[PATCH v4 0/7] Split a folio to any lower order folios

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

Hi all,

File folio supports any order and multi-size THP is upstreamed[1], so both
file and anonymous folios can be >0 order. Currently, split_huge_page()
only splits a huge page to order-0 pages, but splitting to orders higher than
0 is going to better utilize large folios. In addition, Large Block
Sizes in XFS support would benefit from it[2]. This patchset adds support for
splitting a large folio to any lower order folios and uses it during file
folio truncate operations.

For Patch 6, Hugh did not like my approach to minimize the number of
folios for truncate[3]. I would like to get more feedback, especially
from FS people, on it to decide whether to keep it or not.

The patchset is on top of mm-everything-2024-02-13-01-26.

Changelog 
===

Since v3
---
1. Excluded shmem folios and pagecache folios without FS support from
splitting to any order (per Hugh Dickins).
2. Allowed splitting anonymous large folio to any lower order since
multi-size THP is upstreamed.
3. Adapted selftests code to new framework.

Since v2
---
1. Fixed an issue in __split_page_owner() introduced during my rebase

Since v1
---
1. Changed split_page_memcg() and split_page_owner() parameter to use order
2. Used folio_test_pmd_mappable() in place of the equivalent code

Details
===

* Patch 1 changes split_page_memcg() to use order instead of nr_pages
* Patch 2 changes split_page_owner() to use order instead of nr_pages
* Patch 3 and 4 add new_order parameter split_page_memcg() and
  split_page_owner() and prepare for upcoming changes.
* Patch 5 adds split_huge_page_to_list_to_order() to split a huge page
  to any lower order. The original split_huge_page_to_list() calls
  split_huge_page_to_list_to_order() with new_order = 0.
* Patch 6 uses split_huge_page_to_list_to_order() in large pagecache folio
  truncation instead of split the large folio all the way down to order-0.
* Patch 7 adds a test API to debugfs and test cases in
  split_huge_page_test selftests.

Comments and/or suggestions are welcome.

[1] https://lore.kernel.org/all/20231207161211.2374093-1-ryan.roberts@arm.com/
[2] https://lore.kernel.org/linux-mm/qzbcjn4gcyxla4gwuj6smlnwknz2wvo5wrjctin6eengjfqjei@lzkxv3iy6bol/
[3] https://lore.kernel.org/linux-mm/9dd96da-efa2-5123-20d4-4992136ef3ad@google.com/

Zi Yan (7):
  mm/memcg: use order instead of nr in split_page_memcg()
  mm/page_owner: use order instead of nr in split_page_owner()
  mm: memcg: make memcg huge page split support any order split.
  mm: page_owner: add support for splitting to any order in split
    page_owner.
  mm: thp: split huge page to any lower order pages (except order-1).
  mm: truncate: split huge page cache page to a non-zero order if
    possible.
  mm: huge_memory: enable debugfs to split huge pages to any order.

 include/linux/huge_mm.h                       |  21 +-
 include/linux/memcontrol.h                    |   4 +-
 include/linux/page_owner.h                    |  10 +-
 mm/huge_memory.c                              | 149 +++++++++---
 mm/memcontrol.c                               |  10 +-
 mm/page_alloc.c                               |   8 +-
 mm/page_owner.c                               |   8 +-
 mm/truncate.c                                 |  21 +-
 .../selftests/mm/split_huge_page_test.c       | 223 +++++++++++++++++-
 9 files changed, 382 insertions(+), 72 deletions(-)

-- 
2.43.0