Re: [PATCH v2] mm, zone_device: replace {get, put}_zone_device_page() with a single reference

[Dan Williams]

On Fri, Apr 28, 2017 at 11:00 AM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>> On Fri, Apr 28, 2017 at 10:34 AM, Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>> >> Kirill points out that the calls to {get,put}_dev_pagemap() can be
>> >> removed from the mm fast path if we take a single get_dev_pagemap()
>> >> reference to signify that the page is alive and use the final put of the
>> >> page to drop that reference.
>> >>
>> >> This does require some care to make sure that any waits for the
>> >> percpu_ref to drop to zero occur *after* devm_memremap_page_release(),
>> >> since it now maintains its own elevated reference.
>> >
>> > This is NAK from HMM point of view as i need those call. So if you remove
>> > them now i will need to add them back as part of HMM.
>>
>> I thought you only need them at page free time? You can still hook
>> __put_page().
>
> No, i need a hook when page refcount reach 1, not 0. That being said
> i don't care about put_dev_pagemap(page->pgmap); so that part of the
> patch is fine from HMM point of view but i definitly need to hook my-
> self in the general put_page() function.
>
> So i will have to undo part of this patch for HMM (put_page() will
> need to handle ZONE_DEVICE page differently).

Ok, I'd rather this go in now since it fixes the existing use case,
and unblocks the get_user_pages_fast() conversion to generic code.
That also gives Kirill and -mm folks a chance to review what HMM wants
to do on top of the page_ref infrastructure.  The
{get,put}_zone_device_page interface went in in 4.5 right before
page_ref went in during 4.6, so it was just an oversight that
{get,put}_zone_device_page were not removed earlier.