diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 1c90005ab343..f102e88fc785 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -3253,26 +3253,64 @@ static void calc_sectors(struct r10conf *conf, sector_t size)
 	}
 }
 
+enum geo_type {geo_new, geo_old, geo_start};
+static int setup_geo(struct geom *geo, struct mddev *mddev, enum geo_type new)
+{
+	int nc, fc, fo;
+	int layout, chunk, disks;
+	switch (new) {
+	case geo_old:
+		layout = mddev->layout;
+		chunk = mddev->chunk_sectors;
+		disks = mddev->raid_disks - mddev->delta_disks;
+		break;
+	case geo_new:
+		layout = mddev->new_layout;
+		chunk = mddev->new_chunk_sectors;
+		disks = mddev->raid_disks;
+		break;
+	default: /* avoid 'may be unused' warnings */
+	case geo_start: /* new when starting reshape - raid_disks not
+			 * updated yet. */
+		layout = mddev->new_layout;
+		chunk = mddev->new_chunk_sectors;
+		disks = mddev->raid_disks + mddev->delta_disks;
+		break;
+	}
+	if (layout >> 17)
+		return -1;
+	if (chunk < (PAGE_SIZE >> 9) ||
+	    !is_power_of_2(chunk))
+		return -2;
+	nc = layout & 255;
+	fc = (layout >> 8) & 255;
+	fo = layout & (1<<16);
+	geo->raid_disks = disks;
+	geo->near_copies = nc;
+	geo->far_copies = fc;
+	geo->far_offset = fo;
+	geo->chunk_mask = chunk - 1;
+	geo->chunk_shift = ffz(~chunk);
+	return nc*fc;
+}
+
 static struct r10conf *setup_conf(struct mddev *mddev)
 {
 	struct r10conf *conf = NULL;
-	int nc, fc, fo;
 	int err = -EINVAL;
+	struct geom geo;
+	int copies;
 
-	if (mddev->new_chunk_sectors < (PAGE_SIZE >> 9) ||
-	    !is_power_of_2(mddev->new_chunk_sectors)) {
+	copies = setup_geo(&geo, mddev, geo_new);
+
+	if (copies == -2) {
 		printk(KERN_ERR "md/raid10:%s: chunk size must be "
 		       "at least PAGE_SIZE(%ld) and be a power of 2.\n",
 		       mdname(mddev), PAGE_SIZE);
 		goto out;
 	}
 
-	nc = mddev->new_layout & 255;
-	fc = (mddev->new_layout >> 8) & 255;
-	fo = mddev->new_layout & (1<<16);
-
-	if ((nc*fc) <2 || (nc*fc) > mddev->raid_disks ||
-	    (mddev->new_layout >> 17)) {
+	if (copies < 2 || copies > mddev->raid_disks) {
 		printk(KERN_ERR "md/raid10:%s: unsupported raid10 layout: 0x%8x\n",
 		       mdname(mddev), mddev->new_layout);
 		goto out;
@@ -3292,15 +3330,8 @@ static struct r10conf *setup_conf(struct mddev *mddev)
 	if (!conf->tmppage)
 		goto out;
 
-
-	conf->geo.raid_disks = mddev->raid_disks;
-	conf->geo.near_copies = nc;
-	conf->geo.far_copies = fc;
-	conf->copies = nc*fc;
-	conf->geo.far_offset = fo;
-	conf->geo.chunk_mask = mddev->new_chunk_sectors - 1;
-	conf->geo.chunk_shift = ffz(~mddev->new_chunk_sectors);
-
+	conf->geo = geo;
+	conf->copies = copies;
 	conf->r10bio_pool = mempool_create(NR_RAID10_BIOS, r10bio_pool_alloc,
 					   r10bio_pool_free, conf);
 	if (!conf->r10bio_pool)