md: allow a reshape operation to be reversed.

Currently a reshape operation always progresses from the start
of the array to the end unless the number of devices is being
reduced, in which case it progressed in the opposite direction.

To reverse a partial reshape which changes the number of devices
you can stop the array and re-assemble with the raid-disks numbers
reversed and it will undo.

However for a reshape that does not change the number of devices
it is not possible to reverse the reshape in the middle - you have to
wait until it completes.

So add a 'reshape_direction' attribute with is either 'forwards' or
'backwards' and can be explicitly set when delta_disks is zero.

This will become more important when we allow the data_offset to
change in a reshape. Then the explicit statement of what direction is
being used will be more useful.

This can be enabled in raid5 trivially as it already supports
reverse reshape and just needs to use a different trigger to request it.

Signed-off-by: NeilBrown <[e-mail]>

drivers/md/md.c

···
 	init_waitqueue_head(&mddev->sb_wait);
 	init_waitqueue_head(&mddev->recovery_wait);
 	mddev->reshape_position = MaxSector;
+	mddev->reshape_backwards = 0;
 	mddev->resync_min = 0;
 	mddev->resync_max = MaxSector;
 	mddev->level = LEVEL_NONE;
···
 		mddev->events = ev1;
 		mddev->bitmap_info.offset = 0;
 		mddev->bitmap_info.default_offset = MD_SB_BYTES >> 9;
+		mddev->reshape_backwards = 0;
 
 		if (mddev->minor_version >= 91) {
 			mddev->reshape_position = sb->reshape_position;
···
 			mddev->new_level = sb->new_level;
 			mddev->new_layout = sb->new_layout;
 			mddev->new_chunk_sectors = sb->new_chunk >> 9;
+			if (mddev->delta_disks < 0)
+				mddev->reshape_backwards = 1;
 		} else {
 			mddev->reshape_position = MaxSector;
 			mddev->delta_disks = 0;
···
 		mddev->events = ev1;
 		mddev->bitmap_info.offset = 0;
 		mddev->bitmap_info.default_offset = 1024 >> 9;
-		
+		mddev->reshape_backwards = 0;
+
 		mddev->recovery_cp = le64_to_cpu(sb->resync_offset);
 		memcpy(mddev->uuid, sb->set_uuid, 16);
 
···
 			mddev->new_level = le32_to_cpu(sb->new_level);
 			mddev->new_layout = le32_to_cpu(sb->new_layout);
 			mddev->new_chunk_sectors = le32_to_cpu(sb->new_chunk);
+			if (mddev->delta_disks < 0 ||
+			    (mddev->delta_disks == 0 &&
+			     (le32_to_cpu(sb->feature_map)
+			      & MD_FEATURE_RESHAPE_BACKWARDS)))
+				mddev->reshape_backwards = 1;
 		} else {
 			mddev->reshape_position = MaxSector;
 			mddev->delta_disks = 0;
···
 		sb->delta_disks = cpu_to_le32(mddev->delta_disks);
 		sb->new_level = cpu_to_le32(mddev->new_level);
 		sb->new_chunk = cpu_to_le32(mddev->new_chunk_sectors);
+		if (mddev->delta_disks == 0 &&
+		    mddev->reshape_backwards)
+			sb->feature_map
+				|= cpu_to_le32(MD_FEATURE_RESHAPE_BACKWARDS);
 	}
 
