md/raid1: prioritize adding disk to 'removed' mirror

New disk should be added to "removed" position first instead of to be a
replacement. Commit 6090368a ("md/raid10: prioritize adding disk to
'removed' mirror") has fixed this issue for raid10. Fix it for raid1 now.
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20230627014332.3810102-1-linan666@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/raid1.c

@@ -1764,7 +1764,7 @@ static int raid1_add_disk(struct mddev *mddev, struct md_rdev *rdev)
 {
 	struct r1conf *conf = mddev->private;
 	int err = -EEXIST;
-	int mirror = 0;
+	int mirror = 0, repl_slot = -1;
 	struct raid1_info *p;
 	int first = 0;
 	int last = conf->raid_disks - 1;
@@ -1807,17 +1807,21 @@ static int raid1_add_disk(struct mddev *mddev, struct md_rdev *rdev)
 			break;
 		}
 		if (test_bit(WantReplacement, &p->rdev->flags) &&
-		    p[conf->raid_disks].rdev == NULL) {
-			/* Add this device as a replacement */
-			clear_bit(In_sync, &rdev->flags);
-			set_bit(Replacement, &rdev->flags);
-			rdev->raid_disk = mirror;
-			err = 0;
-			conf->fullsync = 1;
-			rcu_assign_pointer(p[conf->raid_disks].rdev, rdev);
-			break;
-		}
+		    p[conf->raid_disks].rdev == NULL && repl_slot < 0)
+			repl_slot = mirror;
 	}
+
+	if (err && repl_slot >= 0) {
+		/* Add this device as a replacement */
+		p = conf->mirrors + repl_slot;
+		clear_bit(In_sync, &rdev->flags);
+		set_bit(Replacement, &rdev->flags);
+		rdev->raid_disk = repl_slot;
+		err = 0;
+		conf->fullsync = 1;
+		rcu_assign_pointer(p[conf->raid_disks].rdev, rdev);
+	}
+
 	print_conf(conf);
 	return err;
 }