Re: [PATCH v12 13/13] dm: add power-of-2 target for zoned devices with non power-of-2 zone sizes

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Aug 23 2022 at  8:18P -0400,
Pankaj Raghav <p.raghav@samsung.com> wrote:

> Only zoned devices with power-of-2(po2) number of sectors per zone(zone
> size) were supported in linux but now non power-of-2(npo2) zone sizes
> support has been added to the block layer.
> 
> Filesystems such as F2FS and btrfs have support for zoned devices with
> po2 zone size assumption. Before adding native support for npo2 zone
> sizes, it was suggested to create a dm target for npo2 zone size device to
> appear as a po2 zone size target so that file systems can initially
> work without any explicit changes by using this target.
> 
> The design of this target is very simple: remap the device zone size to
> the zone capacity and change the zone size to be the nearest power of 2
> value.
> 
> For e.g., a device with a zone size/capacity of 3M will have an equivalent
> target layout as follows:
> 
> Device layout :-
> zone capacity = 3M
> zone size = 3M
> 
> |--------------|-------------|
> 0             3M            6M
> 
> Target layout :-
> zone capacity=3M
> zone size = 4M
> 
> |--------------|---|--------------|---|
> 0             3M  4M             7M  8M
> 
> The area between target's zone capacity and zone size will be emulated
> in the target.
> The read IOs that fall in the emulated gap area will return 0 filled
> bio and all the other IOs in that area will result in an error.
> If a read IO span across the emulated area boundary, then the IOs are
> split across them. All other IO operations that span across the emulated
> area boundary will result in an error.
> 
> The target can be easily created as follows:
> dmsetup create <label> --table '0 <size_sects> po2zone /dev/nvme<id>'
> 
> Note that the target does not support partial mapping of the underlying
> device.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> Suggested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> Suggested-by: Damien Le Moal <damien.lemoal@wdc.com>
> Suggested-by: Hannes Reinecke <hare@suse.de>


This target needs more review from those who Suggested-by it.

And the header and docs needs to address:

1) why is a partial mapping of the underlying device disallowed?
2) why is it assumed all IO is read-only? (talk to me and others like
   we don't know the inherent limitations of this class of zoned hw)

On a code level:
1) are you certain you're properly failing all writes?
   - are writes allowed to the "zone capacity area" but _not_
     allowed to the "emulated zone area"? (if yes, _please document_). 
2) yes, you absolutely need to implement the .status target_type hook
   (for both STATUS and TABLE).
3) really not loving the nested return (of DM_MAPIO_SUBMITTED or
   DM_MAPIO_REMAPPED) from methods called from dm_po2z_map().  Would
   prefer to not have to do a depth-first search to see where and when
   dm_po2z_map() returns a DM_MAPIO_XXX unless there is a solid
   justification for it.  To me it just obfuscates the DM interface a
   bit too much. 

Otherwise, pretty clean code and nothing weird going on.

I look forward to seeing your next (final?) revision of this patchset.

Thanks,
Mike