On Wed, Jul 26, 2017 at 06:57:58PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgold...@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.
 
> Signed-off-by: Goldwyn Rodrigues <rgold...@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?
 
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?

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