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>

1 files changed, 5 insertions, 8 deletions

diff --git a/drivers/nvme/host/rdma.c b/drivers/nvme/host/rdma.c
index b037d0cb2a7e..251101bf982f 100644
--- a/drivers/nvme/host/rdma.c
+++ b/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);