linux-kernel - Re: [PATCH 1/3] block: Introduce __blk_segment_map

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Date:	Mon, 18 Jun 2012 14:31:03 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Asias He <asias@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...nel.org>
Subject: Re: [PATCH 1/3] block: Introduce __blk_segment_map_sg() helper

Hello, Asias.

On Mon, Jun 18, 2012 at 02:53:08PM +0800, Asias He wrote:
> Split the mapping code in blk_rq_map_sg() to a helper
> __blk_segment_map_sg(), so that other mapping function, e.g.
> blk_bio_map_sg(), can share the code.
> 
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Shaohua Li <shli@...nel.org>
> Cc: linux-kernel@...r.kernel.org
> Suggested-by: Tejun Heo <tj@...nel.org>
> Suggested-by: Jens Axboe <axboe@...nel.dk>
> Signed-off-by: Asias He <asias@...hat.com>
> ---
>  block/blk-merge.c |   80 ++++++++++++++++++++++++++++++-----------------------
>  1 file changed, 45 insertions(+), 35 deletions(-)
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 160035f..576b68e 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -110,6 +110,49 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
>  	return 0;
>  }
>  
> +static void
> +__blk_segment_map_sg(struct request_queue *q, struct bio_vec *bvec,
> +		     struct scatterlist *sglist, struct bio_vec **bvprv,
> +		     struct scatterlist **sg, int *nsegs, int *cluster)
> +{
> +
> +	int nbytes = bvec->bv_len;
> +
> +	if (*bvprv && *cluster) {
> +		if ((*sg)->length + nbytes > queue_max_segment_size(q))
> +			goto new_segment;
> +
> +		if (!BIOVEC_PHYS_MERGEABLE(*bvprv, bvec))
> +			goto new_segment;
> +		if (!BIOVEC_SEG_BOUNDARY(q, *bvprv, bvec))
> +			goto new_segment;
> +
> +		(*sg)->length += nbytes;
> +	} else {
> +new_segment:
> +		if (!*sg)
> +			*sg = sglist;
> +		else {
> +			/*
> +			 * If the driver previously mapped a shorter
> +			 * list, we could see a termination bit
> +			 * prematurely unless it fully inits the sg
> +			 * table on each mapping. We KNOW that there
> +			 * must be more entries here or the driver
> +			 * would be buggy, so force clear the
> +			 * termination bit to avoid doing a full
> +			 * sg_init_table() in drivers for each command.
> +			 */
> +			(*sg)->page_link &= ~0x02;
> +			*sg = sg_next(*sg);
> +		}
> +
> +		sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
> +		(*nsegs)++;
> +	}
> +	*bvprv = bvec;
> +}

I *hope* this is a bit prettier.  e.g. Do we really need to pass in
@sglist and keep using "goto new_segment"?

Thanks.

-- 
tejun
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