Jason Gunthorpe wrote: > 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. Yes, this is how I see it too. Without serious help from folks that want to kill struct-page usage with DAX the next step will be dynamic compound page metadata initialization whenever a PMD entry is installed.
