[PATCH RFC v3 0/3] fix NVMe multipath partition diskstats

Currently the NVMe multipath partition diskstats are not maintained.
This is because the functionality to update the diskstats only updates
for the whole disk (and not per partition).

As an example:
$ ./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

Notice how all values are 0 for nvme1n1p1.

This series addresses that problem by first updating the gendisk code to
create the partition table for hidden disks, as suggested by Keith.

This is done by cloning the partition table from the multipath disk.

Secondly the NVMe multipath functionality to handle the diskstats is
modified to handle the gendisk partition, by looking up this partition
from the per-path disk partition.

Setting as RFC as I am not too happy with the block partition cloning
code.

Difference to v2:
- don't scan per-path disk, and clone from multipath disk instead

John Garry (3):
  block: support linking disks to clone partitions
  block: don't show partition diskstats for GENHD_FL_HIDDEN
  nvme-multipath: fix diskstats for partitions

 block/blk.h                   |  1 +
 block/genhd.c                 | 35 +++++++++++++++++++++++-
 block/partitions/core.c       | 45 +++++++++++++++++++++++++++++++
 drivers/nvme/host/core.c      |  1 +
 drivers/nvme/host/multipath.c | 51 ++++++++++++++++++++++++++++-------
 include/linux/blkdev.h        |  8 ++++++
 6 files changed, 130 insertions(+), 11 deletions(-)

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2.43.7