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

quoted

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.

Thinking further.. I'm left confused about just what the heck this
target is assuming.

E.g.: feels like its exposing a readonly end of the zone is very
bi-polar... yet no hint to upper layer it shouldn't write to that
read-only end (the "emulated zone").. but there has to be some zoned
magic assumed?  And I'm just naive?

You are absolutely right about "zoned magic". Applications that use a zoned
block device are aware of the zone capacity and zone size. BLKREPORTZONE
ioctl is typically used to get the zone information from a zoned block device.

This target adjusts the zone report so that zone size and zone capacity are
modified correctly (see dm_po2z_report_zones() and
dm_po2z_report_zones_cb() functions).