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

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues 

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.

Signed-off-by: Goldwyn Rodrigues 
---
 block/blk-core.c   | 4 ++--
 include/linux/blkdev.h | 6 ++
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 048be4aa6024..8de633f8c633 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2044,10 +2044,10 @@ generic_make_request_checks(struct bio *bio)
 
/*
 * For a REQ_NOWAIT based request, return -EOPNOTSUPP
-* if queue is not a request based queue.
+* if queue cannot handle nowait bio's
 */
 
-   if ((bio->bi_opf & REQ_NOWAIT) && !queue_is_rq_based(q))
+   if ((bio->bi_opf & REQ_NOWAIT) && !blk_queue_supports_nowait(q))
goto not_supported;
 
if (should_fail_request(>bi_disk->part0, bio->bi_iter.bi_size))
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 02fa42d24b52..1d0da2a9cf46 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -631,6 +631,7 @@ struct request_queue {
 #define QUEUE_FLAG_REGISTERED  26  /* queue has been registered to a disk 
*/
 #define QUEUE_FLAG_SCSI_PASSTHROUGH 27 /* queue supports SCSI commands */
 #define QUEUE_FLAG_QUIESCED28  /* queue has been quiesced */
+#define QUEUE_FLAG_NOWAIT  29  /* stack device driver supports 
REQ_NOWAIT */
 
 #define QUEUE_FLAG_DEFAULT ((1 << QUEUE_FLAG_IO_STAT) |\
 (1 << QUEUE_FLAG_STACKABLE)|   \
@@ -759,6 +760,11 @@ static inline bool queue_is_rq_based(struct request_queue 
*q)
return q->request_fn || q->mq_ops;
 }
 
+static inline bool blk_queue_supports_nowait(struct request_queue *q)
+{
+   return queue_is_rq_based(q) || test_bit(QUEUE_FLAG_NOWAIT, 
>queue_flags);
+}
+
 static inline unsigned int blk_queue_cluster(struct request_queue *q)
 {
return q->limits.cluster;
-- 
2.14.2

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

[Jan Kara]

On Thu 10-08-17 06:49:53, Goldwyn Rodrigues wrote:
> On 08/09/2017 09:17 PM, Jens Axboe wrote:
> > On 08/09/2017 08:07 PM, Goldwyn Rodrigues wrote:
>  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).
> >
> > this doesn't answer my question. So if a bio returns -EAGAIN, part
> > of the bio probably already dispatched to disk (if the bio is
> > splitted to 2 bios, one returns -EAGAIN, the other one doesn't
> > block and dispatch to disk), what will application be going to do?
> > I think this is different to other IO errors. FOr other IO errors,
> > application will handle the error, while we ask app to retry the
> > whole bio here and app doesn't know part of bio is already written
> > to disk.
> 
>  It is the same as for other I/O errors as well, such as EIO. You do
>  not know which bio of all submitted bio's returned the error EIO.
>  The application would and should consider the whole I/O as failed.
> 
>  The user application does not know of bios, or how it is going to be
>  split in the underlying layers. It knows at the system call level.
>  In this case, the EAGAIN will be returned to the user for the whole
>  I/O not as a part of the I/O. It is up to application to try the I/O
>  again with or without RWF_NOWAIT set. In direct I/O, it is bubbled
>  out using dio->io_error. You can read about it at the patch header
>  for the initial patchset at [1].
> 
>  Use case: It is for applications having two threads, a compute
>  thread and an I/O thread. It would try to push AIO as much as
>  possible in the compute thread using RWF_NOWAIT, and if it fails,
>  would pass it on to I/O thread which would perform without
>  RWF_NOWAIT. End result if done right is you save on context switches
>  and all the synchronization/messaging machinery to perform I/O.
> 
>  [1] http://marc.info/?l=linux-block=149789003305876=2
> >>>
> >>> Yes, I knew the concept, but I didn't see previous patches mentioned
> >>> the -EAGAIN actually should be taken as a real IO error. This means a
> >>> lot to applications and make the API hard to use. I'm wondering if we
> >>> should disable bio split for NOWAIT bio, which will make the -EAGAIN
> >>> only mean 'try again'.
> >>
> >> Don't take it as EAGAIN, but read it as EWOULDBLOCK. Why do you say
> >> the API is hard to use? Do you have a case to back it up?
> > 
> > Because it is hard to use, and potentially suboptimal. Let's say you're
> > doing a 1MB write, we hit EWOULDBLOCK for the last split. Do we return a
> > short write, or do we return EWOULDBLOCK? If the latter, then that
> > really sucks from an API point of view.
> > 
> >> No, not splitting the bio does not make sense here. I do not see any
> >> advantage in it, unless you can present a case otherwise.
> > 
> > It ties back into the "hard to use" that I do agree with IFF we don't
> > return the short write. It's hard for an application to use that
> > efficiently, if we write 1MB-128K but get EWOULDBLOCK, the re-write the
> > full 1MB from a different context.
> > 
> 
> It returns the error code only and not short reads/writes. But isn't
> that true for all system calls in case of error?
> 
> For aio, there are two result fields in io_event out of which one could
> be used for error while the other be used for amount of writes/reads
> performed. However, only one is used. This will not work with
> pread()/pwrite() calls though because of the limitation of return values.
> 
> Finally, what if the EWOULDBLOCK is returned for an earlier bio (say
> offset 128k) for a 1MB pwrite(), while the rest of the 7 128K are
> successful. What short return value should the system call return?

