+		// TODO check and try to fix metadata

Please avoid //-style comments.

+static u64
+xfs_dax_ddev_offset(
+	struct xfs_mount	*mp,
+	struct dax_device	*dax_dev,
+	u64			disk_offset)
+{
+	xfs_buftarg_t *targp;
+
+	if (mp->m_ddev_targp->bt_daxdev == dax_dev)
+		targp = mp->m_ddev_targp;
+	else if (mp->m_logdev_targp->bt_daxdev == dax_dev)
+		targp = mp->m_logdev_targp;
+	else
+		targp = mp->m_rtdev_targp;
+
+	return disk_offset - targp->bt_dax_part_off;

This is only called for the data device.  So I think we can simplify
this and open code the logic in xfs_dax_notify_ddev_failure.

+void
+xfs_notify_failure_register(
+	struct xfs_mount	*mp,
+	struct dax_device	*dax_dev)
+{
+	if (dax_dev && !fs_dax_get_holder(dax_dev))
+		fs_dax_register_holder(dax_dev, mp, &xfs_dax_holder_operations);
+}
+
+void
+xfs_notify_failure_unregister(
+	struct dax_device	*dax_dev)
+{
+	if (dax_dev)
+		fs_dax_unregister_holder(dax_dev);
+}

Why do we need these wrappers?  Also instead of the fs_dax_get_holder
here, fs_dax_register_holder needs to return an error if there already
is a holder.