blk-cgroup: improve policy registration error handling

This patch improve the returned error code of blkcg_policy_register().

1. Move the validation check for cpd/pd_alloc_fn and cpd/pd_free_fn
   function pairs to the start of blkcg_policy_register(). This ensures
   we immediately return -EINVAL if the function pairs are not correctly
   provided, rather than returning -ENOSPC after locking and unlocking
   mutexes unnecessarily.

   Those locks should not contention any problems, as error of policy
   registration is a super cold path.

2. Return -ENOMEM when cpd_alloc_fn() failed.

Co-authored-by: Wen Tao <wentao@uniontech.com>
Signed-off-by: Wen Tao <wentao@uniontech.com>
Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/3E333A73B6B6DFC0+20250317022924.150907-1-chenlinxuan@uniontech.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -1727,27 +1727,27 @@ int blkcg_policy_register(struct blkcg_policy *pol)
 	struct blkcg *blkcg;
 	int i, ret;
 
+	/*
+	 * 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))
+		return -EINVAL;
+
 	mutex_lock(&blkcg_pol_register_mutex);
 	mutex_lock(&blkcg_pol_mutex);
 
 	/* find an empty slot */
-	ret = -ENOSPC;
 	for (i = 0; i < BLKCG_MAX_POLS; i++)
 		if (!blkcg_policy[i])
 			break;
 	if (i >= BLKCG_MAX_POLS) {
 		pr_warn("blkcg_policy_register: BLKCG_MAX_POLS too small\n");
+		ret = -ENOSPC;
 		goto err_unlock;
 	}
 
-	/*
-	 * 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;
-
 	/* register @pol */
 	pol->plid = i;
 	blkcg_policy[pol->plid] = pol;
@@ -1758,8 +1758,10 @@ int blkcg_policy_register(struct blkcg_policy *pol)
 			struct blkcg_policy_data *cpd;
 
 			cpd = pol->cpd_alloc_fn(GFP_KERNEL);
-			if (!cpd)
+			if (!cpd) {
+				ret = -ENOMEM;
 				goto err_free_cpds;
+			}
 
 			blkcg->cpd[pol->plid] = cpd;
 			cpd->blkcg = blkcg;