[PATCH 6/6] ublk: lift need_map check out of ublk_{,un}map_io()

[Caleb Sander Mateos <csander@purestorage.com>]

ublk_map_io() and ublk_unmap_io() are no-ops for ublk devices that
enable user copy or zero copy. However, the implementation is a bit
convoluted, returning the full request data length and relying on the
caller to check the return value against the request length.
UBLK_F_SHMEM_ZC recently added branches to skip the ublk_{,un}map_io()
call for I/Os using a shared-memory buffer. This is a more logical place
for the need_map check, so move it there from ublk_{,un}map_io().

Checking need_map early also avoids the expensive pointer-chasing for
the ublk_iod_is_shmem_zc() check in __ublk_complete_rq() in the common
case of a ublk device using user copy or zero copy.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
---
 drivers/block/ublk_drv.c | 21 +++++++--------------
 1 file changed, 7 insertions(+), 14 deletions(-)

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 1da45e382253..d4443a43229e 100644
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -1464,19 +1464,15 @@ static inline bool ublk_need_unmap_req(const struct request *req)
 {
 	return blk_rq_has_data(req) &&
 	       (req_op(req) == REQ_OP_READ || req_op(req) == REQ_OP_DRV_IN);
 }
 
-static unsigned int ublk_map_io(const struct ublk_queue *ubq,
-				const struct request *req,
+static unsigned int ublk_map_io(const struct request *req,
 				const struct ublk_io *io)
 {
 	const unsigned int rq_bytes = blk_rq_bytes(req);
 
-	if (!ublk_need_map_io(ubq))
-		return rq_bytes;
-
 	/*
 	 * no zero copy, we delay copy WRITE request data into ublksrv
 	 * context and the big benefit is that pinning pages in current
 	 * context is pretty fast, see ublk_pin_user_pages
 	 */
@@ -1488,19 +1484,15 @@ static unsigned int ublk_map_io(const struct ublk_queue *ubq,
 		return ublk_copy_user_pages(req, 0, &iter, dir);
 	}
 	return rq_bytes;
 }
 
-static unsigned int ublk_unmap_io(bool need_map,
-		const struct request *req,
+static unsigned int ublk_unmap_io(const struct request *req,
 		const struct ublk_io *io)
 {
 	const unsigned int rq_bytes = blk_rq_bytes(req);
 
-	if (!need_map)
-		return rq_bytes;
-
 	if (ublk_need_unmap_req(req)) {
 		struct iov_iter iter;
 		const int dir = ITER_SOURCE;
 
 		import_ubuf(dir, u64_to_user_ptr(io->buf.addr), io->res, &iter);
@@ -1591,15 +1583,16 @@ static inline void __ublk_complete_rq(struct request *req, struct ublk_io *io,
 	if (req_op(req) != REQ_OP_READ && req_op(req) != REQ_OP_WRITE &&
 	    req_op(req) != REQ_OP_DRV_IN)
 		goto exit;
 
 	/* shmem zero copy: no data to unmap, pages already shared */
-	if (ublk_iod_is_shmem_zc(req->mq_hctx->driver_data, req->tag))
+	if (!need_map ||
+	    ublk_iod_is_shmem_zc(req->mq_hctx->driver_data, req->tag))
 		goto exit;
 
 	/* for READ request, writing data in iod->addr to rq buffers */
-	unmapped_bytes = ublk_unmap_io(need_map, req, io);
+	unmapped_bytes = ublk_unmap_io(req, io);
 
 	/*
 	 * Extremely impossible since we got data filled in just before
 	 *
 	 * Re-read simply for this unlikely case.
@@ -1755,14 +1748,14 @@ static bool ublk_start_io(const struct ublk_queue *ubq, struct request *req,
 			  struct ublk_io *io)
 {
 	unsigned mapped_bytes;
 
 	/* shmem zero copy: skip data copy, pages already shared */
-	if (ublk_iod_is_shmem_zc(ubq, req->tag))
+	if (!ublk_need_map_io(ubq) || ublk_iod_is_shmem_zc(ubq, req->tag))
 		return true;
 
-	mapped_bytes = ublk_map_io(ubq, req, io);
+	mapped_bytes = ublk_map_io(req, io);
 
 	/* partially mapped, update io descriptor */
 	if (unlikely(mapped_bytes != blk_rq_bytes(req))) {
 		/*
 		 * Nothing mapped, retry until we succeed.
-- 
2.54.0