Re: [PATCH RFC] nvme-multipath: fix diskstats for partitions

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

On 24/07/2026 17:56, John Garry wrote:
>> This is the idea, assuming we can get the partition tables of the head
>> and path disks to be aligned:
>> 
>> ---
>> --- a/drivers/nvme/host/multipath.c
>> +++ b/drivers/nvme/host/multipath.c
>> @@ -543,7 +543,10 @@ 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);
>> +               struct block_device *bdev;
>> +
>> +               bdev = xa_load(&ns->disk->part_tbl, bdev_partno(bio->bi_bdev));
>> +               bio_set_dev(bio, bdev);
>>                  /*
>>                   * Use BIO_REMAPPED to skip bio_check_eod() when this bio
>>                   * enters submit_bio_noacct() for the per-path device. The EOD
>> --
> 
> ok, got it.
> 
> And I think that we would need to do the reverse lookup in
> nvme_mpath_start_request() and nvme_mpath_end_request() to get the head
> disk partition, like:
> bdev = xa_load(&ns->head->disk->part_tbl, bdev_partno(bio->bi_bdev));
> 
> I suppose that the tricky part now would be have the per-path disk scan
> run but keep those per-path disks hidden.
> 
> I experimented by stop setting GENHD_FL_HIDDEN for the per-path disk,
> and the diskstats look ok, FWIW.

Here's an attempt to get the disk partition tables for hidden disks:

--->8----

 From d7e124982215648ae914a1fb48f34f915f94eb60 Mon Sep 17 00:00:00 2001
From: John Garry <john.garry@linux.dev>
Date: Thu, 30 Jul 2026 13:14:32 +0100
Subject: [PATCH] block: scan partitions for hidden disks

If a disk is hidden (GENHD_FL_HIDDEN flags set), we currently do not 
scan the disk partition table.

GENHD_FL_HIDDEN is used in NVMe multipath support to hide the per-path
disk.

However, it would be useful there to actually have the per-path disk
partition table available for situations where we want to send bios to
specific per-path disk partition.

Change GENHD_FL_HIDDEN to scan partitions. For anyone wanting to avoid
scanning the partition (and keep the disk hidden), flag GENHD_FL_NO_PART 
can still be used.

Signed-off-by: John Garry <john.g.garry@oracle.com>

diff --git a/block/genhd.c b/block/genhd.c
index df2c3c69b467..64469943fbfe 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -417,6 +417,10 @@ static void add_disk_final(struct gendisk *disk)
  		 */
  		dev_set_uevent_suppress(ddev, 0);
  		disk_uevent(disk, KOBJ_ADD);
+	} else {
+		bdev_add(disk->part0, ddev->devt);
+		if (get_capacity(disk))
+			disk_scan_partitions(disk, BLK_OPEN_READ);
  	}

  	blk_apply_bdi_limits(disk->bdi, &disk->queue->limits);
@@ -487,8 +491,7 @@ static int __add_disk(struct device *parent, struct 
gendisk *disk,
  	dev_set_name(ddev, "%s", disk->disk_name);
  	if (fwnode)
  		device_set_node(ddev, fwnode);
-	if (!(disk->flags & GENHD_FL_HIDDEN))
-		ddev->devt = MKDEV(disk->major, disk->first_minor);
+	ddev->devt = MKDEV(disk->major, disk->first_minor);
  	ret = device_add(ddev);
  	if (ret)
  		goto out_free_ext_minor;
diff --git a/block/partitions/core.c b/block/partitions/core.c
index b5c59b79ca7c..a48896c6c791 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -152,7 +152,8 @@ static struct parsed_partitions 
*check_partition(struct gendisk *hd)

  	}
  	if (res > 0) {
-		printk(KERN_INFO "%s", seq_buf_str(&state->pp_buf));
+		if (!(hd->flags & GENHD_FL_HIDDEN))
+			printk(KERN_INFO "%s", seq_buf_str(&state->pp_buf));

  		kfree(state->pp_buf.buffer);
  		return state;
@@ -164,7 +165,7 @@ static struct parsed_partitions 
*check_partition(struct gendisk *hd)
  	 */
  	if (err)
  		res = err;
-	if (res) {
+	if (res && !(hd->flags & GENHD_FL_HIDDEN)) {
  		seq_buf_puts(&state->pp_buf,
  			     " unable to read partition table\n");
  		printk(KERN_INFO "%s", seq_buf_str(&state->pp_buf));
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 453c1f0b2dd0..869fd05019d2 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1790,10 +1790,6 @@ static void nvme_enable_aen(struct nvme_ctrl *ctrl)

  static int nvme_ns_open(struct nvme_ns *ns)
  {
-
-	/* should never be called due to GENHD_FL_HIDDEN */
-	if (WARN_ON_ONCE(nvme_ns_head_multipath(ns->head)))
-		goto fail;
  	if (!nvme_get_ns(ns))
  		goto fail;
  	if (!try_module_get(ns->ctrl->ops->module))
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 9213a5716f95..a270cf1394c7 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -260,7 +260,7 @@ static inline unsigned int disk_openers(struct 
gendisk *disk)
   */
  static inline bool disk_has_partscan(struct gendisk *disk)
  {
-	return !(disk->flags & (GENHD_FL_NO_PART | GENHD_FL_HIDDEN)) &&
+	return !(disk->flags & GENHD_FL_NO_PART) &&
  		!test_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
  }

-- 
2.43.0

---8<---

With that, /sys/class/block/ looks a bit different (with the per-path 
partitions):

#ls -l /sys/class/block/ | grep nvme0
lrwxrwxrwx    1 root     root             0 Jul 30 12:39 nvme0c0n1 ->
../../devices/virtual/nvme-fabrics/ctl/nvme0/nvme0c0n1
lrwxrwxrwx    1 root     root             0 Jul 30 12:39 nvme0c0n1p1
-> ../../devices/virtual/nvme-fabrics/ctl/nvme0/nvme0c0n1/nvme0c0n1p1
lrwxrwxrwx    1 root     root             0 Jul 30 12:39 nvme0c1n1 ->
../../devices/virtual/nvme-fabrics/ctl/nvme1/nvme0c1n1
lrwxrwxrwx    1 root     root             0 Jul 30 12:39 nvme0c1n1p1
-> ../../devices/virtual/nvme-fabrics/ctl/nvme1/nvme0c1n1/nvme0c1n1p1
lrwxrwxrwx    1 root     root             0 Jul 30 12:39 nvme0n1 ->
../../devices/virtual/nvme-subsystem/nvme-subsys0/nvme0n1
lrwxrwxrwx    1 root     root             0 Jul 30 12:39 nvme0n1p1 ->
../../devices/virtual/nvme-subsystem/nvme-subsys0/nvme0n1/nvme0n1p1

Thoughts?