Re: [dm-devel] [RFC PATCH V2 3/3] dm: support bio polling

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

On Thu, Jun 17, 2021 at 06:35:49PM +0800, Ming Lei wrote:
> Support bio(REQ_POLLED) polling in the following approach:
> 
> 1) only support io polling on normal READ/WRITE, and other abnormal IOs
> still fallback on IRQ mode, so the target io is exactly inside the dm
> io.
> 
> 2) hold one refcnt on io->io_count after submitting this dm bio with
> REQ_POLLED
> 
> 3) support dm native bio splitting, any dm io instance associated with
> current bio will be added into one list which head is bio->bi_end_io
> which will be recovered before ending this bio
> 
> 4) implement .poll_bio() callback, call bio_poll() on the single target
> bio inside the dm io which is retrieved via bio->bi_bio_drv_data; call
> dec_pending() after the target io is done in .poll_bio()
> 
> 4) enable QUEUE_FLAG_POLL if all underlying queues enable QUEUE_FLAG_POLL,
> which is based on Jeffle's previous patch.
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

...

> @@ -938,8 +945,12 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
>  		end_io_acct(io);
>  		free_io(md, io);
>  
> -		if (io_error == BLK_STS_DM_REQUEUE)
> +		if (io_error == BLK_STS_DM_REQUEUE) {
> +			/* not poll any more in case of requeue */
> +			if (bio->bi_opf & REQ_POLLED)
> +				bio->bi_opf &= ~REQ_POLLED;

It becomes not necessary to clear REQ_POLLED before requeuing since
every dm_io is added into the hlist_head which is reused from
bio->bi_end_io, so all dm-io(include the one to be requeued) will be
polled.

Thanks,
Ming

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