This is indeed tricky. If an application submits 1MB write, I don't think
we can afford to just write arbitrary subset of it. That just IMHO too much
violates how writes traditionally behaved. Even short writes trigger bugs
in various applications but I'm willing to require that applications using
NOWAIT IO can handle these. However writing arbitrary subset looks like a
nasty catch. IMHO we should not submit further bios until we are sure
current one does not return EWOULDBLOCK when splitting a larger one...

Honza
-- 
Jan Kara 
SUSE Labs, CR

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

[Jens Axboe]

On 08/10/2017 05:49 AM, Goldwyn Rodrigues wrote:
> 
> 
> On 08/09/2017 09:17 PM, Jens Axboe wrote:
>> On 08/09/2017 08:07 PM, Goldwyn Rodrigues wrote:
> 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).
>>
>> this doesn't answer my question. So if a bio returns -EAGAIN, part
>> of the bio probably already dispatched to disk (if the bio is
>> splitted to 2 bios, one returns -EAGAIN, the other one doesn't
>> block and dispatch to disk), what will application be going to do?
>> I think this is different to other IO errors. FOr other IO errors,
>> application will handle the error, while we ask app to retry the
>> whole bio here and app doesn't know part of bio is already written
>> to disk.
>
> It is the same as for other I/O errors as well, such as EIO. You do
> not know which bio of all submitted bio's returned the error EIO.
> The application would and should consider the whole I/O as failed.
>
> The user application does not know of bios, or how it is going to be
> split in the underlying layers. It knows at the system call level.
> In this case, the EAGAIN will be returned to the user for the whole
> I/O not as a part of the I/O. It is up to application to try the I/O
> again with or without RWF_NOWAIT set. In direct I/O, it is bubbled
> out using dio->io_error. You can read about it at the patch header
> for the initial patchset at [1].
>
> Use case: It is for applications having two threads, a compute
> thread and an I/O thread. It would try to push AIO as much as
> possible in the compute thread using RWF_NOWAIT, and if it fails,
> would pass it on to I/O thread which would perform without
> RWF_NOWAIT. End result if done right is you save on context switches
> and all the synchronization/messaging machinery to perform I/O.
>
> [1] http://marc.info/?l=linux-block=149789003305876=2

 Yes, I knew the concept, but I didn't see previous patches mentioned
 the -EAGAIN actually should be taken as a real IO error. This means a
 lot to applications and make the API hard to use. I'm wondering if we
 should disable bio split for NOWAIT bio, which will make the -EAGAIN
 only mean 'try again'.
>>>
>>> Don't take it as EAGAIN, but read it as EWOULDBLOCK. Why do you say
>>> the API is hard to use? Do you have a case to back it up?
>>
>> Because it is hard to use, and potentially suboptimal. Let's say you're
>> doing a 1MB write, we hit EWOULDBLOCK for the last split. Do we return a
>> short write, or do we return EWOULDBLOCK? If the latter, then that
>> really sucks from an API point of view.
>>
>>> No, not splitting the bio does not make sense here. I do not see any
>>> advantage in it, unless you can present a case otherwise.
>>
>> It ties back into the "hard to use" that I do agree with IFF we don't
>> return the short write. It's hard for an application to use that
>> efficiently, if we write 1MB-128K but get EWOULDBLOCK, the re-write the
>> full 1MB from a different context.
>>
> 
> It returns the error code only and not short reads/writes. But isn't
> that true for all system calls in case of error?

