dm: prepare for allocating blk-mq clone requests in target

For blk-mq request-based DM the responsibility of allocating a cloned
request will be transfered from DM core to the target type.

To prepare for conditionally using this new model the original
request's 'special' now points to the dm_rq_target_io because the
clone is allocated later in the block layer rather than in DM core.

Signed-off-by: Keith Busch <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm.c

···
  * the md may be freed in dm_put() at the end of this function.
  * Or do dm_get() before calling this function and dm_put() later.
  */
-static void rq_completed(struct mapped_device *md, int rw, int run_queue)
+static void rq_completed(struct mapped_device *md, int rw, bool run_queue)
 {
 	atomic_dec(&md->pending[rw]);
 
···
 
 /*
  * Complete the clone and the original request.
- * Must be called without queue lock.
+ * Must be called without clone's queue lock held,
+ * see end_clone_request() for more details.
  */
 static void dm_end_request(struct request *clone, int error)
 {
···
 
 static void dm_unprep_request(struct request *rq)
 {
-	struct request *clone = rq->special;
+	struct dm_rq_target_io *tio = rq->special;
+	struct request *clone = tio->clone;
 
 	rq->special = NULL;
 	rq->cmd_flags &= ~REQ_DONTPREP;
···
 /*
  * Requeue the original request of a clone.
  */
-static void dm_requeue_unmapped_request(struct request *clone)
+static void dm_requeue_unmapped_original_request(struct mapped_device *md,
+						 struct request *rq)
 {
-	int rw = rq_data_dir(clone);
-	struct dm_rq_target_io *tio = clone->end_io_data;
-	struct mapped_device *md = tio->md;
-	struct request *rq = tio->orig;
+	int rw = rq_data_dir(rq);
 	struct request_queue *q = rq->q;
 	unsigned long flags;
 
···
 	blk_requeue_request(q, rq);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 
-	rq_completed(md, rw, 0);
+	rq_completed(md, rw, false);
+}
+
+static void dm_requeue_unmapped_request(struct request *clone)
+{
+	struct dm_rq_target_io *tio = clone->end_io_data;
+
+	dm_requeue_unmapped_original_request(tio->md, tio->orig);
 }
 
 static void __stop_queue(struct request_queue *q)
···
 static void dm_softirq_done(struct request *rq)
 {
 	bool mapped = true;
-	struct request *clone = rq->completion_data;
-	struct dm_rq_target_io *tio = clone->end_io_data;
+	struct dm_rq_target_io *tio = rq->special;
+	struct request *clone = tio->clone;
 
 	if (rq->cmd_flags & REQ_FAILED)
 		mapped = false;
···
  * Complete the clone and the original request with the error status
  * through softirq context.
  */
-static void dm_complete_request(struct request *clone, int error)
+static void dm_complete_request(struct request *rq, int error)
 {
-	struct dm_rq_target_io *tio = clone->end_io_data;
-	struct request *rq = tio->orig;
+	struct dm_rq_target_io *tio = rq->special;
 
 	tio->error = error;
-	rq->completion_data = clone;
 	blk_complete_request(rq);
 }
 
···
  * Target's rq_end_io() function isn't called.
  * This may be used when the target's map_rq() function fails.
  */
-static void dm_kill_unmapped_request(struct request *clone, int error)
+static void dm_kill_unmapped_request(struct request *rq, int error)
 {
-	struct dm_rq_target_io *tio = clone->end_io_data;
-	struct request *rq = tio->orig;
-
 	rq->cmd_flags |= REQ_FAILED;
-	dm_complete_request(clone, error);
+	dm_complete_request(rq, error);
 }
 
 /*
- * Called with the queue lock held
+ * Called with the clone's queue lock held
  */
 static void end_clone_request(struct request *clone, int error)
 {
+	struct dm_rq_target_io *tio = clone->end_io_data;
+
 	/*
 	 * For just cleaning up the information of the queue in which
 	 * the clone was dispatched.
···
 
 	/*
 	 * Actual request completion is done in a softirq context which doesn't
-	 * hold the queue lock.  Otherwise, deadlock could occur because:
+	 * hold the clone's queue lock.  Otherwise, deadlock could occur because:
 	 *     - another request may be submitted by the upper level driver
 	 *       of the stacking during the completion
 	 *     - the submission which requires queue lock may be done
-	 *       against this queue
+	 *       against this clone's queue
 	 */
-	dm_complete_request(clone, error);
+	dm_complete_request(tio->orig, error);
 }
 
 /*
···
 		_dm_request(q, bio);
 }
 
-static void dm_dispatch_request(struct request *rq)
+static void dm_dispatch_clone_request(struct request *clone, struct request *rq)
 {
 	int r;
 
-	if (blk_queue_io_stat(rq->q))
-		rq->cmd_flags |= REQ_IO_STAT;
+	if (blk_queue_io_stat(clone->q))
+		clone->cmd_flags |= REQ_IO_STAT;
 
-	rq->start_time = jiffies;
-	r = blk_insert_cloned_request(rq->q, rq);
+	clone->start_time = jiffies;
+	r = blk_insert_cloned_request(clone->q, clone);
 	if (r)
+		/* must complete clone in terms of original request */
 		dm_complete_request(rq, r);
 }
 
···
 	return 0;
 }
 
-static struct request *__clone_rq(struct request *rq, struct mapped_device *md,
-				  struct dm_rq_target_io *tio, gfp_t gfp_mask)
+static struct request *clone_rq(struct request *rq, struct mapped_device *md,
+				struct dm_rq_target_io *tio, gfp_t gfp_mask)
 {
 	struct request *clone = alloc_clone_request(md, gfp_mask);
 
