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