It's not a hard error. If you wrote 896K in the example above, I'd
really expect the return value to be 896*1024. The API is hard to use
efficiently, if that's not the case.

> For aio, there are two result fields in io_event out of which one could
> be used for error while the other be used for amount of writes/reads
> performed. However, only one is used. This will not work with
> pread()/pwrite() calls though because of the limitation of return values.

Don't invent something new for this, the mechanism already exists for
returning a short read or write. That's how all of them have worked for
decades.

> Finally, what if the EWOULDBLOCK is returned for an earlier bio (say
> offset 128k) for a 1MB pwrite(), while the rest of the 7 128K are
> successful. What short return value should the system call return?

It should return 128*1024, since that's how much was successfully done
from the start offset. But yes, this is exactly the point that I brought
up, and why contesting Shaohua's suggestion to perhaps treat splits
differently should not be discarded so quickly.

-- 
Jens Axboe

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

[Jens Axboe]

On 08/10/2017 05:38 AM, Goldwyn Rodrigues wrote:
> 
> 
> On 08/09/2017 09:18 PM, Jens Axboe wrote:
>> On 08/08/2017 02:36 PM, Jens Axboe wrote:
>>> On 08/08/2017 02:32 PM, Shaohua Li wrote:
> 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_QUIESCED31/* queue has been quiesced */
> +#define QUEUE_FLAG_NOWAIT  32/* stack device driver supports 
> REQ_NOWAIT */
>>>
>>> Does this work on 32-bit, where sizeof(unsigned long) == 32?
>>
>> I didn't get an answer to this one.
>>
> 
> Oh, I assumed the question is rhetorical.
> No, it will not work on 32-bit. I was planning to change the field
> queue_flags to u64. Is that okay?

No, besides that would not work with set/test_bit() and friends. Grab
a free bit instead.

-- 
Jens Axboe

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

[Goldwyn Rodrigues]

On 08/09/2017 09:17 PM, Jens Axboe wrote:
> On 08/09/2017 08:07 PM, Goldwyn Rodrigues wrote:
 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).
>
> this doesn't answer my question. So if a bio returns -EAGAIN, part
> of the bio probably already dispatched to disk (if the bio is
> splitted to 2 bios, one returns -EAGAIN, the other one doesn't
> block and dispatch to disk), what will application be going to do?
> I think this is different to other IO errors. FOr other IO errors,
> application will handle the error, while we ask app to retry the
> whole bio here and app doesn't know part of bio is already written
> to disk.

 It is the same as for other I/O errors as well, such as EIO. You do
 not know which bio of all submitted bio's returned the error EIO.
 The application would and should consider the whole I/O as failed.

 The user application does not know of bios, or how it is going to be
 split in the underlying layers. It knows at the system call level.
 In this case, the EAGAIN will be returned to the user for the whole
 I/O not as a part of the I/O. It is up to application to try the I/O
 again with or without RWF_NOWAIT set. In direct I/O, it is bubbled
 out using dio->io_error. You can read about it at the patch header
 for the initial patchset at [1].

 Use case: It is for applications having two threads, a compute
 thread and an I/O thread. It would try to push AIO as much as
 possible in the compute thread using RWF_NOWAIT, and if it fails,
 would pass it on to I/O thread which would perform without
 RWF_NOWAIT. End result if done right is you save on context switches
 and all the synchronization/messaging machinery to perform I/O.

 [1] http://marc.info/?l=linux-block=149789003305876=2
>>>
>>> Yes, I knew the concept, but I didn't see previous patches mentioned
>>> the -EAGAIN actually should be taken as a real IO error. This means a
>>> lot to applications and make the API hard to use. I'm wondering if we
>>> should disable bio split for NOWAIT bio, which will make the -EAGAIN
>>> only mean 'try again'.
>>
>> Don't take it as EAGAIN, but read it as EWOULDBLOCK. Why do you say
>> the API is hard to use? Do you have a case to back it up?
> 
> Because it is hard to use, and potentially suboptimal. Let's say you're
> doing a 1MB write, we hit EWOULDBLOCK for the last split. Do we return a
> short write, or do we return EWOULDBLOCK? If the latter, then that
> really sucks from an API point of view.
> 
>> No, not splitting the bio does not make sense here. I do not see any
>> advantage in it, unless you can present a case otherwise.
> 
> It ties back into the "hard to use" that I do agree with IFF we don't
> return the short write. It's hard for an application to use that
> efficiently, if we write 1MB-128K but get EWOULDBLOCK, the re-write the
> full 1MB from a different context.
> 

It returns the error code only and not short reads/writes. But isn't
that true for all system calls in case of error?

