Re: [PATCH 1/9] QUEUE_FLAG_NOWAIT to indicate device supports nowait

[Shaohua Li <shli@kernel.org>]

On Wed, Aug 09, 2017 at 06:44:55AM -0500, Goldwyn Rodrigues wrote:
> 
> 
> On 08/08/2017 03:32 PM, Shaohua Li wrote:
> > On Wed, Jul 26, 2017 at 06:57:58PM -0500, Goldwyn Rodrigues wrote:
> >> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> >>
> >> Nowait is a feature of direct AIO, where users can request
> >> to return immediately if the I/O is going to block. This translates
> >> to REQ_NOWAIT in bio.bi_opf flags. While request based devices
> >> don't wait, stacked devices such as md/dm will.
> >>
> >> In order to explicitly mark stacked devices as supported, we
> >> set the QUEUE_FLAG_NOWAIT in the queue_flags and return -EAGAIN
> >> whenever the device would block.
> > 
> > probably you should route this patch to Jens first, DM/MD are different trees.
> 
> Yes, I have sent it to linux-block as well, and he has commented as well.
> 
> 
> >  
> >> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> >> ---
> >>  block/blk-core.c       | 3 ++-
> >>  include/linux/blkdev.h | 2 ++
> >>  2 files changed, 4 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/block/blk-core.c b/block/blk-core.c
> >> index 970b9c9638c5..1c9a981d88e5 100644
> >> --- a/block/blk-core.c
> >> +++ b/block/blk-core.c
> >> @@ -2025,7 +2025,8 @@ generic_make_request_checks(struct bio *bio)
> >>  	 * if queue is not a request based queue.
> >>  	 */
> >>  
> >> -	if ((bio->bi_opf & REQ_NOWAIT) && !queue_is_rq_based(q))
> >> +	if ((bio->bi_opf & REQ_NOWAIT) && !queue_is_rq_based(q) &&
> >> +	    !blk_queue_supports_nowait(q))
> >>  		goto not_supported;
> >>  
> >>  	part = bio->bi_bdev->bd_part;
> >> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> >> index 25f6a0cb27d3..fae021ebec1b 100644
> >> --- a/include/linux/blkdev.h
> >> +++ b/include/linux/blkdev.h
> >> @@ -633,6 +633,7 @@ struct request_queue {
> >>  #define QUEUE_FLAG_REGISTERED  29	/* queue has been registered to a disk */
> >>  #define QUEUE_FLAG_SCSI_PASSTHROUGH 30	/* queue supports SCSI commands */
> >>  #define QUEUE_FLAG_QUIESCED    31	/* queue has been quiesced */
> >> +#define QUEUE_FLAG_NOWAIT      32	/* stack device driver supports REQ_NOWAIT */
> >>  
> >>  #define QUEUE_FLAG_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
> >>  				 (1 << QUEUE_FLAG_STACKABLE)	|	\
> >> @@ -732,6 +733,7 @@ static inline void queue_flag_clear(unsigned int flag, struct request_queue *q)
> >>  #define blk_queue_dax(q)	test_bit(QUEUE_FLAG_DAX, &(q)->queue_flags)
> >>  #define blk_queue_scsi_passthrough(q)	\
> >>  	test_bit(QUEUE_FLAG_SCSI_PASSTHROUGH, &(q)->queue_flags)
> >> +#define blk_queue_supports_nowait(q)	test_bit(QUEUE_FLAG_NOWAIT, &(q)->queue_flags)
> > 
> > Should this bit consider under layer disks? For example, one raid array disk
> > doesn't support NOWAIT, shouldn't we disable NOWAIT for the array?
> 
> Yes, it should. I will add a check before setting the flag. Thanks.
> Request-based devices don't wait. So, they would not have this flag set.
> It is only the bio-based, with the  make_request_fn hook which need this.
> 
> >  
> > I have another generic question. If a bio is splitted into 2 bios, one bio
> > doesn't need to wait but the other need to wait. We will return -EAGAIN for the
> > second bio, so the whole bio will return -EAGAIN, but the first bio is already
> > dispatched to disk. Is this correct behavior?
> > 
> 
> No, from a multi-device point of view, this is inconsistent. I have
> tried the request bio returns -EAGAIN before the split, but I shall
> check again. Where do you see this happening?

No, this isn't multi-device specific, any driver can do it. Please see blk_queue_split.