Re: [PATCH] sd: Optimal I/O size is in bytes, not sectors

[Fam Zheng <famz@redhat.com>]

On Wed, 01/20 11:01, Martin K. Petersen wrote:
> Commit ca369d51b3e1 ("block/sd: Fix device-imposed transfer length
> limits") accidentally switched optimal I/O size reporting from bytes to
> block layer sectors.
> 
> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
> Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  drivers/scsi/sd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index 4e08d1cd704d..ec163d08f6c3 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -2893,7 +2893,7 @@ static int sd_revalidate_disk(struct gendisk *disk)
>  	    sdkp->opt_xfer_blocks <= SD_DEF_XFER_BLOCKS &&
>  	    sdkp->opt_xfer_blocks * sdp->sector_size >= PAGE_CACHE_SIZE)
>  		rw_max = q->limits.io_opt =
> -			logical_to_sectors(sdp, sdkp->opt_xfer_blocks);
> +			sdkp->opt_xfer_blocks * sdp->sector_size;

Hi Martin,

This looks wrong to me, maybe I'm missing the obvious? Here
sdkp->opt_xfer_blocks is in block size unit, and rw_max is in byte unit.

Following is:

	else
		rw_max = BLK_DEF_MAX_SECTORS;

Which seems in sector unit, and is already making above change suspicious, and
further down:

	/* Combine with controller limits */
	q->limits.max_sectors = min(rw_max, queue_max_hw_sectors(q));

looks like a unit mismatch to me.  IIUC q->limits.max_sectors _is_ in sector
unit, similar to queue_max_hw_sectors().

Is the error reported by Christian fixed just because we are setting an
incorrect high max?

(I noticed this when I see I/O error because a virtio-scsi guest starts to
issue large reads that are rejected by host device.)

Fam