 	if (rdev->badblocks.count == 0)
···
 		mddev->new_chunk_sectors = mddev->chunk_sectors;
 		mddev->raid_disks -= mddev->delta_disks;
 		mddev->delta_disks = 0;
+		mddev->reshape_backwards = 0;
 		module_put(pers->owner);
 		printk(KERN_WARNING "md: %s: %s would not accept array\n",
 		       mdname(mddev), clevel);
···
 	mddev->layout = mddev->new_layout;
 	mddev->chunk_sectors = mddev->new_chunk_sectors;
 	mddev->delta_disks = 0;
+	mddev->reshape_backwards = 0;
 	mddev->degraded = 0;
 	if (mddev->pers->sync_request == NULL) {
 		/* this is now an array without redundancy, so
···
 		int olddisks = mddev->raid_disks - mddev->delta_disks;
 		mddev->delta_disks = n - olddisks;
 		mddev->raid_disks = n;
+		mddev->reshape_backwards = (mddev->delta_disks < 0);
 	} else
 		mddev->raid_disks = n;
 	return rv ? rv : len;
···
 		return -EINVAL;
 	mddev->reshape_position = new;
 	mddev->delta_disks = 0;
+	mddev->reshape_backwards = 0;
 	mddev->new_level = mddev->level;
 	mddev->new_layout = mddev->layout;
 	mddev->new_chunk_sectors = mddev->chunk_sectors;
···
 static struct md_sysfs_entry md_reshape_position =
 __ATTR(reshape_position, S_IRUGO|S_IWUSR, reshape_position_show,
        reshape_position_store);
+
+static ssize_t
+reshape_direction_show(struct mddev *mddev, char *page)
+{
+	return sprintf(page, "%s\n",
+		       mddev->reshape_backwards ? "backwards" : "forwards");
+}
+
+static ssize_t
+reshape_direction_store(struct mddev *mddev, const char *buf, size_t len)
+{
+	int backwards = 0;
+	if (cmd_match(buf, "forwards"))
+		backwards = 0;
+	else if (cmd_match(buf, "backwards"))
+		backwards = 1;
+	else
+		return -EINVAL;
+	if (mddev->reshape_backwards == backwards)
+		return len;
+
+	/* check if we are allowed to change */
+	if (mddev->delta_disks)
+		return -EBUSY;
+
+	if (mddev->persistent &&
+	    mddev->major_version == 0)
+		return -EINVAL;
+
+	mddev->reshape_backwards = backwards;
+	return len;
+}
+
+static struct md_sysfs_entry md_reshape_direction =
+__ATTR(reshape_direction, S_IRUGO|S_IWUSR, reshape_direction_show,
+       reshape_direction_store);
 
 static ssize_t
 array_size_show(struct mddev *mddev, char *page)
···
 	&md_safe_delay.attr,
 	&md_array_state.attr,
 	&md_reshape_position.attr,
+	&md_reshape_direction.attr,
 	&md_array_size.attr,
 	&max_corr_read_errors.attr,
 	NULL,
···
 	mddev->events = 0;
 	mddev->can_decrease_events = 0;
 	mddev->delta_disks = 0;
+	mddev->reshape_backwards = 0;
 	mddev->new_level = LEVEL_NONE;
 	mddev->new_layout = 0;
 	mddev->new_chunk_sectors = 0;
···
 	mddev->new_chunk_sectors = mddev->chunk_sectors;
 	mddev->new_layout = mddev->layout;
 	mddev->delta_disks = 0;
+	mddev->reshape_backwards = 0;
 
 	return 0;
 }
···
 	if (mddev->sync_thread || mddev->reshape_position != MaxSector)
 		return -EBUSY;
 	mddev->delta_disks = raid_disks - mddev->raid_disks;
+	if (mddev->delta_disks < 0)
+		mddev->reshape_backwards = 1;
+	else if (mddev->delta_disks > 0)
+		mddev->reshape_backwards = 0;
 
 	rv = mddev->pers->check_reshape(mddev);
-	if (rv < 0)
+	if (rv < 0) {
 		mddev->delta_disks = 0;
+		mddev->reshape_backwards = 0;
+	}
 	return rv;
 }
 

drivers/md/md.h

···
 	sector_t			reshape_position;
 	int				delta_disks, new_level, new_layout;
 	int				new_chunk_sectors;
+	int				reshape_backwards;
 
 	atomic_t			plug_cnt;	/* If device is expecting
 							 * more bios soon.

drivers/md/raid5.c

···
 			 * to check again.
 			 */
 			spin_lock_irq(&conf->device_lock);
-			if (mddev->delta_disks < 0
+			if (mddev->reshape_backwards
 			    ? logical_sector < conf->reshape_progress
 			    : logical_sector >= conf->reshape_progress) {
 				disks = conf->previous_raid_disks;
 				previous = 1;
 			} else {
-				if (mddev->delta_disks < 0
+				if (mddev->reshape_backwards
 				    ? logical_sector < conf->reshape_safe
 				    : logical_sector >= conf->reshape_safe) {
 					spin_unlock_irq(&conf->device_lock);
···
 				 */
 				int must_retry = 0;
 				spin_lock_irq(&conf->device_lock);
-				if (mddev->delta_disks < 0
+				if (mddev->reshape_backwards
 				    ? logical_sector >= conf->reshape_progress
 				    : logical_sector < conf->reshape_progress)
 					/* mismatch, need to try again */
···
 