···
 
 static void map_tio_request(struct kthread_work *work);
 
-static struct request *clone_rq(struct request *rq, struct mapped_device *md,
-				gfp_t gfp_mask)
+static struct dm_rq_target_io *prep_tio(struct request *rq,
+					struct mapped_device *md, gfp_t gfp_mask)
 {
-	struct request *clone;
 	struct dm_rq_target_io *tio;
 
 	tio = alloc_rq_tio(md, gfp_mask);
···
 	memset(&tio->info, 0, sizeof(tio->info));
 	init_kthread_work(&tio->work, map_tio_request);
 
-	clone = __clone_rq(rq, md, tio, GFP_ATOMIC);
-	if (!clone) {
+	if (!clone_rq(rq, md, tio, gfp_mask)) {
 		free_rq_tio(tio);
 		return NULL;
 	}
 
-	return clone;
+	return tio;
 }
 
 /*
···
 static int dm_prep_fn(struct request_queue *q, struct request *rq)
 {
 	struct mapped_device *md = q->queuedata;
-	struct request *clone;
+	struct dm_rq_target_io *tio;
 
 	if (unlikely(rq->special)) {
 		DMWARN("Already has something in rq->special.");
 		return BLKPREP_KILL;
 	}
 
-	clone = clone_rq(rq, md, GFP_ATOMIC);
-	if (!clone)
+	tio = prep_tio(rq, md, GFP_ATOMIC);
+	if (!tio)
 		return BLKPREP_DEFER;
 
-	rq->special = clone;
+	rq->special = tio;
 	rq->cmd_flags |= REQ_DONTPREP;
 
 	return BLKPREP_OK;
···
  * 0  : the request has been processed (not requeued)
  * !0 : the request has been requeued
  */
-static int map_request(struct dm_target *ti, struct request *clone,
+static int map_request(struct dm_target *ti, struct request *rq,
 		       struct mapped_device *md)
 {
 	int r, requeued = 0;
-	struct dm_rq_target_io *tio = clone->end_io_data;
+	struct dm_rq_target_io *tio = rq->special;
+	struct request *clone = tio->clone;
 
 	r = ti->type->map_rq(ti, clone, &tio->info);
 	switch (r) {
···
 	case DM_MAPIO_REMAPPED:
 		/* The target has remapped the I/O so dispatch it */
 		trace_block_rq_remap(clone->q, clone, disk_devt(dm_disk(md)),
-				     blk_rq_pos(tio->orig));
-		dm_dispatch_request(clone);
+				     blk_rq_pos(rq));
+		dm_dispatch_clone_request(clone, rq);
 		break;
 	case DM_MAPIO_REQUEUE:
 		/* The target wants to requeue the I/O */
···
 		}
 
 		/* The target wants to complete the I/O */
-		dm_kill_unmapped_request(clone, r);
+		dm_kill_unmapped_request(rq, r);
 		break;
 	}
 
···
 {
 	struct dm_rq_target_io *tio = container_of(work, struct dm_rq_target_io, work);
 
-	map_request(tio->ti, tio->clone, tio->md);
+	map_request(tio->ti, tio->orig, tio->md);
 }
 
-static struct request *dm_start_request(struct mapped_device *md, struct request *orig)
+static void dm_start_request(struct mapped_device *md, struct request *orig)
 {
-	struct request *clone;
-
 	blk_start_request(orig);
-	clone = orig->special;
-	atomic_inc(&md->pending[rq_data_dir(clone)]);
+	atomic_inc(&md->pending[rq_data_dir(orig)]);
 
 	/*
 	 * Hold the md reference here for the in-flight I/O.
···
 	 * See the comment in rq_completed() too.
 	 */
 	dm_get(md);
-
-	return clone;
 }
 
 /*
···
 	int srcu_idx;
 	struct dm_table *map = dm_get_live_table(md, &srcu_idx);
 	struct dm_target *ti;
-	struct request *rq, *clone;
+	struct request *rq;
 	struct dm_rq_target_io *tio;
 	sector_t pos;
 
···
 		ti = dm_table_find_target(map, pos);
 		if (!dm_target_is_valid(ti)) {
 			/*
-			 * Must perform setup, that dm_done() requires,
+			 * Must perform setup, that rq_completed() requires,
 			 * before calling dm_kill_unmapped_request
 			 */
 			DMERR_LIMIT("request attempted access beyond the end of device");
-			clone = dm_start_request(md, rq);
-			dm_kill_unmapped_request(clone, -EIO);
+			dm_start_request(md, rq);
+			dm_kill_unmapped_request(rq, -EIO);
 			continue;
 		}
 
 		if (ti->type->busy && ti->type->busy(ti))
 			goto delay_and_out;
 
-		clone = dm_start_request(md, rq);
+		dm_start_request(md, rq);
 
 		tio = rq->special;
 		/* Establish tio->ti before queuing work (map_tio_request) */
···
 			bioset_free(md->bs);
 			md->bs = p->bs;
 			p->bs = NULL;
-		} else if (dm_table_get_type(t) == DM_TYPE_REQUEST_BASED) {
-			/*
-			 * There's no need to reload with request-based dm
-			 * because the size of front_pad doesn't change.
-			 * Note for future: If you are to reload bioset,
-			 * prep-ed requests in the queue may refer
-			 * to bio from the old bioset, so you must walk
-			 * through the queue to unprep.
-			 */
 		}
+		/*
+		 * There's no need to reload with request-based dm
+		 * because the size of front_pad doesn't change.
+		 * Note for future: If you are to reload bioset,
+		 * prep-ed requests in the queue may refer
+		 * to bio from the old bioset, so you must walk
+		 * through the queue to unprep.
+		 */
 		goto out;
 	}