block: Introduce __blk_segment_map_sg() helper

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: Rusty Russell <rusty@rustcorp.com.au>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: Shaohua Li <shli@kernel.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Suggested-by: Jens Axboe <axboe@kernel.dk>
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -110,43 +110,28 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
 	return 0;
 }
 
-/*
- * map a request to scatterlist, return number of sg entries setup. Caller
- * must make sure sg can hold rq->nr_phys_segments entries
- */
-int blk_rq_map_sg(struct request_queue *q, struct request *rq,
-		  struct scatterlist *sglist)
+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)
 {
-	struct bio_vec *bvec, *bvprv;
-	struct req_iterator iter;
-	struct scatterlist *sg;
-	int nsegs, cluster;
 
-	nsegs = 0;
-	cluster = blk_queue_cluster(q);
-
-	/*
-	 * for each bio in rq
-	 */
-	bvprv = NULL;
-	sg = NULL;
-	rq_for_each_segment(bvec, rq, iter) {
 	int nbytes = bvec->bv_len;
 
-		if (bvprv && cluster) {
-			if (sg->length + nbytes > queue_max_segment_size(q))
+	if (*bvprv && *cluster) {
+		if ((*sg)->length + nbytes > queue_max_segment_size(q))
 			goto new_segment;
 
-			if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
+		if (!BIOVEC_PHYS_MERGEABLE(*bvprv, bvec))
 			goto new_segment;
-			if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
+		if (!BIOVEC_SEG_BOUNDARY(q, *bvprv, bvec))
 			goto new_segment;
 
-			sg->length += nbytes;
+		(*sg)->length += nbytes;
 	} else {
 new_segment:
-			if (!sg)
-				sg = sglist;
+		if (!*sg)
+			*sg = sglist;
 		else {
 			/*
 			 * If the driver previously mapped a shorter
@@ -158,14 +143,39 @@ int blk_rq_map_sg(struct request_queue *q, struct request *rq,
 			 * termination bit to avoid doing a full
 			 * sg_init_table() in drivers for each command.
 			 */
-				sg->page_link &= ~0x02;
-				sg = sg_next(sg);
+			(*sg)->page_link &= ~0x02;
+			*sg = sg_next(*sg);
 		}
 
-			sg_set_page(sg, bvec->bv_page, nbytes, bvec->bv_offset);
-			nsegs++;
+		sg_set_page(*sg, bvec->bv_page, nbytes, bvec->bv_offset);
+		(*nsegs)++;
 	}
-		bvprv = bvec;
+	*bvprv = bvec;
+}
+
+/*
+ * map a request to scatterlist, return number of sg entries setup. Caller
+ * must make sure sg can hold rq->nr_phys_segments entries
+ */
+int blk_rq_map_sg(struct request_queue *q, struct request *rq,
+		  struct scatterlist *sglist)
+{
+	struct bio_vec *bvec, *bvprv;
+	struct req_iterator iter;
+	struct scatterlist *sg;
+	int nsegs, cluster;
+
+	nsegs = 0;
+	cluster = blk_queue_cluster(q);
+
+	/*
+	 * for each bio in rq
+	 */
+	bvprv = NULL;
+	sg = NULL;
+	rq_for_each_segment(bvec, rq, iter) {
+		__blk_segment_map_sg(q, bvec, sglist, &bvprv, &sg,
+				     &nsegs, &cluster);
 	} /* segments in rq */