 	if (sector_nr == 0) {
 		/* If restarting in the middle, skip the initial sectors */
-		if (mddev->delta_disks < 0 &&
+		if (mddev->reshape_backwards &&
 		    conf->reshape_progress < raid5_size(mddev, 0, 0)) {
 			sector_nr = raid5_size(mddev, 0, 0)
 				- conf->reshape_progress;
-		} else if (mddev->delta_disks >= 0 &&
+		} else if (!mddev->reshape_backwards &&
 			   conf->reshape_progress > 0)
 			sector_nr = conf->reshape_progress;
 		sector_div(sector_nr, new_data_disks);
···
 	sector_div(readpos, data_disks);
 	safepos = conf->reshape_safe;
 	sector_div(safepos, data_disks);
-	if (mddev->delta_disks < 0) {
+	if (mddev->reshape_backwards) {
 		writepos -= min_t(sector_t, reshape_sectors, writepos);
 		readpos += reshape_sectors;
 		safepos += reshape_sectors;
···
 	 * Maybe that number should be configurable, but I'm not sure it is
 	 * worth it.... maybe it could be a multiple of safemode_delay???
 	 */
-	if ((mddev->delta_disks < 0
+	if ((mddev->reshape_backwards
 	     ? (safepos > writepos && readpos < writepos)
 	     : (safepos < writepos && readpos > writepos)) ||
 	    time_after(jiffies, conf->reshape_checkpoint + 10*HZ)) {
···
 		sysfs_notify(&mddev->kobj, NULL, "sync_completed");
 	}
 
-	if (mddev->delta_disks < 0) {
+	if (mddev->reshape_backwards) {
 		BUG_ON(conf->reshape_progress == 0);
 		stripe_addr = writepos;
 		BUG_ON((mddev->dev_sectors &
···
 		list_add(&sh->lru, &stripes);
 	}
 	spin_lock_irq(&conf->device_lock);
-	if (mddev->delta_disks < 0)
+	if (mddev->reshape_backwards)
 		conf->reshape_progress -= reshape_sectors * new_data_disks;
 	else
 		conf->reshape_progress += reshape_sectors * new_data_disks;
···
 				       mdname(mddev));
 				return -EINVAL;
 			}
-		} else if (mddev->delta_disks < 0
+		} else if (mddev->reshape_backwards
 		    ? (here_new * mddev->new_chunk_sectors <=
 		       here_old * mddev->chunk_sectors)
 		    : (here_new * mddev->new_chunk_sectors >=
···
 	conf->chunk_sectors = mddev->new_chunk_sectors;
 	conf->prev_algo = conf->algorithm;
 	conf->algorithm = mddev->new_layout;
-	if (mddev->delta_disks < 0)
+	if (mddev->reshape_backwards)
 		conf->reshape_progress = raid5_size(mddev, 0, 0);
 	else
 		conf->reshape_progress = 0;
···
 		mddev->chunk_sectors = conf->chunk_sectors;
 		mddev->reshape_position = MaxSector;
 		mddev->delta_disks = 0;
+		mddev->reshape_backwards = 0;
 	}
 }
 

include/linux/raid/md_p.h

···
 					    * active device with same 'role'.
 					    * 'recovery_offset' is also set.
 					    */
+#define	MD_FEATURE_RESHAPE_BACKWARDS	32 /* Reshape doesn't change number
+					    * of devices, but is going
+					    * backwards anyway.
+					    */
 #define	MD_FEATURE_ALL			(MD_FEATURE_BITMAP_OFFSET	\
 					|MD_FEATURE_RECOVERY_OFFSET	\
 					|MD_FEATURE_RESHAPE_ACTIVE	\
 					|MD_FEATURE_BAD_BLOCKS		\
-					|MD_FEATURE_REPLACEMENT)
+					|MD_FEATURE_REPLACEMENT		\
+					|MD_FEATURE_RESHAPE_BACKWARDS)
 
 #endif