Re: can we finally kill off CONFIG_FS_DAX_LIMITED

[David Hildenbrand <david@redhat.com>]

On 23.08.21 16:05, Gerald Schaefer wrote:
> 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.
> 

add_memory() is for adding system RAM. I don't think that's what you 
want in the case of DCSS. Supporting ZONE_DEVICE cleanly would be ideal.

-- 
Thanks,

David / dhildenb