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

[Felix Kuehling]

Am 2021-08-17 um 1:50 a.m. schrieb Christoph Hellwig:
> On Mon, Aug 16, 2021 at 03:00:49PM -0400, Felix Kuehling wrote:
>> Am 2021-08-15 um 11:40 a.m. schrieb Christoph Hellwig:
>>> On Fri, Aug 13, 2021 at 01:31:45AM -0500, Alex Sierra wrote:
>>>> Add MEMORY_DEVICE_GENERIC case to free_zone_device_page callback.
>>>> Device generic type memory case is now able to free its pages properly.
>>> How is this going to work for the two existing MEMORY_DEVICE_GENERIC
>>> that now change behavior?  And which don't have a ->page_free callback
>>> at all?
>> 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?

Thanks,
  Felix