Re: [PATCH 6/6] blk-mq: manage hctx map via xarray

[Christoph Hellwig <hch@infradead.org>]

> -			hctxs[i] = blk_mq_alloc_and_init_hctx(set, q, i,
> -					old_node);
> -			WARN_ON_ONCE(!hctxs[i]);
> +			WARN_ON_ONCE(!blk_mq_alloc_and_init_hctx(set, q, i,
> +						old_node));


Please avoid doing the actual work inside a WARN_ON statement.

>  
>  	for (; j < end; j++) {
> -		struct blk_mq_hw_ctx *hctx = hctxs[j];
> +		struct blk_mq_hw_ctx *hctx = blk_mq_get_hctx(q, j);
>  
> -		if (hctx) {
> +		if (hctx)
>  			blk_mq_exit_hctx(q, set, hctx, j);
> -			hctxs[j] = NULL;
> -		}
>  	}

Instead of a for loop that does xa_loads repeatedly this can just
use xa_for_each_range.  Same for a bunch of other loops like that,
e.g. in blk_mq_unregister_dev or the __blk_mq_register_dev failure
path.

> @@ -919,12 +919,12 @@ static inline void *blk_mq_rq_to_pdu(struct request *rq)
>  static inline struct blk_mq_hw_ctx *blk_mq_get_hctx(struct request_queue *q,
>  		unsigned int hctx_idx)
>  {
> -	return q->queue_hw_ctx[hctx_idx];
> +	return xa_load(&q->hctx_table, hctx_idx);
>  }
>  
>  #define queue_for_each_hw_ctx(q, hctx, i)				\
>  	for ((i) = 0; (i) < (q)->nr_hw_queues &&			\
> -	     ({ hctx = blk_mq_get_hctx((q), (i)); 1; }); (i)++)
> +	     (hctx = blk_mq_get_hctx((q), (i))); (i)++)

This should be using a xa_for_each loop.

With various places converted to loops I'm not even sure we really
want the blk_mq_get_hctx helper at all.