[PATCH RFC v2 2/2] nvme-multipath: fix diskstats for partitions

[John Garry <john.g.garry@oracle.com>]

From: John Garry <john.garry@linux.dev>

Currently diskstats for partitions are never updated:
$ ./fio_read nvme1n1p1 # run traffic on /dev/nvme1n1p1
...
$ more /proc/diskstats | grep nvme1
 259       2 nvme1c1n1 49857 0 400344 768565 0 0 0 0 0 2334 768565 0 0 0 0 0 0
 259       3 nvme1n1 99710 0 800680 1599285 0 0 0 0 0 2346 1599285 0 0 0 0 0 0
 259       5 nvme1n1p1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
 259       4 nvme1c2n1 49853 0 400336 831472 0 0 0 0 0 2315 831472 0 0 0 0 0 0

This is because we only ever update the diskstats for the multipath disk in
nvme_mpath_end_request(), and we never take into account that the original
bi_bdev may been a partition of this disk.

Functions bdev_start_io_acct() and bdev_start_io_acct() do handle
updating diskstats for a partition, in that they also update the whole
disk also (if a partition), so use the partition (if applicable) when
calling those functions.

Change any functionality which used to lookup the gendisk part0 to now
lookup the specific gendisk partition.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 drivers/nvme/host/multipath.c | 31 +++++++++++++++++++++++++------
 1 file changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
index 75dbb58286a32..5c95a2ab7755f 100644
--- a/drivers/nvme/host/multipath.c
+++ b/drivers/nvme/host/multipath.c
@@ -144,6 +144,19 @@ void nvme_mpath_start_freeze(struct nvme_subsystem *subsys)
 			blk_freeze_queue_start(h->disk->queue);
 }
 
+static struct block_device *nvme_to_disk_part(struct gendisk *disk, u8 partno)
+{
+	struct block_device *bdev;
+
+	/* Quick lookup for whole disk */
+	if (!partno)
+		return disk->part0;
+	rcu_read_lock();
+	bdev = xa_load(&disk->part_tbl, partno);
+	rcu_read_unlock();
+	return bdev;
+}
+
 void nvme_failover_req(struct request *req)
 {
 	struct nvme_ns *ns = req->q->queuedata;
@@ -165,8 +178,10 @@ void nvme_failover_req(struct request *req)
 	}
 
 	spin_lock_irqsave(&ns->head->requeue_lock, flags);
-	for (bio = req->bio; bio; bio = bio->bi_next)
-		bio_set_dev(bio, ns->head->disk->part0);
+	for (bio = req->bio; bio; bio = bio->bi_next) {
+		bio_set_dev(bio, nvme_to_disk_part(ns->head->disk,
+			bdev_partno(bio->bi_bdev)));
+	}
 	blk_steal_bios(&ns->head->requeue_list, req);
 	spin_unlock_irqrestore(&ns->head->requeue_lock, flags);
 
@@ -194,8 +209,9 @@ void nvme_mpath_start_request(struct request *rq)
 		return;
 
 	nvme_req(rq)->flags |= NVME_MPATH_IO_STATS;
-	nvme_req(rq)->start_time = bdev_start_io_acct(disk->part0, req_op(rq),
-						      jiffies);
+	nvme_req(rq)->start_time = bdev_start_io_acct(
+			nvme_to_disk_part(disk, bdev_partno(rq->part)),
+			req_op(rq), jiffies);
 }
 EXPORT_SYMBOL_GPL(nvme_mpath_start_request);
 
@@ -208,7 +224,8 @@ void nvme_mpath_end_request(struct request *rq)
 
 	if (!(nvme_req(rq)->flags & NVME_MPATH_IO_STATS))
 		return;
-	bdev_end_io_acct(ns->head->disk->part0, req_op(rq),
+	bdev_end_io_acct(nvme_to_disk_part(ns->head->disk,
+			 bdev_partno(rq->part)), req_op(rq),
 			 blk_rq_bytes(rq) >> SECTOR_SHIFT,
 			 nvme_req(rq)->start_time);
 }
@@ -549,7 +566,9 @@ static void nvme_ns_head_submit_bio(struct bio *bio)
 	srcu_idx = srcu_read_lock(&head->srcu);
 	ns = nvme_find_path(head);
 	if (likely(ns)) {
-		bio_set_dev(bio, ns->disk->part0);
+		bio_set_dev(bio, nvme_to_disk_part(ns->disk,
+				bdev_partno(bio->bi_bdev)));
+
 		/*
 		 * Use BIO_REMAPPED to skip bio_check_eod() when this bio
 		 * enters submit_bio_noacct() for the per-path device. The EOD
-- 
2.43.7