drbd: split __drbd_make_request in before and after drbd_al_begin_io

This is in preparation to be able to defer requests that need to wait
for an activity log transaction to a submitter workqueue.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_req.c

@@ -34,14 +34,14 @@
 static bool drbd_may_do_local_read(struct drbd_conf *mdev, sector_t sector, int size);
 
 /* Update disk stats at start of I/O request */
-static void _drbd_start_io_acct(struct drbd_conf *mdev, struct drbd_request *req, struct bio *bio)
+static void _drbd_start_io_acct(struct drbd_conf *mdev, struct drbd_request *req)
 {
-	const int rw = bio_data_dir(bio);
+	const int rw = bio_data_dir(req->master_bio);
 	int cpu;
 	cpu = part_stat_lock();
 	part_round_stats(cpu, &mdev->vdisk->part0);
 	part_stat_inc(cpu, &mdev->vdisk->part0, ios[rw]);
-	part_stat_add(cpu, &mdev->vdisk->part0, sectors[rw], bio_sectors(bio));
+	part_stat_add(cpu, &mdev->vdisk->part0, sectors[rw], req->i.size >> 9);
 	(void) cpu; /* The macro invocations above want the cpu argument, I do not like
 		       the compiler warning about cpu only assigned but never used... */
 	part_inc_in_flight(&mdev->vdisk->part0, rw);
@@ -1020,12 +1020,16 @@ drbd_submit_req_private_bio(struct drbd_request *req)
 		bio_endio(bio, -EIO);
 }
 
-void __drbd_make_request(struct drbd_conf *mdev, struct bio *bio, unsigned long start_time)
+/* returns the new drbd_request pointer, if the caller is expected to
+ * drbd_send_and_submit() it (to save latency), or NULL if we queued the
+ * request on the submitter thread.
+ * Returns ERR_PTR(-ENOMEM) if we cannot allocate a drbd_request.
+ */
+struct drbd_request *
+drbd_request_prepare(struct drbd_conf *mdev, struct bio *bio, unsigned long start_time)
 {
-	const int rw = bio_rw(bio);
-	struct bio_and_error m = { NULL, };
+	const int rw = bio_data_dir(bio);
 	struct drbd_request *req;
-	bool no_remote = false;
 
 	/* allocate outside of all locks; */
 	req = drbd_req_new(mdev, bio);
@@ -1035,7 +1039,7 @@ void __drbd_make_request(struct drbd_conf *mdev, struct bio *bio, unsigned long
 		 * if user cannot handle io errors, that's not our business. */
 		dev_err(DEV, "could not kmalloc() req\n");
 		bio_endio(bio, -ENOMEM);
-		return;
+		return ERR_PTR(-ENOMEM);
 	}
 	req->start_time = start_time;
 
@@ -1057,6 +1061,15 @@ void __drbd_make_request(struct drbd_conf *mdev, struct bio *bio, unsigned long
 		drbd_al_begin_io(mdev, &req->i, true);
 	}
 
+	return req;
+}
+
+static void drbd_send_and_submit(struct drbd_conf *mdev, struct drbd_request *req)
+{
+	const int rw = bio_rw(req->master_bio);
+	struct bio_and_error m = { NULL, };
+	bool no_remote = false;
+
 	spin_lock_irq(&mdev->tconn->req_lock);
 	if (rw == WRITE) {
 		/* This may temporarily give up the req_lock,
@@ -1079,7 +1092,7 @@ void __drbd_make_request(struct drbd_conf *mdev, struct bio *bio, unsigned long
 	}
 
 	/* Update disk stats */
-	_drbd_start_io_acct(mdev, req, bio);
+	_drbd_start_io_acct(mdev, req);
 
 	/* We fail READ/READA early, if we can not serve it.
 	 * We must do this before req is registered on any lists.
@@ -1137,7 +1150,14 @@ void __drbd_make_request(struct drbd_conf *mdev, struct bio *bio, unsigned long
 
 	if (m.bio)
 		complete_master_bio(mdev, &m);
-	return;
+}
+
+void __drbd_make_request(struct drbd_conf *mdev, struct bio *bio, unsigned long start_time)
+{
+	struct drbd_request *req = drbd_request_prepare(mdev, bio, start_time);
+	if (IS_ERR_OR_NULL(req))
+		return;
+	drbd_send_and_submit(mdev, req);
 }
 
 void drbd_make_request(struct request_queue *q, struct bio *bio)