[PATCH 1/1] virtio-blk: avoid preallocating big SGL for data

@@ -24,6 +24,12 @@
 /* The maximum number of sg elements that fit into a virtqueue */
 #define VIRTIO_BLK_MAX_SG_ELEMS 32768
 
+#ifdef CONFIG_ARCH_NO_SG_CHAIN
+#define VIRTIO_BLK_INLINE_SG_CNT	0
+#else
+#define VIRTIO_BLK_INLINE_SG_CNT	2
+#endif
+
 static int virtblk_queue_count_set(const char *val,
 		const struct kernel_param *kp)
 {

@@ -99,7 +105,7 @@ struct virtio_blk {
 struct virtblk_req {
 	struct virtio_blk_outhdr out_hdr;
 	u8 status;
-	struct scatterlist sg[];
+	struct sg_table sg_table;
 };
 
 static inline blk_status_t virtblk_result(struct virtblk_req *vbr)

@@ -188,6 +194,8 @@ static inline void virtblk_request_done(struct request *req)
 {
 	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
 
+	sg_free_table_chained(&vbr->sg_table, VIRTIO_BLK_INLINE_SG_CNT);
+
 	if (req->rq_flags & RQF_SPECIAL_PAYLOAD) {
 		kfree(page_address(req->special_vec.bv_page) +
 		      req->special_vec.bv_offset);

@@ -291,7 +299,15 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 			return BLK_STS_RESOURCE;
 	}
 
-	num = blk_rq_map_sg(hctx->queue, req, vbr->sg);
+	vbr->sg_table.sgl = (struct scatterlist *)(vbr + 1);
+	err = sg_alloc_table_chained(&vbr->sg_table,
+				     blk_rq_nr_phys_segments(req),
+				     vbr->sg_table.sgl,
+				     VIRTIO_BLK_INLINE_SG_CNT);
+	if (err)
+		return BLK_STS_RESOURCE;
+
+	num = blk_rq_map_sg(hctx->queue, req, vbr->sg_table.sgl);
 	if (num) {
 		if (rq_data_dir(req) == WRITE)
 			vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_OUT);

@@ -300,7 +316,7 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 	}
 
 	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
-	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg, num);
+	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg_table.sgl, num);
 	if (err) {
 		virtqueue_kick(vblk->vqs[qid].vq);
 		/* Don't stop the queue if -ENOMEM: we may have failed to

@@ -309,6 +325,8 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 		if (err == -ENOSPC)
 			blk_mq_stop_hw_queue(hctx);
 		spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
+		sg_free_table_chained(&vbr->sg_table,
+				      VIRTIO_BLK_INLINE_SG_CNT);
 		switch (err) {
 		case -ENOSPC:
 			return BLK_STS_DEV_RESOURCE;

@@ -687,16 +705,6 @@ static const struct attribute_group *virtblk_attr_groups[] = {
 	NULL,
 };
 
-static int virtblk_init_request(struct blk_mq_tag_set *set, struct request *rq,
-		unsigned int hctx_idx, unsigned int numa_node)
-{
-	struct virtio_blk *vblk = set->driver_data;
-	struct virtblk_req *vbr = blk_mq_rq_to_pdu(rq);
-
-	sg_init_table(vbr->sg, vblk->sg_elems);
-	return 0;
-}
-
 static int virtblk_map_queues(struct blk_mq_tag_set *set)
 {
 	struct virtio_blk *vblk = set->driver_data;

@@ -709,7 +717,6 @@ static const struct blk_mq_ops virtio_mq_ops = {
 	.queue_rq	= virtio_queue_rq,
 	.commit_rqs	= virtio_commit_rqs,
 	.complete	= virtblk_request_done,
-	.init_request	= virtblk_init_request,
 	.map_queues	= virtblk_map_queues,
 };
 

@@ -805,7 +812,7 @@ static int virtblk_probe(struct virtio_device *vdev)
 	vblk->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
 	vblk->tag_set.cmd_size =
 		sizeof(struct virtblk_req) +
-		sizeof(struct scatterlist) * sg_elems;
+		sizeof(struct scatterlist) * VIRTIO_BLK_INLINE_SG_CNT;
 	vblk->tag_set.driver_data = vblk;
 	vblk->tag_set.nr_hw_queues = vblk->num_vqs;