Re: can we finally kill off CONFIG_FS_DAX_LIMITED

Gerald Schaefer
2021-08-23T14:06:07Z

On Fri, 20 Aug 2021 07:43:40 +0200
Christoph Hellwig <hch@lst.de> wrote:

> Hi all,
>
> looking at the recent ZONE_DEVICE related changes we still have a
> horrible maze of different code paths. I already suggested to
> depend on ARCH_HAS_PTE_SPECIAL for ZONE_DEVICE there, which all modern
> architectures have anyway. But the other odd special case is
> CONFIG_FS_DAX_LIMITED which is just used for the xpram driver. Does
> this driver still see use? If so can we make it behave like the
> other DAX drivers and require a pgmap? I think the biggest missing
> part would be to implement ARCH_HAS_PTE_DEVMAP for s390.

Puh, yes, that seems to be needed in order to enable ZONE_DEVICE, and
then we could use devm_memremap_pages(), at least that was my plan
some time ago. However, either the ARCH_HAS_PTE_DEVMAP dependency
is new, or I overlooked it before, but we do not have any free bits
in the pte left, so this is not going to work.

Would it strictly be necessary to implement ZONE_DEVICE, or would
it be enough if we would use e.g. add_memory() instead of just
adding the DCSS memory directly to the kernel mapping via
vmem_add_mapping()? That way we might at least get the struct pages,
but somehow it doesn't feel completely right.