Re: [PATCH v5 04/12] block: add poll_capable method to support bio-based IO polling

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Mar 03 2021 at  6:57am -0500,
Jeffle Xu <jefflexu@linux.alibaba.com> wrote:

> 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

s/cacheing/caching/

> 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 out-of-date.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>

Ideally QUEUE_FLAG_POLL would be authoritative.. but I appreciate the
problem you've described.  Though I do wonder if this should be solved
by bio-based's fops->poll() method clearing the request_queue's
QUEUE_FLAG_POLL if it finds an underlying device doesn't have
QUEUE_FLAG_POLL set.  Though making bio-based's fops->poll() always need
to validate the an underlying device does support polling is pretty
unfortunate.

Either way, queue_poll_store() will need to avoid blk-mq specific poll
checking for bio-based devices.

Mike

> ---
>  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 0f4f0c8a7825..367c1d9a55c6 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 9dc83c30e7bc..7df40792c032 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1867,6 +1867,7 @@ static inline void blk_ksm_unregister(struct request_queue *q) { }
>  struct block_device_operations {
>  	blk_qc_t (*submit_bio) (struct bio *bio);
>  	int (*poll)(struct request_queue *q, blk_qc_t cookie);
> +	bool (*poll_capable)(struct gendisk *disk);
>  	int (*open) (struct block_device *, fmode_t);
>  	void (*release) (struct gendisk *, fmode_t);
>  	int (*rw_page)(struct block_device *, sector_t, struct page *, unsigned int);
> -- 
> 2.27.0
>