block: improve handling of the magic discard payload

Instead of allocating a single unused biovec for discard requests, send
them down without any payload.  Instead we allow the driver to add a
"special" payload using a biovec embedded into struct request (unioned
over other fields never used while in the driver), and overloading
the number of segments for this case.

This has a couple of advantages:

 - we don't have to allocate the bio_vec
 - the amount of special casing for discard requests in the block
   layer is significantly reduced
 - using this same scheme for other request types is trivial,
   which will be important for implementing the new WRITE_ZEROES
   op on devices where it actually requires a payload (e.g. SCSI)
 - we can get rid of playing games with the request length, as
   we'll never touch it and completions will work just fine
 - it will allow us to support ranged discard operations in the
   future by merging non-contiguous discard bios into a single
   request
 - last but not least it removes a lot of code

This patch is the common base for my WIP series for ranges discards and to
remove discard_zeroes_data in favor of always using REQ_OP_WRITE_ZEROES,
so it would be good to get it in quickly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/nvme/host/core.c

@@ -239,8 +239,6 @@ static inline int nvme_setup_discard(struct nvme_ns *ns, struct request *req,
 		struct nvme_command *cmnd)
 {
 	struct nvme_dsm_range *range;
-	struct page *page;
-	int offset;
 	unsigned int nr_bytes = blk_rq_bytes(req);
 
 	range = kmalloc(sizeof(*range), GFP_ATOMIC);
@@ -257,17 +255,10 @@ static inline int nvme_setup_discard(struct nvme_ns *ns, struct request *req,
 	cmnd->dsm.nr = 0;
 	cmnd->dsm.attributes = cpu_to_le32(NVME_DSMGMT_AD);
 
-	req->completion_data = range;
-	page = virt_to_page(range);
-	offset = offset_in_page(range);
-	blk_add_request_payload(req, page, offset, sizeof(*range));
-
-	/*
-	 * we set __data_len back to the size of the area to be discarded
-	 * on disk. This allows us to report completion on the full amount
-	 * of blocks described by the request.
-	 */
-	req->__data_len = nr_bytes;
+	req->special_vec.bv_page = virt_to_page(range);
+	req->special_vec.bv_offset = offset_in_page(range);
+	req->special_vec.bv_len = sizeof(*range);
+	req->rq_flags |= RQF_SPECIAL_PAYLOAD;
 
 	return BLK_MQ_RQ_QUEUE_OK;
 }

drivers/nvme/host/nvme.h

@@ -236,8 +236,10 @@ static inline unsigned nvme_map_len(struct request *rq)
 
 static inline void nvme_cleanup_cmd(struct request *req)
 {
-	if (req_op(req) == REQ_OP_DISCARD)
-		kfree(req->completion_data);
+	if (req->rq_flags & RQF_SPECIAL_PAYLOAD) {
+		kfree(page_address(req->special_vec.bv_page) +
+		      req->special_vec.bv_offset);
+	}
 }
 
 static inline int nvme_error_status(u16 status)

drivers/nvme/host/pci.c

@@ -302,14 +302,14 @@ static void __nvme_submit_cmd(struct nvme_queue *nvmeq,
 static __le64 **iod_list(struct request *req)
 {
 	struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
-	return (__le64 **)(iod->sg + req->nr_phys_segments);
+	return (__le64 **)(iod->sg + blk_rq_nr_phys_segments(req));
 }
 
 static int nvme_init_iod(struct request *rq, unsigned size,
 		struct nvme_dev *dev)
 {
 	struct nvme_iod *iod = blk_mq_rq_to_pdu(rq);
-	int nseg = rq->nr_phys_segments;
+	int nseg = blk_rq_nr_phys_segments(rq);
 
 	if (nseg > NVME_INT_PAGES || size > NVME_INT_BYTES(dev)) {
 		iod->sg = kmalloc(nvme_iod_alloc_size(dev, size, nseg), GFP_ATOMIC);
@@ -339,8 +339,6 @@ static void nvme_free_iod(struct nvme_dev *dev, struct request *req)
 	__le64 **list = iod_list(req);
 	dma_addr_t prp_dma = iod->first_dma;
 
