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

[Zi Yan <ziy@nvidia.com>]

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.

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.



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(-)

-- 
2.51.0