Re: [PATCH 06/11] block: optimize generic_make_request for direct to blk-mq issue

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

On Sat, Apr 25, 2020 at 07:09:39PM +0200, Christoph Hellwig wrote:
> Don't bother with the on-stack bio list if we know that we directly
> issue to a request based driver that can't re-inject bios.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/blk-core.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 732d5b8d3cd25..e8c48203b2c55 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1008,6 +1008,18 @@ generic_make_request_checks(struct bio *bio)
>  	return false;
>  }
>  
> +static inline blk_qc_t __direct_make_request(struct bio *bio)
> +{
> +	struct request_queue *q = bio->bi_disk->queue;
> +	blk_qc_t ret;
> +
> +	if (unlikely(bio_queue_enter(bio)))
> +		return BLK_QC_T_NONE;
> +	ret = blk_mq_make_request(q, bio);
> +	blk_queue_exit(q);
> +	return ret;
> +}
> +
>  static blk_qc_t do_make_request(struct bio *bio,
>  		struct bio_list bio_list_on_stack[2])
>  {
> @@ -1116,7 +1128,10 @@ blk_qc_t generic_make_request(struct bio *bio)
>  		return BLK_QC_T_NONE;
>  	}
>  
> -	return __generic_make_request(bio);
> +	if (bio->bi_disk->queue->make_request_fn)
> +		return __generic_make_request(bio);
> +	return __direct_make_request(bio);
> +

blk_mq_make_request() still calls into generic_make_request(), so bio
may be added to current->bio_list, then looks __direct_make_request()
can't cover recursive bio submission any more.

Thanks,
Ming