Re: [PATCH v2] mm/migrate: rename PAGE_ migration flags to FOLIO_

David Hildenbrand (Arm)
2026-03-25T15:04:37Z

On 3/25/26 16:00, Zi Yan wrote:
> On 25 Mar 2026, at 10:53, David Hildenbrand (Arm) wrote:
>
>> On 3/25/26 15:21, Zi Yan wrote:
>>>
>>>
>>> Hi David,
>>
>> Hi,
>>
>>>
>>> In terms of folio_change_private(), I did not think it is related to
>>> folio_{attach,detach}_private(), since the latter change folio refcount during
>>> the operation. If folio_change_private() is related to attach/detach,
>>> I imagine it would check folio refcount before touches ->private. But
>>> that is my interpretation.
>>
>> I mean, given that
>>
>> a) It's located in pagemap.h in between folio_attach_private() and
>> folio_detach_private()
>>
>> b) It clearly states that "The page must previously have had data
>> attached and the data must be detached before the folio will be freed."
>>
>> This is the wrong API to use?
>>
>> Sure, it sets folio->private but in different context.
>>
>> I can spot one user in mm/hugetlb.c, that likely also should not be
>> using this API, because there likely was no previous attach/detach.
>>
>>>
>>> BTW, do you know why we have set_page_private() but no folio_set_private()?
>>> I would suggest folio_set_private() if it exists.
>>
>> folio_set_private() sets ... PG_private. :)
>>
>> folio_test_private() checks PG_private and folio_get_private() returns
>> page->private.
>>
>> A cursed interface.
>
> Oh man. folio_get_private() should be renamed to folio_get_private_data(),
> so that we can have folio_set_private_data().

Likely we should strive towards only using folio->private (and the API)
really for fs-private data (i.e., the pagemap.h interface), and add
proper custom members for all other use cases.

For page->private it's a different discussion (requires more work I
guess, because there are many more use cases.

--
Cheers,

David