Re: [PATCH 03/23] sd: implement REQ_OP_WRITE_ZEROES

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Thu, 2017-03-23 at 10:33 -0400, Christoph Hellwig wrote:
> diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
> index af632e350ab4..b6f70a09a301 100644
> --- a/drivers/scsi/sd.c
> +++ b/drivers/scsi/sd.c
> @@ -748,7 +748,7 @@ static int sd_setup_unmap_cmnd(struct scsi_cmnd *cmd)
>  	return scsi_init_io(cmd);
>  }
>  
> -static int sd_setup_write_same16_cmnd(struct scsi_cmnd *cmd)
> +static int sd_setup_write_same16_cmnd(struct scsi_cmnd *cmd, bool unmap)
>  {
>  	struct scsi_device *sdp = cmd->device;
>  	struct request *rq = cmd->request;
> @@ -765,13 +765,14 @@ static int sd_setup_write_same16_cmnd(struct scsi_cmnd *cmd)
>  
>  	cmd->cmd_len = 16;
>  	cmd->cmnd[0] = WRITE_SAME_16;
> -	cmd->cmnd[1] = 0x8; /* UNMAP */
> +	if (unmap)
> +		cmd->cmnd[1] = 0x8; /* UNMAP */
>  	put_unaligned_be64(sector, &cmd->cmnd[2]);
>  	put_unaligned_be32(nr_sectors, &cmd->cmnd[10]);

Hello Christoph,

A quote from SBC: "An OPTIMAL UNMAP GRANULARITY field set to a non-zero value
indicates the optimal granularity in logical blocks for unmap requests (e.g.,
an UNMAP command or a WRITE SAME (16) command with the UNMAP bit set to one).
An unmap request with a number of logical blocks that is not a multiple of
this value may result in unmap operations on fewer LBAs than requested."

This means that just like the start and end of a discard must be aligned on a
discard_granularity boundary, WRITE SAME commands with the UNMAP bit set must
also respect that granularity. I think this means that either
__blkdev_issue_zeroout() has to be modified such that it rejects unaligned
REQ_OP_WRITE_ZEROES operations or that blk_bio_write_same_split() has to be
modified such that it generates REQ_OP_WRITEs for the unaligned start and tail.

Bart.