[BLOCK] Update read/write block io statistics at completion time

Right now we do it at queueing time, which works alright for reads
(since they are usually sync), but not for async writes since we can
queue io a lot faster than we can complete it. This makes the vmstat
output look extremely bursty.
Signed-off-by: Jens Axboe <axboe@suse.de>

drivers/block/ll_rw_blk.c

@@ -2387,16 +2387,12 @@ static void drive_stat_acct(struct request *rq, int nr_sectors, int new_io)
 	if (!blk_fs_request(rq) || !rq->rq_disk)
 		return;
 
-	if (rw == READ) {
-		__disk_stat_add(rq->rq_disk, read_sectors, nr_sectors);
-		if (!new_io)
+	if (!new_io) {
+		if (rw == READ)
 			__disk_stat_inc(rq->rq_disk, read_merges);
-	} else if (rw == WRITE) {
-		__disk_stat_add(rq->rq_disk, write_sectors, nr_sectors);
-		if (!new_io)
+		else
 			__disk_stat_inc(rq->rq_disk, write_merges);
-	}
-	if (new_io) {
+	} else {
 		disk_round_stats(rq->rq_disk);
 		rq->rq_disk->in_flight++;
 	}
@@ -3048,6 +3044,13 @@ static int __end_that_request_first(struct request *req, int uptodate,
 				(unsigned long long)req->sector);
 	}
 
+	if (blk_fs_request(req) && req->rq_disk) {
+		if (rq_data_dir(req) == READ)
+			__disk_stat_add(req->rq_disk, read_sectors, nr_bytes >> 9);
+		else
+			__disk_stat_add(req->rq_disk, write_sectors, nr_bytes >> 9);
+	}
+
 	total_bytes = bio_nbytes = 0;
 	while ((bio = req->bio) != NULL) {
 		int nbytes;