Re: [PATCH 1/5] block: Implement support for WRITE SAME

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Jan 30 2012 at  7:31pm -0500,
Martin K. Petersen <martin.petersen@oracle.com> wrote:

> From: "Martin K. Petersen" <martin.petersen@oracle.com>
> 
> The WRITE SAME command supported on some SCSI devices allows the same
> block to be efficiently replicated throughout a block range. Only a
> single logical block is transferred from the host and the storage device
> writes the same data to all blocks described by the I/O.
> 
> This patch implements support for WRITE SAME in the block layer. The
> blkdev_issue_write_same() function can be used by filesystems and block
> drivers to replicate a buffer across a block range. This can be used to
> efficiently initialize software RAID devices, etc.
> 
> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

...

> diff --git a/block/blk-lib.c b/block/blk-lib.c
> index 2b461b4..19f4754 100644
> --- a/block/blk-lib.c
> +++ b/block/blk-lib.c
> @@ -115,6 +115,85 @@ int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>  EXPORT_SYMBOL(blkdev_issue_discard);
>  
>  /**
> + * blkdev_issue_write_same - queue a write same operation
> + * @bdev:	target blockdev
> + * @sector:	start sector
> + * @nr_sects:	number of sectors to write
> + * @gfp_mask:	memory allocation flags (for bio_alloc)
> + * @buffer:	pointer to buffer containing data to write
> + * @length:	buffer length. Must be equal to bdev logical block size.
> + *
> + * Description:
> + *    Issue a write same request for the sectors in question.
> + */
> +int blkdev_issue_write_same(struct block_device *bdev, sector_t sector,
> +			    sector_t nr_sects, gfp_t gfp_mask, void *buffer,
> +			    unsigned int length)
> +{
> +	DECLARE_COMPLETION_ONSTACK(wait);
> +	struct request_queue *q = bdev_get_queue(bdev);
> +	unsigned int max_write_same_sectors;
> +	struct bio_batch bb;
> +	struct bio *bio;
> +	int ret = 0;
> +
> +	if (!q)
> +		return -ENXIO;
> +
> +	max_write_same_sectors = q->limits.max_write_same_sectors;
> +
> +	if (max_write_same_sectors == 0)
> +		return -EOPNOTSUPP;
> +
> +	BUG_ON(length != bdev_logical_block_size(bdev));

Why require @length to be passed in if we can easily determine the only
correct length, via bdev_logical_block_size, locally?

Mike