Re: [PATCH v4 1/7] mm/zone_device: Add order argument to folio_free callback

[Balbir Singh <balbirs@nvidia.com>]

On 1/12/26 10:51, Zi Yan wrote:
> On 11 Jan 2026, at 19:19, Balbir Singh wrote:
> 
>> On 1/12/26 08:35, Matthew Wilcox wrote:
>>> On Sun, Jan 11, 2026 at 09:55:40PM +0100, Francois Dugast wrote:
>>>> The core MM splits the folio before calling folio_free, restoring the
>>>> zone pages associated with the folio to an initialized state (e.g.,
>>>> non-compound, pgmap valid, etc...). The order argument represents the
>>>> folio’s order prior to the split which can be used driver side to know
>>>> how many pages are being freed.
>>>
>>> This really feels like the wrong way to fix this problem.
>>>
> 
> Hi Matthew,
> 
> I think the wording is confusing, since the actual issue is that:
> 
> 1. zone_device_page_init() calls prep_compound_page() to form a large folio,
> 2. but free_zone_device_folio() never reverse the course,
> 3. the undo of prep_compound_page() in free_zone_device_folio() needs to
>    be done before driver callback ->folio_free(), since once ->folio_free()
>    is called, the folio can be reallocated immediately,
> 4. after the undo of prep_compound_page(), folio_order() can no longer provide
>    the original order information, thus, folio_free() needs that for proper
>    device side ref manipulation.
> 
> So this is not used for "split" but undo of prep_compound_page(). It might
> look like a split to non core MM people, since it changes a large folio
> to a bunch of base pages. BTW, core MM has no compound_page_dctor() but
> open codes it in free_pages_prepare() by resetting page flags, page->mapping,
> and so on. So it might be why the undo prep_compound_page() is missed
> by non core MM people.
> 
>>
>> This stems from a special requirement, freeing is done in two phases
>>
>> 1. Free the folio -> inform the driver (which implies freeing the backing device memory)
>> 2. Return the folio back, split it back to single order folios
> 
> Hi Balbir,
> 
> Please refrain from using "split" here, since it confuses MM people. A folio
> is split when it is still in use, but in this case, the folio has been freed
> and needs to be restored to "free page" state.
> 

Yeah, the word split came from the initial version that called it folio_split_unref()
and I was also thinking of the split callback for zone device folios, but I agree
(re)initialization is a better term.

>>
>> The current code does not do 2. 1 followed by 2 does not work for
>> Francois since the backing memory can get reused before we reach step 2.
>> The proposed patch does 2 followed 1, but doing 2 means we've lost the
>> folio order and thus the old order is passed in. Although, I wonder if the
>> backing folio's zone_device_data can be used to encode any order information
>> about the device side allocation.
>>
>> @Francois, I hope I did not miss anything in the explanation above.
>>
>>> I think someone from the graphics side really needs to take the lead on
>>> understanding what the MM is doing (both currently and in the future).
>>> I'm happy to work with you, but it feels like there's a lot of churn right
>>> now because there's a lot of people working on this without understanding
>>> the MM side of things (and conversely, I don't think (m)any people on the
>>> MM side really understand what graphics cards are trying to accomplish).
>>>
>>
>> I suspect you are referring to folio specialization and/or downsizing?
>>
>>> Who is that going to be?  I'm happy to get on the phone with someone.
>>
>> Happy to work with you, but I am not the authority on graphics, I can speak
>> to zone device folios. I suspect we'd need to speak to more than one person.
>>
> 
> --
> Best Regards,
> Yan, Zi