On 08/09/2017 10:02 AM, Shaohua Li wrote: > 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@xxxxxxxx> >>>> >>>> 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@xxxxxxxx> >>>> --- >>>> 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. > In that case, the bio end_io function is chained and the bio of the split will replicate the error to the parent (if not already set). -- Goldwyn