[PATCH v3 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 people would like to support flexible orders
for anonymous folio[1] too. Currently, split_huge_page() only splits a huge
page to order-0 pages, but splitting to orders higher than 0 is also useful.
This patchset adds support for splitting a huge page to any lower order pages
and uses it during file folio truncate operations.

The patchset is on top of mm-everything-2023-03-27-21-20.

Changelog 
===
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/linux-mm/Y%2FblF0GIunm+pRIC@casper.infradead.org/

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.
  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                       |  10 +-
 include/linux/memcontrol.h                    |   4 +-
 include/linux/page_owner.h                    |  10 +-
 mm/huge_memory.c                              | 137 ++++++++---
 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       | 225 +++++++++++++++++-
 9 files changed, 365 insertions(+), 68 deletions(-)

-- 
2.39.2