For aio, there are two result fields in io_event out of which one could
be used for error while the other be used for amount of writes/reads
performed. However, only one is used. This will not work with
pread()/pwrite() calls though because of the limitation of return values.

Finally, what if the EWOULDBLOCK is returned for an earlier bio (say
offset 128k) for a 1MB pwrite(), while the rest of the 7 128K are
successful. What short return value should the system call return?

-- 
Goldwyn

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

[Goldwyn Rodrigues]

On 08/09/2017 09:18 PM, Jens Axboe wrote:
> On 08/08/2017 02:36 PM, Jens Axboe wrote:
>> On 08/08/2017 02:32 PM, Shaohua Li wrote:
 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_QUIESCED31 /* queue has been quiesced */
 +#define QUEUE_FLAG_NOWAIT  32 /* stack device driver supports 
 REQ_NOWAIT */
>>
>> Does this work on 32-bit, where sizeof(unsigned long) == 32?
> 
> I didn't get an answer to this one.
> 

Oh, I assumed the question is rhetorical.
No, it will not work on 32-bit. I was planning to change the field
queue_flags to u64. Is that okay?

-- 
Goldwyn

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

[Jens Axboe]

On 08/09/2017 08:07 PM, Goldwyn Rodrigues wrote:
>>> 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).

 this doesn't answer my question. So if a bio returns -EAGAIN, part
 of the bio probably already dispatched to disk (if the bio is
 splitted to 2 bios, one returns -EAGAIN, the other one doesn't
 block and dispatch to disk), what will application be going to do?
 I think this is different to other IO errors. FOr other IO errors,
 application will handle the error, while we ask app to retry the
 whole bio here and app doesn't know part of bio is already written
 to disk.
>>>
>>> It is the same as for other I/O errors as well, such as EIO. You do
>>> not know which bio of all submitted bio's returned the error EIO.
>>> The application would and should consider the whole I/O as failed.
>>>
>>> The user application does not know of bios, or how it is going to be
>>> split in the underlying layers. It knows at the system call level.
>>> In this case, the EAGAIN will be returned to the user for the whole
>>> I/O not as a part of the I/O. It is up to application to try the I/O
>>> again with or without RWF_NOWAIT set. In direct I/O, it is bubbled
>>> out using dio->io_error. You can read about it at the patch header
>>> for the initial patchset at [1].
>>>
>>> Use case: It is for applications having two threads, a compute
>>> thread and an I/O thread. It would try to push AIO as much as
>>> possible in the compute thread using RWF_NOWAIT, and if it fails,
>>> would pass it on to I/O thread which would perform without
>>> RWF_NOWAIT. End result if done right is you save on context switches
>>> and all the synchronization/messaging machinery to perform I/O.
>>>
>>> [1] http://marc.info/?l=linux-block=149789003305876=2
>>
>> Yes, I knew the concept, but I didn't see previous patches mentioned
>> the -EAGAIN actually should be taken as a real IO error. This means a
>> lot to applications and make the API hard to use. I'm wondering if we
>> should disable bio split for NOWAIT bio, which will make the -EAGAIN
>> only mean 'try again'.
> 
> Don't take it as EAGAIN, but read it as EWOULDBLOCK. Why do you say
> the API is hard to use? Do you have a case to back it up?

Because it is hard to use, and potentially suboptimal. Let's say you're
doing a 1MB write, we hit EWOULDBLOCK for the last split. Do we return a
short write, or do we return EWOULDBLOCK? If the latter, then that
really sucks from an API point of view.

> No, not splitting the bio does not make sense here. I do not see any
> advantage in it, unless you can present a case otherwise.

It ties back into the "hard to use" that I do agree with IFF we don't
return the short write. It's hard for an application to use that
efficiently, if we write 1MB-128K but get EWOULDBLOCK, the re-write the
full 1MB from a different context.

-- 
Jens Axboe

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

[Jens Axboe]

On 08/08/2017 02:36 PM, Jens Axboe wrote:
> On 08/08/2017 02:32 PM, Shaohua Li wrote:
>>> 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_QUIESCED31  /* queue has been quiesced */
>>> +#define QUEUE_FLAG_NOWAIT  32  /* stack device driver supports 
>>> REQ_NOWAIT */
> 
> Does this work on 32-bit, where sizeof(unsigned long) == 32?

I didn't get an answer to this one.

