On 10/05/2011 09:54 PM, Christoph Hellwig wrote:
> Add a helper to map a bio to a scatterlist, modelled after blk_rq_map_sg.
> This helper is useful for any driver that wants to create a scatterlist
> from its ->make_request method.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
>
I have some questions.
- Could we later use this bio_map_sg() to implement blk_rq_map_sg() and
remove some duplicated code?
- Don't you need to support a chained bio (bio->next != NULL)? Because
I did not see any looping in the last patch
[PATCH 5/5] virtio-blk: implement ->make_request
Or is it that ->make_request() is a single bio at a time?
If so could we benefit from both bio-chaining and sg-chaning to
make bigger IOs?
Thanks
Boaz
> Index: linux-2.6/block/blk-merge.c
> ===================================================================
> --- linux-2.6.orig/block/blk-merge.c 2011-10-04 11:49:32.857014742 -0400
> +++ linux-2.6/block/blk-merge.c 2011-10-04 13:37:51.305630951 -0400
> @@ -199,6 +199,69 @@ new_segment:
> }
> EXPORT_SYMBOL(blk_rq_map_sg);
>
> +/*
> + * map a bio to a scatterlist, return number of sg entries setup. Caller
> + * must make sure sg can hold bio->bi_phys_segments entries
> + */
> +int bio_map_sg(struct request_queue *q, struct bio *bio,
> + struct scatterlist *sglist)
> +{
> + struct bio_vec *bvec, *bvprv;
> + struct scatterlist *sg;
> + int nsegs, cluster;
> + unsigned long i;
> +
> + nsegs = 0;
> + cluster = blk_queue_cluster(q);
> +
> + bvprv = NULL;
> + sg = NULL;
> + bio_for_each_segment(bvec, bio, i) {
> + 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;
> + } /* segments in bio */
> +
> + if (sg)
> + sg_mark_end(sg);
> +
> + BUG_ON(bio->bi_phys_segments && nsegs > bio->bi_phys_segments);
> + return nsegs;
> +}
> +EXPORT_SYMBOL(bio_map_sg);
> +
> static inline int ll_new_hw_segment(struct request_queue *q,
> struct request *req,
> struct bio *bio)
> Index: linux-2.6/include/linux/blkdev.h
> ===================================================================
> --- linux-2.6.orig/include/linux/blkdev.h 2011-10-04 13:37:13.216148915 -0400
> +++ linux-2.6/include/linux/blkdev.h 2011-10-04 13:37:51.317613617 -0400
> @@ -854,6 +854,8 @@ extern void blk_queue_flush_queueable(st
> extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
>
> extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
> +extern int bio_map_sg(struct request_queue *q, struct bio *bio,
> + struct scatterlist *sglist);
> extern void blk_dump_rq_flags(struct request *, char *);
> extern long nr_blockdev_pages(void);
>
>
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