Re: [patch] block: clarify documentation for blk_{start|finish}_plug

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Jan 08, 2019 at 04:57:34PM -0500, Jeff Moyer wrote:
> There was some confusion about what these functions did.  Make it clear
> that this is a hint for upper layers to pass to the block layer, and
> that it does not guarantee that I/O will not be submitted between a
> start and finish plug.
> 
> Reported-by: "Darrick J. Wong" <darrick.wong@oracle.com>
> Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index c78042975737..f2732f106a2e 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1683,6 +1683,15 @@ EXPORT_SYMBOL(kblockd_mod_delayed_work_on);
>   * @plug:	The &struct blk_plug that needs to be initialized
>   *
>   * Description:
> + *   blk_start_plug() indicates to the block layer an intent by the caller
> + *   to submit multiple I/O requests in a batch.  The block layer may use
> + *   this hint to defer submitting I/Os from the caller until blk_finish_plug()
> + *   is called.  However, the block layer may choose to submit requests
> + *   before a call to blk_finish_plug() if the number of queued I/Os
> + *   exceeds %BLK_MAX_REQUEST_COUNT, or if the size of the I/O is larger than
> + *   %BLK_PLUG_FLUSH_SIZE.  The queued I/Os may also be submitted early if
> + *   the task schedules (see below).

Makes sense to my poor brain,

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D

>   *   Tracking blk_plug inside the task_struct will help with auto-flushing the
>   *   pending I/O should the task end up blocking between blk_start_plug() and
>   *   blk_finish_plug(). This is important from a performance perspective, but
> @@ -1765,6 +1774,16 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>  		blk_mq_flush_plug_list(plug, from_schedule);
>  }
>  
> +/**
> + * blk_finish_plug - mark the end of a batch of submitted I/O
> + * @plug:	The &struct blk_plug passed to blk_start_plug()
> + *
> + * Description:
> + * Indicate that a batch of I/O submissions is complete.  This function
> + * must be paired with an initial call to blk_start_plug().  The intent
> + * is to allow the block layer to optimize I/O submission.  See the
> + * documentation for blk_start_plug() for more information.
> + */
>  void blk_finish_plug(struct blk_plug *plug)
>  {
>  	if (plug != current->plug)