Re: [PATCH 6/8] fs: add IOCB flags related to passing back dio completions

[Jens Axboe <axboe@kernel.dk>]

On 7/21/23 9:48?AM, Darrick J. Wong wrote:
> On Thu, Jul 20, 2023 at 12:13:08PM -0600, Jens Axboe wrote:
>> Async dio completions generally happen from hard/soft IRQ context, which
>> means that users like iomap may need to defer some of the completion
>> handling to a workqueue. This is less efficient than having the original
>> issuer handle it, like we do for sync IO, and it adds latency to the
>> completions.
>>
>> Add IOCB_DIO_DEFER, which the issuer can set if it is able to safely
>> punt these completions to a safe context. If the dio handler is aware
>> of this flag, assign a callback handler in kiocb->dio_complete and
>> associated data io kiocb->private. The issuer will then call this handler
>> with that data from task context.
>>
>> No functional changes in this patch.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>  include/linux/fs.h | 34 ++++++++++++++++++++++++++++++++--
>>  1 file changed, 32 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/fs.h b/include/linux/fs.h
>> index 6867512907d6..2c589418a078 100644
>> --- a/include/linux/fs.h
>> +++ b/include/linux/fs.h
>> @@ -338,6 +338,20 @@ enum rw_hint {
>>  #define IOCB_NOIO		(1 << 20)
>>  /* can use bio alloc cache */
>>  #define IOCB_ALLOC_CACHE	(1 << 21)
>> +/*
>> + * IOCB_DIO_DEFER can be set by the iocb owner, to indicate that the
>> + * iocb completion can be passed back to the owner for execution from a safe
>> + * context rather than needing to be punted through a workqueue. If this
>> + * flag is set, the completion handling may set iocb->dio_complete to a
>> + * handler, which the issuer will then call from task context to complete
>> + * the processing of the iocb. iocb->private should then also be set to
>> + * the argument being passed to this handler. Note that while this provides
> 
> Who should be setting iocb->private?  Can I suggest rewording this to:
> 
> "If this flag is set, the bio completion handling may set
> iocb->dio_complete to a handler function and iocb->private to context
> information for that handler.  The issuer should call the handler with
> that context information from task context to complete the processing of
> the iocb."
> 
> Assuming I've understood what this does from the next patch? :)

Yep this is definitely better - thanks, I'll update it!

>> + * a task context for the dio_complete() callback, it should only be used
>> + * on the completion side for non-IO generating completions. It's fine to
>> + * call blocking functions from this callback, but they should not wait for
>> + * unrelated IO (like cache flushing, new IO generation, etc).
>> + */
>> +#define IOCB_DIO_DEFER		(1 << 22)
> 
> Sorry to nitpick names here, but "defer" feels a little vague to me.
> Defer what?  And to whom?
> 
> This flag means "defer iocb completion to the caller", right?  If so,
> wouldn't this be better named IOCB_DIO_CALLER_COMP?

That is probably better indeed. Naming is hard! CALLER_COMP or
ISSUER_COMP would be better and more descriptive. I'll go with your
suggestion.

>>  /* for use in trace events */
>>  #define TRACE_IOCB_STRINGS \
>> @@ -351,7 +365,8 @@ enum rw_hint {
>>  	{ IOCB_WRITE,		"WRITE" }, \
>>  	{ IOCB_WAITQ,		"WAITQ" }, \
>>  	{ IOCB_NOIO,		"NOIO" }, \
>> -	{ IOCB_ALLOC_CACHE,	"ALLOC_CACHE" }
>> +	{ IOCB_ALLOC_CACHE,	"ALLOC_CACHE" }, \
>> +	{ IOCB_DIO_DEFER,	"DIO_DEFER" }
>>  
>>  struct kiocb {
>>  	struct file		*ki_filp;
>> @@ -360,7 +375,22 @@ struct kiocb {
>>  	void			*private;
>>  	int			ki_flags;
>>  	u16			ki_ioprio; /* See linux/ioprio.h */
>> -	struct wait_page_queue	*ki_waitq; /* for async buffered IO */
>> +	union {
>> +		/*
>> +		 * Only used for async buffered reads, where it denotes the
>> +		 * page waitqueue associated with completing the read. Valid
>> +		 * IFF IOCB_WAITQ is set.
>> +		 */
>> +		struct wait_page_queue	*ki_waitq;
>> +		/*
>> +		 * Can be used for O_DIRECT IO, where the completion handling
>> +		 * is punted back to the issuer of the IO. May only be set
>> +		 * if IOCB_DIO_DEFER is set by the issuer, and the issuer must
>> +		 * then check for presence of this handler when ki_complete is
>> +		 * invoked.
> 
> Might want to reiterate in the comment that kiocb.private should be
> passed as @data.

OK, will do.

-- 
Jens Axboe