Re: [PATCH v6 08/13] mm: call pgmap->ops->page_free for DEVICE_GENERIC pages

On Tue, Aug 17, 2021 at 11:44:54AM -0400, Felix Kuehling wrote:

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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.