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, 2 insertions, 2 deletions

diff --git a/drivers/nvme/target/loop.c b/drivers/nvme/target/loop.c
index 57ded6b3ed8a..9aaa70071ae5 100644
--- a/drivers/nvme/target/loop.c
+++ b/drivers/nvme/target/loop.c
@@ -185,13 +185,13 @@ static int nvme_loop_queue_rq(struct blk_mq_hw_ctx *hctx,
 	if (blk_rq_bytes(req)) {
 		iod->sg_table.sgl = iod->first_sgl;
 		ret = sg_alloc_table_chained(&iod->sg_table,
-			req->nr_phys_segments, iod->sg_table.sgl);
+				blk_rq_nr_phys_segments(req),
+				iod->sg_table.sgl);
 		if (ret)
 			return BLK_MQ_RQ_QUEUE_BUSY;
 
 		iod->req.sg = iod->sg_table.sgl;
 		iod->req.sg_cnt = blk_rq_map_sg(req->q, req, iod->sg_table.sgl);
-		BUG_ON(iod->req.sg_cnt > req->nr_phys_segments);
 	}
 
 	blk_mq_start_request(req);