[PATCH v5 0/7] support non power of 2 zoned devices

[Pankaj Raghav]

Hello,
  The previous revision ended up leading to a new direction to add npo2
  device support as a dm target instead of adding support to filesystems
  directly[0]. I would like to hear some inputs from the community,
  especially from Christoph and Mike Snitzer about this approach.

- Background and Motivation:

The zone storage implementation in Linux, introduced since v4.10, first
targetted SMR drives which have a power of 2 (po2) zone size alignment
requirement. The po2 zone size was further imposed implicitly by the
block layer's blk_queue_chunk_sectors(), used to prevent IO merging
across chunks beyond the specified size, since v3.16 through commit
762380ad9322 ("block: add notion of a chunk size for request merging").
But this same general block layer po2 requirement for blk_queue_chunk_sectors()
was removed on v5.10 through commit 07d098e6bbad ("block: allow 'chunk_sectors'
to be non-power-of-2").

NAND, which is the media used in newer zoned storage devices, does not
naturally align to po2. In these devices, zone cap is not the same as the
po2 zone size. When the zone cap != zone size, then unmapped LBAs are
introduced to cover the space between the zone cap and zone size. po2
requirement does not make sense for these type of zone storage devices.
This patch series aims to remove these unmapped LBAs for zoned devices when
zone cap is npo2. This is done by relaxing the po2 zone size constraint
in the kernel and allowing zoned device with npo2 zone sizes if zone cap
== zone size.

Removing the po2 requirement from zone storage should be possible
now provided that no userspace regression and no performance regressions are
introduced. Stop-gap patches have been already merged into f2fs-tools to
proactively not allow npo2 zone sizes until proper support is added [1].

There were two efforts previously to add support to npo2 devices: 1) via
device level emulation [2] but that was rejected with a final conclusion
to add support for non po2 zoned device in the complete stack[3] 2)
adding support to the complete stack by removing the constraint in the
block layer and NVMe layer with support to btrfs, zonefs, etc which was
rejected with a conclusion to add a dm target for FS support [0]
to reduce the regression impact.

- Patchset description:

The support is planned to be added in two phases:
- Add npo2 support to block, nvme layer and necessary stop gap patches
  in the filesystems
- Add dm target for npo2 devices so that they are presented as a po2
  device to filesystems

This patchset addresses the first phase for adding support to npo2
devices.

Patches 1-2 deals with removing the po2 constraint from the
block layer.

Patches 3-4 deals with removing the constraint from nvme zns.

Patch 5-6 removes the po2 contraint in null blk

Patches 7 adds conditions to not allow non power of 2 devices in
DM.

The patch series is based on linux-next tag: next-20220520

- Future work
Add DM target for npo2 devices to be presented as a po2 device.

[0] https://lore.kernel.org/lkml/PH0PR04MB74166C87F694B150A5AE0F009BD09@PH0PR04MB7416.namprd04.prod.outlook.com/
[1] https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git/commit/?h=dev-test&id=6afcf6493578e77528abe65ab8b12f3e1c16749f
[2] https://lore.kernel.org/all/20220310094725.GA28499@lst.de/T/
[3] https://lore.kernel.org/all/20220315135245.eqf4tqngxxb7ymqa@unifi/

Changes since v1:
- Put the function declaration and its usage in the same commit (Bart)
- Remove bdev_zone_aligned function (Bart)
- Change the name from blk_queue_zone_aligned to blk_queue_is_zone_start
  (Damien)
- q is never null in from bdev_get_queue (Damien)
- Add condition during bringup and check for zsze == zcap for npo2
  drives (Damien)
- Rounddown operation should be made generic to work in 32 bits arch
  (bart)
- Add comments where generic calculation is directly used instead having
  special handling for po2 zone sizes (Hannes)
- Make the minimum zone size alignment requirement for btrfs to be 1M
  instead of BTRFS_STRIPE_LEN(David)

Changes since v2:
- Minor formatting changes

Changes since v3:
- Make superblock mirror align with the existing superblock log offsets
  (David)
- DM change return value and remove extra newline
- Optimize null blk zone index lookup with shift for po2 zone size

Changes since v4:
- Remove direct filesystems support for npo2 devices (Johannes, Hannes,
  Damien)

Luis Chamberlain (1):
  dm-zoned: ensure only power of 2 zone sizes are allowed

Pankaj Raghav (6):
  block: make blkdev_nr_zones and blk_queue_zone_no generic for npo2
    zsze
  block: allow blk-zoned devices to have non-power-of-2 zone size
  nvme: zns: Allow ZNS drives that have non-power_of_2 zone size
  nvmet: Allow ZNS target to support non-power_of_2 zone sizes
  null_blk: allow non power of 2 zoned devices
  null_blk: use zone_size_sects_shift for power of 2 zoned devices

 block/blk-core.c                  |  3 +--
 block/blk-zoned.c                 | 38 +++++++++++++++++++++++--------
 drivers/block/null_blk/main.c     |  5 ++--
 drivers/block/null_blk/null_blk.h |  6 +++++
 drivers/block/null_blk/zoned.c    | 20 ++++++++++------
 drivers/md/dm-zone.c              | 12 ++++++++++
 drivers/nvme/host/zns.c           | 21 +++++++++--------
 drivers/nvme/target/zns.c         |  2 +-
 include/linux/blkdev.h            | 36 ++++++++++++++++++++++++++++-
 9 files changed, 111 insertions(+), 32 deletions(-)

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2.25.1