ub->mutex);
+	ublk_put_device(ub);
+	return ret;
+}
+
 static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 		unsigned int issue_flags)
 {
@@ -2005,6 +2129,12 @@ static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 	case UBLK_CMD_SET_PARAMS:
 		ret = ublk_ctrl_set_params(cmd);
 		break;
+	case UBLK_CMD_START_USER_RECOVERY:
+		ret = ublk_ctrl_start_recovery(cmd);
+		break;
+	case UBLK_CMD_END_USER_RECOVERY:
+		ret = ublk_ctrl_end_recovery(cmd);
+		break;
 	default:
 		break;
 	}
-- 
2.27.0