Re: [PATCH v3 2/7] block: Change elevator_init_mq() to always succeed

[Ming Lei <ming.lei@redhat.com>]

On Wed, Sep 04, 2019 at 05:42:42PM +0900, Damien Le Moal wrote:
> If the default elevator chosen is mq-deadline, elevator_init_mq() may
> return an error if mq-deadline initialization fails, leading to
> blk_mq_init_allocated_queue() returning an error, which in turn will
> cause the block device initialization to fail and the device not being
> exposed.
> 
> Instead of taking such extreme measure, handle mq-deadline
> initialization failures in the same manner as when mq-deadline is not
> available (no module to load), that is, default to the "none" scheduler.
> With this change, elevator_init_mq() return type can be changed to void.
> 
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/blk-mq.c   |  8 +-------
>  block/blk.h      |  2 +-
>  block/elevator.c | 23 ++++++++++++-----------
>  3 files changed, 14 insertions(+), 19 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 13923630e00a..ee4caf0c0807 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2842,8 +2842,6 @@ static unsigned int nr_hw_queues(struct blk_mq_tag_set *set)
>  struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
>  						  struct request_queue *q)
>  {
> -	int ret = -ENOMEM;
> -
>  	/* mark the queue as mq asap */
>  	q->mq_ops = set->ops;
>  
> @@ -2904,14 +2902,10 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
>  	blk_mq_add_queue_tag_set(set, q);
>  	blk_mq_map_swqueue(q);
>  
> -	ret = elevator_init_mq(q);
> -	if (ret)
> -		goto err_tag_set;
> +	elevator_init_mq(q);
>  
>  	return q;
>  
> -err_tag_set:
> -	blk_mq_del_queue_tag_set(q);
>  err_hctxs:
>  	kfree(q->queue_hw_ctx);
>  	q->nr_hw_queues = 0;
> diff --git a/block/blk.h b/block/blk.h
> index e4619fc5c99a..ed347f7a97b1 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -184,7 +184,7 @@ void blk_account_io_done(struct request *req, u64 now);
>  
>  void blk_insert_flush(struct request *rq);
>  
> -int elevator_init_mq(struct request_queue *q);
> +void elevator_init_mq(struct request_queue *q);
>  int elevator_switch_mq(struct request_queue *q,
>  			      struct elevator_type *new_e);
>  void __elevator_exit(struct request_queue *, struct elevator_queue *);
> diff --git a/block/elevator.c b/block/elevator.c
> index 4721834815bb..2944c129760c 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -628,34 +628,35 @@ static inline bool elv_support_iosched(struct request_queue *q)
>  
>  /*
>   * For blk-mq devices supporting IO scheduling, we default to using mq-deadline,
> - * if available, for single queue devices. If deadline isn't available OR we
> - * have multiple queues, default to "none".
> + * if available, for single queue devices. If deadline isn't available OR
> + * deadline initialization fails OR we have multiple queues, default to "none".
>   */
> -int elevator_init_mq(struct request_queue *q)
> +void elevator_init_mq(struct request_queue *q)
>  {
>  	struct elevator_type *e;
> -	int err = 0;
> +	int err;
>  
>  	if (!elv_support_iosched(q))
> -		return 0;
> +		return;
>  
>  	if (q->nr_hw_queues != 1)
> -		return 0;
> +		return;
>  
>  	WARN_ON_ONCE(test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags));
>  
>  	if (unlikely(q->elevator))
> -		goto out;
> +		return;
>  
>  	e = elevator_get(q, "mq-deadline", false);
>  	if (!e)
> -		goto out;
> +		return;
>  
>  	err = blk_mq_init_sched(q, e);
> -	if (err)
> +	if (err) {
> +		pr_warn("\"%s\" elevator initialization failed, "
> +			"falling back to \"none\"\n", e->elevator_name);
>  		elevator_put(e);
> -out:
> -	return err;
> +	}
>  }

Looks fine:

Reviewed-by: Ming Lei <ming.lei@redhat.com>

BTW, blk_mq_init_sched()'s failure patch should have restored
q->nr_request. And that could be done in another standalone patch.

-- 
Ming