raid1: allow user to force reads from a specific disk

@@ -418,19 +418,37 @@ static int raid1_end_write_request(struc
 static int read_balance(conf_t *conf, r1bio_t *r1_bio)
 {
 	const unsigned long this_sector = r1_bio->sector;
-	int new_disk = conf->last_used, disk = new_disk;
-	int wonly_disk = -1;
+	int new_disk = conf->read_from_disk, disk = conf->last_used;
+	int wonly_disk = -1, forced_read = 0;
 	const int sectors = r1_bio->sectors;
 	sector_t new_distance, current_distance;
 	mdk_rdev_t *rdev;
 
 	rcu_read_lock();
+
+	if (new_disk != -1) {
+		/* user has forced reads to one disk */
+		forced_read = 1;
+		if (new_disk >= 0 && new_disk < conf->raid_disks)
+			goto rb_out;
+	}
+
+	new_disk = conf->last_used;
 	/*
 	 * Check if we can balance. We can balance on the whole
 	 * device if no resync is going on, or below the resync window.
 	 * We take the first readable disk when above the resync window.
 	 */
  retry:
+	/*
+	 * If we reach this point and user has forced reads from one disk,
+	 * disable the forced reads because they cannot be done.  User can
+	 * check the "read_from_disk" attribute after doing IO to see if
+	 * all the reads were really done from the correct disk.
+	 */
+	if (forced_read)
+		conf->read_from_disk = -1;
+
 	if (conf->mddev->recovery_cp < MaxSector &&
 	    (this_sector + sectors >= conf->next_resync)) {
 		/* Choose the first operation device, for consistancy */

@@ -518,7 +536,6 @@ static int read_balance(conf_t *conf, r1
 
  rb_out:
 
-
 	if (new_disk >= 0) {
 		rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
 		if (!rdev)

@@ -1802,6 +1819,52 @@ static sector_t sync_request(mddev_t *md
 	return nr_sectors;
 }
 
+static ssize_t
+raid1_show_read_from_disk(mddev_t *mddev, char *page)
+{
+	conf_t *conf = mddev_to_conf(mddev);
+	if (conf)
+		return sprintf(page, "%d\n", conf->read_from_disk);
+	else
+		return 0;
+}
+
+static ssize_t
+raid1_store_read_from_disk(mddev_t *mddev, const char *page, size_t len)
+{
+	conf_t *conf = mddev_to_conf(mddev);
+	char *end;
+	int new;
+
+	if (len >= PAGE_SIZE)
+		return -EINVAL;
+	if (!conf)
+		return -ENODEV;
+
+	new = simple_strtoul(page, &end, 10);
+	if (!*page || (*end && *end != '\n') )
+		return -EINVAL;
+	if (new < -1 || new >= conf->raid_disks)
+		return -EINVAL;
+	conf->read_from_disk = new;
+
+	return len;
+}
+
+static struct md_sysfs_entry
+raid1_read_from_disk = __ATTR(read_from_disk, S_IRUGO | S_IWUSR,
+				raid1_show_read_from_disk,
+				raid1_store_read_from_disk);
+
+static struct attribute *raid1_attrs[] =  {
+	&raid1_read_from_disk.attr,
+	NULL,
+};
+static struct attribute_group raid1_attrs_group = {
+	.name = NULL,
+	.attrs = raid1_attrs,
+};
+
 static int run(mddev_t *mddev)
 {
 	conf_t *conf;

@@ -1913,6 +1976,7 @@ static int run(mddev_t *mddev)
 		      !test_bit(In_sync, &conf->mirrors[j].rdev->flags)) ; j++)
 		/* nothing */;
 	conf->last_used = j;
+	conf->read_from_disk = -1; /* default: balance reads */
 
 
 	mddev->thread = md_register_thread(raid1d, mddev, "%s_raid1");

@@ -1930,6 +1994,8 @@ static int run(mddev_t *mddev)
 	/*
 	 * Ok, everything is just fine now
 	 */
+	sysfs_create_group(&mddev->kobj, &raid1_attrs_group);
+
 	mddev->array_size = mddev->size;
 
 	mddev->queue->unplug_fn = raid1_unplug;

@@ -32,6 +32,7 @@ struct r1_private_data_s {
 	int			raid_disks;
 	int			working_disks;
 	int			last_used;
+	int			read_from_disk;
 	sector_t		next_seq_sect;
 	spinlock_t		device_lock;