-	nvme_cleanup_cmd(req);
-
 	if (iod->npages == 0)
 		dma_pool_free(dev->prp_small_pool, list[0], prp_dma);
 	for (i = 0; i < iod->npages; i++) {
@@ -510,7 +508,7 @@ static int nvme_map_data(struct nvme_dev *dev, struct request *req,
 			DMA_TO_DEVICE : DMA_FROM_DEVICE;
 	int ret = BLK_MQ_RQ_QUEUE_ERROR;
 
-	sg_init_table(iod->sg, req->nr_phys_segments);
+	sg_init_table(iod->sg, blk_rq_nr_phys_segments(req));
 	iod->nents = blk_rq_map_sg(q, req, iod->sg);
 	if (!iod->nents)
 		goto out;
@@ -566,6 +564,7 @@ static void nvme_unmap_data(struct nvme_dev *dev, struct request *req)
 		}
 	}
 
+	nvme_cleanup_cmd(req);
 	nvme_free_iod(dev, req);
 }
 
@@ -596,20 +595,20 @@ static int nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
 		}
 	}
 
-	map_len = nvme_map_len(req);
-	ret = nvme_init_iod(req, map_len, dev);
+	ret = nvme_setup_cmd(ns, req, &cmnd);
 	if (ret != BLK_MQ_RQ_QUEUE_OK)
 		return ret;
 
-	ret = nvme_setup_cmd(ns, req, &cmnd);
+	map_len = nvme_map_len(req);
+	ret = nvme_init_iod(req, map_len, dev);
 	if (ret != BLK_MQ_RQ_QUEUE_OK)
-		goto out;
+		goto out_free_cmd;
 
-	if (req->nr_phys_segments)
+	if (blk_rq_nr_phys_segments(req))
 		ret = nvme_map_data(dev, req, map_len, &cmnd);
 
 	if (ret != BLK_MQ_RQ_QUEUE_OK)
-		goto out;
+		goto out_cleanup_iod;
 
 	blk_mq_start_request(req);
 
@@ -620,14 +619,16 @@ static int nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
 		else
 			ret = BLK_MQ_RQ_QUEUE_ERROR;
 		spin_unlock_irq(&nvmeq->q_lock);
-		goto out;
+		goto out_cleanup_iod;
 	}
 	__nvme_submit_cmd(nvmeq, &cmnd);
 	nvme_process_cq(nvmeq);
 	spin_unlock_irq(&nvmeq->q_lock);
 	return BLK_MQ_RQ_QUEUE_OK;
-out:
+out_cleanup_iod:
 	nvme_free_iod(dev, req);
+out_free_cmd:
+	nvme_cleanup_cmd(req);
 	return ret;
 }
 

drivers/nvme/host/rdma.c

@@ -952,8 +952,7 @@ static int nvme_rdma_map_data(struct nvme_rdma_queue *queue,
 	struct nvme_rdma_request *req = blk_mq_rq_to_pdu(rq);
 	struct nvme_rdma_device *dev = queue->device;
 	struct ib_device *ibdev = dev->dev;
-	int nents, count;
-	int ret;
+	int count, ret;
 
 	req->num_sge = 1;
 	req->inline_data = false;
@@ -965,16 +964,14 @@ static int nvme_rdma_map_data(struct nvme_rdma_queue *queue,
 		return nvme_rdma_set_sg_null(c);
 
 	req->sg_table.sgl = req->first_sgl;
-	ret = sg_alloc_table_chained(&req->sg_table, rq->nr_phys_segments,
-				req->sg_table.sgl);
+	ret = sg_alloc_table_chained(&req->sg_table,
+			blk_rq_nr_phys_segments(rq), req->sg_table.sgl);
 	if (ret)
 		return -ENOMEM;
 
-	nents = blk_rq_map_sg(rq->q, rq, req->sg_table.sgl);
-	BUG_ON(nents > rq->nr_phys_segments);
-	req->nents = nents;
+	req->nents = blk_rq_map_sg(rq->q, rq, req->sg_table.sgl);
 
-	count = ib_dma_map_sg(ibdev, req->sg_table.sgl, nents,
+	count = ib_dma_map_sg(ibdev, req->sg_table.sgl, req->nents,
 		    rq_data_dir(rq) == WRITE ? DMA_TO_DEVICE : DMA_FROM_DEVICE);
 	if (unlikely(count <= 0)) {
 		sg_free_table_chained(&req->sg_table, true);