quoted

Definitely, but we should try figuring out what's going on here. I
assume on x86-64 it behaves differently?

Yes, we should root cause. I highly suspect that there is somewhere
alignment miscalculations happen that cause this memory waste with the
offset 16M. I am also not sure why the 2M label size was increased,
and  why 16M is now an alignment requirement.

This appears to be because even if we set vmemmap to be outside of the
dax device, the alignment calculates the maximum size of vmemmap for
this device, and subtracts it from the devdax size.
See [1], line 795 is where this offset is calculated.

This also explains why with 64K pages, the 16M offset worked: because
fewer struct pages were able to fit within 16M - label size.

[1] https://soleen.com/source/xref/linux/drivers/nvdimm/pfn_devs.c?r=b7b3c01b&mo=18459&fi=718#795