[PATCH v2 1/3] blk-cgroup: check for pd_(alloc|free)_fn in blkcg_activate_policy()

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 block/blk-cgroup.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 37e6cc91d576..dcd5e857650a 100644
--- a/block/blk-cgroup.c
+++ b/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:
@@ -1733,7 +1741,10 @@ 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))
 		goto err_unlock;
-- 
2.39.2