Re: [RFC PATCH 0/5] Separate compound page from folio

[Zi Yan <ziy@nvidia.com>]

On 2 Feb 2026, at 23:30, Balbir Singh wrote:

> On 1/30/26 14:48, Zi Yan wrote:
>> Hi all,
>>
>> Based on my discussion with Jason about device private folio
>> reinitialization[1], I realize that the concepts of compound page and folio
>> are mixed together and confusing, as people think a compound page is equal
>> to a folio. This is not true, since a compound page means a group of
>> pages is managed as a whole and it can be something other than a folio,
>> for example, a slab page. To avoid further confusing people, this
>> patchset separates compound page from folio by moving any folio related
>> code out of compound page functions.
>>
>> The code is on top of mm-new (2026-01-28-20-27) and all mm selftests
>> passed.
>>
>> The key change is that a compound page no longer sets:
>> 1. folio->_nr_pages,
>> 2. folio->_large_mapcount,
>> 3. folio->_nr_pages_mapped,
>> 4. folio->_mm_ids,
>> 5. folio->_mm_id_mapcount,
>> 6. folio->_pincount,
>> 7. folio->_entire_mapcount,
>> 8. folio->_deferred_list.
>>
>> Since these fields are only used by folios that are rmappable. The code
>> setting these fields is moved to page_rmappable_folio(). To make the
>> code move, this patchset also needs to changes several places, where
>> folio and compound page are used interchangably or unusual folio use:
>>
>> 1. in io_mem_alloc_compound(), a compound page is allocated, but later
>>    it is mapped via vm_insert_pages() like a rmappable folio;
>> 2. __split_folio_to_order() sets large_rmappable flag directly instead
>>    of using page_rmappable_folio() for after-split folios;
>> 3. hugetlb unsets large_rmappable to escape deferred_list unqueue
>>    operation.
>>
>> At last, the page freeing path is also changed to have different checks
>> for compound page and folio.
>>
>
> Thanks for doing this!
>
>
>> One thing to note is that for compound page, I do not store compound
>> order in folio->_nr_pages, which overlaps with page[1].memcg_data and
>> use 1 << compound_order() instead, since I do not want to add a new
>> union to struct page and compound_nr() is not as widely used as
>> folio_nr_pages(). But let me know if there is a performance concern for
>> this.
>>
>> Comments and suggestions are welcome.
>>
>
> What does this mean for treating compound pages as folios, does this break
> code that makes any assumptions about their interop?

Yes. All folio initialization code is moved from prep_compound_page() to
page_rmappable_folio(), so such users will see warnings on some folio fields
are not set properly. They should call page_rmappable_folio() on compound
pages they are planning to use as folios. A common use case is to
vm_insert_page(s)() on subpages of a folio.

For in-tree users, I am converting them all in this series.

>
>>
>>
>> Link: https://lore.kernel.org/all/F7E3DF24-A37B-40A0-A507-CEF4AB76C44D@nvidia.com/ [1]
>>
>> Zi Yan (5):
>>   io_uring: allocate folio in io_mem_alloc_compound() and function
>>     rename
>>   mm/huge_memory: use page_rmappable_folio() to convert after-split
>>     folios
>>   mm/hugetlb: set large_rmappable on hugetlb and avoid deferred_list
>>     handling
>>   mm: only use struct page in compound_nr() and compound_order()
>>   mm: code separation for compound page and folio
>>
>>  include/linux/mm.h | 12 ++++--------
>>  io_uring/memmap.c  | 12 ++++++------
>>  mm/huge_memory.c   |  5 ++---
>>  mm/hugetlb.c       |  8 ++++----
>>  mm/hugetlb_cma.c   |  2 +-
>>  mm/internal.h      | 47 +++++++++++++++++++++++++++-------------------
>>  mm/mm_init.c       |  2 +-
>>  mm/page_alloc.c    | 23 ++++++++++++++++++-----
>>  8 files changed, 64 insertions(+), 47 deletions(-)
>>


Best Regards,
Yan, Zi