-- 
Jens Axboe

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

[Goldwyn Rodrigues]

On 08/09/2017 08:17 PM, Shaohua Li wrote:
> On Wed, Aug 09, 2017 at 05:16:23PM -0500, Goldwyn Rodrigues wrote:
>>
>>
>> On 08/09/2017 03:21 PM, Shaohua Li wrote:
>>> On Wed, Aug 09, 2017 at 10:35:39AM -0500, Goldwyn Rodrigues wrote:


 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 

 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 
 ---
  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_QUIESCED31 /* 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).
>>>
>>> this doesn't answer my question. So if a bio returns -EAGAIN, part of the 
>>> bio
>>> probably already dispatched to disk (if the bio is splitted to 2 bios, one
>>> returns -EAGAIN, the other one doesn't block and dispatch to disk), what 
>>> will
>>> application be going to do? I think this is different to other IO errors. 
>>> FOr
>>> other IO errors, 

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

[Shaohua Li]

On Wed, Aug 09, 2017 at 05:16:23PM -0500, Goldwyn Rodrigues wrote:
> 
> 
> On 08/09/2017 03:21 PM, Shaohua Li wrote:
> > On Wed, Aug 09, 2017 at 10:35:39AM -0500, Goldwyn Rodrigues wrote:
> >>
> >>
> >> 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 
> >>
> >> 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 
> >> ---
> >>  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_QUIESCED31 /* 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).
> > 
> > this doesn't answer my question. So if a bio returns -EAGAIN, part of the 
> > bio
> > probably already dispatched to disk (if the bio is splitted to 2 bios, one
> > returns -EAGAIN, the other one doesn't block and dispatch to disk), what 
> > will
> > application be going to do? I think this is different to other IO errors. 
> > FOr
> > other IO errors, application will handle the error, while we ask 

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

[Goldwyn Rodrigues]

On 08/09/2017 03:21 PM, Shaohua Li wrote:
> On Wed, Aug 09, 2017 at 10:35:39AM -0500, Goldwyn Rodrigues wrote:
>>
>>
>> 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 
>>
>> 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 
>> ---
>>  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_QUIESCED31   /* 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).
> 
> this doesn't answer my question. So if a bio returns -EAGAIN, part of the bio
> probably already dispatched to disk (if the bio is splitted to 2 bios, one
> returns -EAGAIN, the other one doesn't block and dispatch to disk), what will
> application be going to do? I think this is different to other IO errors. FOr
> other IO errors, application will handle the error, while we ask app to retry
> the whole bio here and app doesn't know part of bio is already written to 
> disk.

It is the same as for other I/O errors as well, such as EIO. You do not
know which bio of all submitted bio's returned the error EIO. The
application would and should consider the 

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

[Shaohua Li]

On Wed, Aug 09, 2017 at 10:35:39AM -0500, Goldwyn Rodrigues wrote:
> 
> 
> 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 
> 
>  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 
>  ---
>   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_QUIESCED31   /* 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).

this doesn't answer my question. So if a bio returns -EAGAIN, part of the bio
probably already dispatched to disk (if the bio is splitted to 2 bios, one
returns -EAGAIN, the other one doesn't block and dispatch to disk), what will
application be going to do? I think this is different to other IO errors. FOr
other IO errors, application will handle the error, while we ask app to retry
the whole bio here and app doesn't know part of bio is already written to disk.

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

[Goldwyn Rodrigues]

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 

 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 
 ---
  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_QUIESCED31 /* 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

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

[Shaohua Li]

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 
> >>
> >> 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 
> >> ---
> >>  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_QUIESCED31 /* 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.

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

[Goldwyn Rodrigues]

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 
>>
>> 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 
>> ---
>>  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_QUIESCED31   /* 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?

-- 
Goldwyn

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

[Jens Axboe]

On 08/08/2017 02:32 PM, Shaohua Li wrote:
>> 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_QUIESCED31   /* queue has been quiesced */
>> +#define QUEUE_FLAG_NOWAIT  32   /* stack device driver supports 
>> REQ_NOWAIT */

Does this work on 32-bit, where sizeof(unsigned long) == 32?

-- 
Jens Axboe

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

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues 

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.

Signed-off-by: Goldwyn Rodrigues 
---
 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_QUIESCED31  /* 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)
 
 #define blk_noretry_request(rq) \
((rq)->cmd_flags & (REQ_FAILFAST_DEV|REQ_FAILFAST_TRANSPORT| \
-- 
2.12.3