Re: [PATCH V4 11/12] block: add poll_capable method to support bio-based IO polling

[Hannes Reinecke <hare@suse.de>]

On 3/29/21 5:26 PM, Ming Lei wrote:
> From: Jeffle Xu <jefflexu@linux.alibaba.com>
> 
> This method can be used to check if bio-based device supports IO polling
> or not. For mq devices, checking for hw queue in polling mode is
> adequate, while the sanity check shall be implementation specific for
> bio-based devices. For example, dm device needs to check if all
> underlying devices are capable of IO polling.
> 
> Though bio-based device may have done the sanity check during the
> device initialization phase, cacheing the result of this sanity check
> (such as by cacheing in the queue_flags) may not work. Because for dm
> devices, users could change the state of the underlying devices through
> '/sys/block/<dev>/io_poll', bypassing the dm device above. In this case,
> the cached result of the very beginning sanity check could be
> out-of-date. Thus the sanity check needs to be done every time 'io_poll'
> is to be modified.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
>   block/blk-sysfs.c      | 14 +++++++++++---
>   include/linux/blkdev.h |  1 +
>   2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index db3268d41274..c8e7e4af66cb 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -426,9 +426,17 @@ static ssize_t queue_poll_store(struct request_queue *q, const char *page,
>   	unsigned long poll_on;
>   	ssize_t ret;
>   
> -	if (!q->tag_set || q->tag_set->nr_maps <= HCTX_TYPE_POLL ||
> -	    !q->tag_set->map[HCTX_TYPE_POLL].nr_queues)
> -		return -EINVAL;
> +	if (queue_is_mq(q)) {
> +		if (!q->tag_set || q->tag_set->nr_maps <= HCTX_TYPE_POLL ||
> +		    !q->tag_set->map[HCTX_TYPE_POLL].nr_queues)
> +			return -EINVAL;
> +	} else {
> +		struct gendisk *disk = queue_to_disk(q);
> +
> +		if (!disk->fops->poll_capable ||
> +		    !disk->fops->poll_capable(disk))
> +			return -EINVAL;
> +	}
>   
>   	ret = queue_var_store(&poll_on, page, count);
>   	if (ret < 0)
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index bfab74b45f15..a46f975f2a2f 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1881,6 +1881,7 @@ struct block_device_operations {
>   	int (*report_zones)(struct gendisk *, sector_t sector,
>   			unsigned int nr_zones, report_zones_cb cb, void *data);
>   	char *(*devnode)(struct gendisk *disk, umode_t *mode);
> +	bool (*poll_capable)(struct gendisk *disk);
>   	struct module *owner;
>   	const struct pr_ops *pr_ops;
>   };
> 
I really wonder how this would work for nvme multipath; but I guess it 
doesn't change the current situation.

So:

Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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