Re: [PATCH 4/8] iomap: completed polled IO inline

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Jul 20, 2023 at 12:13:06PM -0600, Jens Axboe wrote:
> Polled IO is only allowed for conditions where task completion is safe
> anyway, so we can always complete it inline. This cannot easily be
> checked with a submission side flag, as the block layer may clear the
> polled flag and turn it into a regular IO instead. Hence we need to
> check this at completion time. If REQ_POLLED is still set, then we know
> that this IO was successfully polled, and is completing in task context.
> 
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  fs/iomap/direct-io.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 9f97d0d03724..c3ea1839628f 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -173,9 +173,19 @@ void iomap_dio_bio_end_io(struct bio *bio)
>  	}
>  
>  	/*
> -	 * Flagged with IOMAP_DIO_INLINE_COMP, we can complete it inline
> +	 * Flagged with IOMAP_DIO_INLINE_COMP, we can complete it inline.
> +	 * Ditto for polled requests - if the flag is still at completion
> +	 * time, then we know the request was actually polled and completion

Glad you added the comment here pointing out that REQ_POLLED must
*still* be set after the bio has been executed, because that was the
only question I had about this patch.

> +	 * is called from the task itself. This is why we need to check it
> +	 * here rather than flag it at issue time.
>  	 */
> -	if (dio->flags & IOMAP_DIO_INLINE_COMP) {
> +	if ((dio->flags & IOMAP_DIO_INLINE_COMP) || (bio->bi_opf & REQ_POLLED)) {
> +		/*
> +		 * For polled IO, we need to clear ->private as it points to
> +		 * the bio being polled for. The completion side uses it to
> +		 * know if a given request has been found yet or not. For
> +		 * non-polled IO, ->private isn't applicable.

Thanks for the clarifying note here too.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D


> +		 */
>  		WRITE_ONCE(iocb->private, NULL);
>  		iomap_dio_complete_work(&dio->aio.work);
>  		goto release_bio;
> -- 
> 2.40.1
>