On Thu, Sep 15, 2022 at 08:35:08PM -0700, Dan Williams wrote: > This hackery continues the status of DAX pages as special cases in the > VM. The thought being carrying the Xarray / mapping infrastructure > forward still allows for the continuation of the page-less DAX effort. > Otherwise, the work to convert DAX pages to behave like typical > vm_normal_page() needs more investigation to untangle transparent huge > page assumptions. I see it differently, ZONE_DEVICE by definition is page-based. As long as DAX is using ZONE_DEVICE it should follow the normal struct page rules, including proper reference counting everywhere. By not doing this DAX is causing all ZONE_DEVICE users to suffer because we haven't really special cased just DAX out of all the other users. If there is some kind of non-struct page future, then it will not be ZONE_DEVICE and it will have its own mechanisms, somehow. So, we should be systematically stripping away all the half-backed non-struct page stuff from ZONE_DEVICE as a matter of principle. DAX included, whatever DAX's future may hold. The pte bit and the missing refcounting in the page table paths is the remaining big issue and I hope we fix it. The main problem is that FS-DAX must create compound pages for the 2M page size. Jason
