Re: [dm-devel] [PATCH v3 09/11] dm: support IO polling for bio-based dm device

[Mikulas Patocka <mpatocka@redhat.com>]

On Mon, 8 Feb 2021, Jeffle Xu wrote:

> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index c2945c90745e..8423f1347bb8 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1657,6 +1657,68 @@ static blk_qc_t dm_submit_bio(struct bio *bio)
>  	return BLK_QC_T_NONE;
>  }
>  
> +static int dm_poll_one_md(struct mapped_device *md);
> +
> +static int dm_poll_one_dev(struct dm_target *ti, struct dm_dev *dev,
> +				sector_t start, sector_t len, void *data)
> +{
> +	int i, *count = data;
> +	struct request_queue *q = bdev_get_queue(dev->bdev);
> +	struct blk_mq_hw_ctx *hctx;
> +
> +	if (queue_is_mq(q)) {
> +		if (!percpu_ref_tryget(&q->q_usage_counter))
> +			return 0;
> +
> +		queue_for_each_poll_hw_ctx(q, hctx, i)
> +			*count += blk_mq_poll_hctx(q, hctx);
> +
> +		percpu_ref_put(&q->q_usage_counter);
> +	} else
> +		*count += dm_poll_one_md(dev->bdev->bd_disk->private_data);

This is fragile, because in the future there may be other bio-based 
drivers that support polling. You should check that "q" is really a device 
mapper device before calling dm_poll_one_md on it.

Mikulas