static int blkif_queue_rw_req()

in xen-blkfront.c [696:853]

• 109 lines of code
• 24 McCabe index (conditional complexity)

static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *rinfo)
{
	struct blkfront_info *info = rinfo->dev_info;
	struct blkif_request *ring_req, *extra_ring_req = NULL;
	struct blkif_request *final_ring_req, *final_extra_ring_req = NULL;
	unsigned long id, extra_id = NO_ASSOCIATED_ID;
	bool require_extra_req = false;
	int i;
	struct setup_rw_req setup = {
		.grant_idx = 0,
		.segments = NULL,
		.rinfo = rinfo,
		.need_copy = rq_data_dir(req) && info->feature_persistent,
	};

	/*
	 * Used to store if we are able to queue the request by just using
	 * existing persistent grants, or if we have to get new grants,
	 * as there are not sufficiently many free.
	 */
	bool new_persistent_gnts = false;
	struct scatterlist *sg;
	int num_sg, max_grefs, num_grant;

	max_grefs = req->nr_phys_segments * GRANTS_PER_PSEG;
	if (max_grefs > BLKIF_MAX_SEGMENTS_PER_REQUEST)
		/*
		 * If we are using indirect segments we need to account
		 * for the indirect grefs used in the request.
		 */
		max_grefs += INDIRECT_GREFS(max_grefs);

	/* Check if we have enough persistent grants to allocate a requests */
	if (rinfo->persistent_gnts_c < max_grefs) {
		new_persistent_gnts = true;

		if (gnttab_alloc_grant_references(
		    max_grefs - rinfo->persistent_gnts_c,
		    &setup.gref_head) < 0) {
			gnttab_request_free_callback(
				&rinfo->callback,
				blkif_restart_queue_callback,
				rinfo,
				max_grefs - rinfo->persistent_gnts_c);
			return 1;
		}
	}

	/* Fill out a communications ring structure. */
	id = blkif_ring_get_request(rinfo, req, &final_ring_req);
	ring_req = &rinfo->shadow[id].req;

	num_sg = blk_rq_map_sg(req->q, req, rinfo->shadow[id].sg);
	num_grant = 0;
	/* Calculate the number of grant used */
	for_each_sg(rinfo->shadow[id].sg, sg, num_sg, i)
	       num_grant += gnttab_count_grant(sg->offset, sg->length);

	require_extra_req = info->max_indirect_segments == 0 &&
		num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST;
	BUG_ON(!HAS_EXTRA_REQ && require_extra_req);

	rinfo->shadow[id].num_sg = num_sg;
	if (num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST &&
	    likely(!require_extra_req)) {
		/*
		 * The indirect operation can only be a BLKIF_OP_READ or
		 * BLKIF_OP_WRITE
		 */
		BUG_ON(req_op(req) == REQ_OP_FLUSH || req->cmd_flags & REQ_FUA);
		ring_req->operation = BLKIF_OP_INDIRECT;
		ring_req->u.indirect.indirect_op = rq_data_dir(req) ?
			BLKIF_OP_WRITE : BLKIF_OP_READ;
		ring_req->u.indirect.sector_number = (blkif_sector_t)blk_rq_pos(req);
		ring_req->u.indirect.handle = info->handle;
		ring_req->u.indirect.nr_segments = num_grant;
	} else {
		ring_req->u.rw.sector_number = (blkif_sector_t)blk_rq_pos(req);
		ring_req->u.rw.handle = info->handle;
		ring_req->operation = rq_data_dir(req) ?
			BLKIF_OP_WRITE : BLKIF_OP_READ;
		if (req_op(req) == REQ_OP_FLUSH || req->cmd_flags & REQ_FUA) {
			/*
			 * Ideally we can do an unordered flush-to-disk.
			 * In case the backend onlysupports barriers, use that.
			 * A barrier request a superset of FUA, so we can
			 * implement it the same way.  (It's also a FLUSH+FUA,
			 * since it is guaranteed ordered WRT previous writes.)
			 */
			if (info->feature_flush && info->feature_fua)
				ring_req->operation =
					BLKIF_OP_WRITE_BARRIER;
			else if (info->feature_flush)
				ring_req->operation =
					BLKIF_OP_FLUSH_DISKCACHE;
			else
				ring_req->operation = 0;
		}
		ring_req->u.rw.nr_segments = num_grant;
		if (unlikely(require_extra_req)) {
			extra_id = blkif_ring_get_request(rinfo, req,
							  &final_extra_ring_req);
			extra_ring_req = &rinfo->shadow[extra_id].req;

			/*
			 * Only the first request contains the scatter-gather
			 * list.
			 */
			rinfo->shadow[extra_id].num_sg = 0;

			blkif_setup_extra_req(ring_req, extra_ring_req);

			/* Link the 2 requests together */
			rinfo->shadow[extra_id].associated_id = id;
			rinfo->shadow[id].associated_id = extra_id;
		}
	}

	setup.ring_req = ring_req;
	setup.id = id;

	setup.require_extra_req = require_extra_req;
	if (unlikely(require_extra_req))
		setup.extra_ring_req = extra_ring_req;

	for_each_sg(rinfo->shadow[id].sg, sg, num_sg, i) {
		BUG_ON(sg->offset + sg->length > PAGE_SIZE);

		if (setup.need_copy) {
			setup.bvec_off = sg->offset;
			setup.bvec_data = kmap_atomic(sg_page(sg));
		}

		gnttab_foreach_grant_in_range(sg_page(sg),
					      sg->offset,
					      sg->length,
					      blkif_setup_rw_req_grant,
					      &setup);

		if (setup.need_copy)
			kunmap_atomic(setup.bvec_data);
	}
	if (setup.segments)
		kunmap_atomic(setup.segments);

	/* Copy request(s) to the ring page. */
	*final_ring_req = *ring_req;
	rinfo->shadow[id].status = REQ_WAITING;
	if (unlikely(require_extra_req)) {
		*final_extra_ring_req = *extra_ring_req;
		rinfo->shadow[extra_id].status = REQ_WAITING;
	}

	if (new_persistent_gnts)
		gnttab_free_grant_references(setup.gref_head);

	return 0;
}