[RFC PATCH 00/11] add shmem mTHP collapse support

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Based on mm/mm-new from today.

This is a follow-up patchset for mTHP collapse to support shmem (or file pages)
mTHP collapse, which is based on Nico's patchset [1].

The strategy for shmem/file mTHP collapse follows the anonymous mTHP collapse,
which is, quoting from Nico:

"while scanning PMD ranges for potential collapse candidates, keep
track of pages in KHUGEPAGED_MIN_MTHP_ORDER chunks via a bitmap. Each bit
represents a utilized region of order KHUGEPAGED_MIN_MTHP_ORDER PTEs.

After the scan is complete, we will perform binary recursion on the bitmap
to determine which mTHP size would be most efficient to collapse to. The
'max_ptes_none' will be scaled by the attempted collapse order to determine
how full a THP must be to be eligible.
"

Moreover, to facilitate the scanning of shmem/file folios, extend the
'cc->mthp_bitmap_temp' bitmap to record whether each index within the
PMD range corresponds to a present page, and then this temp bitmap is used
to determine whether each chunk should be marked as present for mTHP
collapse.

Currently, the collapse_pte_mapped_thp() does not build the mapping for mTHP.
Cause we still expect to establish the mTHP mapping via refault under the
control of fault_around. So collapse_pte_mapped_thp() remains responsible
only for building the mapping for PMD-sized THP, which is reasonable and
makes life easier.

In addition, I have added mTHP collapse selftests, and now all khugepaged
test cases can pass.

[1] https://lore.kernel.org/all/20250819134205.622806-1-npache@redhat.com/

Baolin Wang (11):
  mm: khugepaged: add khugepaged_max_ptes_none check in collapse_file()
  mm: khugepaged: generalize collapse_file for mTHP support
  mm: khugepaged: add an order check for THP statistics
  mm: khugepaged: add shmem/file mTHP collapse support
  mm: shmem: kick khugepaged for enabling none-PMD-sized shmem mTHPs
  mm: khugepaged: allow khugepaged to check all shmem/file large orders
  mm: khugepaged: skip large folios that don't need to be collapsed
  selftests:mm: extend the check_huge() to support mTHP check
  selftests: mm: move gather_after_split_folio_orders() into vm_util.c
    file
  selftests: mm: implement the mTHP hugepage check helper
  selftests: mm: add mTHP collapse test cases

 include/linux/shmem_fs.h                      |   4 +-
 mm/khugepaged.c                               | 177 +++++++++++++----
 mm/shmem.c                                    |  10 +-
 tools/testing/selftests/mm/khugepaged.c       | 162 ++++++++++++----
 tools/testing/selftests/mm/run_vmtests.sh     |   4 +
 .../selftests/mm/split_huge_page_test.c       | 135 +------------
 tools/testing/selftests/mm/uffd-common.c      |   4 +-
 tools/testing/selftests/mm/vm_util.c          | 179 +++++++++++++++++-
 tools/testing/selftests/mm/vm_util.h          |   6 +-
 9 files changed, 455 insertions(+), 226 deletions(-)

-- 
2.43.5