blk-cgroup: Pre-allocate tree node on blkg_conf_prep

Similarly to commit 457e490f ("blkcg: allocate struct blkcg_gq
outside request queue spinlock"), blkg_create can also trigger
occasional -ENOMEM failures at the radix insertion because any
allocation inside blkg_create has to be non-blocking, making it more
likely to fail.  This causes trouble for userspace tools trying to
configure io weights who need to deal with this condition.

This patch reduces the occurrence of -ENOMEMs on this path by preloading
the radix tree element on a GFP_KERNEL context, such that we guarantee
the later non-blocking insertion won't fail.

A similar solution exists in blkcg_init_queue for the same situation.
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -657,6 +657,12 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 			goto fail;
 		}
 
+		if (radix_tree_preload(GFP_KERNEL)) {
+			blkg_free(new_blkg);
+			ret = -ENOMEM;
+			goto fail;
+		}
+
 		rcu_read_lock();
 		spin_lock_irq(&q->queue_lock);
 
@@ -664,7 +670,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		if (IS_ERR(blkg)) {
 			ret = PTR_ERR(blkg);
 			blkg_free(new_blkg);
-			goto fail_unlock;
+			goto fail_preloaded;
 		}
 
 		if (blkg) {
@@ -673,10 +679,12 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 			blkg = blkg_create(pos, q, new_blkg);
 			if (IS_ERR(blkg)) {
 				ret = PTR_ERR(blkg);
-				goto fail_unlock;
+				goto fail_preloaded;
 			}
 		}
 
+		radix_tree_preload_end();
+
 		if (pos == blkcg)
 			goto success;
 	}
@@ -686,6 +694,8 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 	ctx->body = input;
 	return 0;
 
+fail_preloaded:
+	radix_tree_preload_end();
 fail_unlock:
 	spin_unlock_irq(&q->queue_lock);
 	rcu_read_unlock();