blk-cgroup: check for pd_(alloc|free)_fn in blkcg_activate_policy()

Currently all policies implement pd_(alloc|free)_fn, however, this is
not necessary for ioprio that only works for blkcg, not blkg.

There are no functional changes, prepare to cleanup activating ioprio
policy.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20240719071506.158075-2-yukuai1@huaweicloud.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -1554,6 +1554,14 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
 	if (blkcg_policy_enabled(q, pol))
 		return 0;
 
+	/*
+	 * Policy is allowed to be registered without pd_alloc_fn/pd_free_fn,
+	 * for example, ioprio. Such policy will work on blkcg level, not disk
+	 * level, and don't need to be activated.
+	 */
+	if (WARN_ON_ONCE(!pol->pd_alloc_fn || !pol->pd_free_fn))
+		return -EINVAL;
+
 	if (queue_is_mq(q))
 		blk_mq_freeze_queue(q);
 retry:
@@ -1733,9 +1741,12 @@ int blkcg_policy_register(struct blkcg_policy *pol)
 		goto err_unlock;
 	}
 
-	/* Make sure cpd/pd_alloc_fn and cpd/pd_free_fn in pairs */
+	/*
+	 * Make sure cpd/pd_alloc_fn and cpd/pd_free_fn in pairs, and policy
+	 * without pd_alloc_fn/pd_free_fn can't be activated.
+	 */
 	if ((!pol->cpd_alloc_fn ^ !pol->cpd_free_fn) ||
-		(!pol->pd_alloc_fn ^ !pol->pd_free_fn))
+	    (!pol->pd_alloc_fn ^ !pol->pd_free_fn))
 		goto err_unlock;
 
 	/* register @pol */