On Tue, Aug 17, 2021 at 11:44:54AM -0400, Felix Kuehling wrote: > >> That's a good catch. Existing drivers shouldn't need a page_free > >> callback if they didn't have one before. That means we need to add a > >> NULL-pointer check in free_device_page. > > Also the other state clearing (__ClearPageWaiters/mem_cgroup_uncharge/ > > ->mapping = NULL). > > > > In many ways this seems like you want to bring back the DEVICE_PUBLIC > > pgmap type that was removed a while ago due to the lack of users > > instead of overloading the generic type. > > I think so. I'm not clear about how DEVICE_PUBLIC differed from what > DEVICE_GENERIC is today. As I understand it, DEVICE_PUBLIC was removed > because it was unused and also known to be broken in some ways. > DEVICE_GENERIC seemed close enough to what we need, other than not being > supported in the migration helpers. > > Would you see benefit in re-introducing DEVICE_PUBLIC as a distinct > memory type from DEVICE_GENERIC? What would be the benefits of making > that distinction? The old DEVICE_PUBLIC mostly different in that it allowed the page to be returned from vm_normal_page, which I think was horribly buggy. But the point is not to bring back these old semantics. The idea is to be able to differeniate between your new coherent on-device memory and the existing DEVICE_GENERIC. That is call the code in free_devmap_managed_page that is currently only used for device private pages also for your new public device pages without affecting the devdax and xen use cases.
