Re: [RFC PATCH v2 00/11] add shmem mTHP collapse support

[Nico Pache]

On Wed, Jun 10, 2026 at 4:29 AM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
>
> (Note: this patchset is not targeting v7.2, but posted for early feedback.)
>
> This is a follow-up patchset for mTHP collapse to support shmem mTHP collapse,
> which is based on Nico's patchset[1].
>
> The shmem mTHP collapse strategy follows the anonymous mTHP collapse approach:
> track present pages via a bitmap while scanning PMD ranges for collapse candidates,
> then use the bitmap after the scan completes to determine the most efficient
> mTHP order to collapse to. Built on the basic framework added for anonymous
> mTHP collapse, the shmem mTHP collapse implementation is straightforward
> (Thanks for Nico's work).

As promised I will review this series :)

>
> In addition, I have added some anon/shmem mTHP collapse selftests, and now all
> khugepaged test cases can pass.

Thank you for doing that!!

I was just working on adding the anon mTHP selftests. Any chance we
can separate out those bits and just send a series for adding anon
mTHP selftests without the file-related changes? Then your series adds
the bits you need here for shmem collapse? Thank you for doing all the
heavy lifting there to generalize those functions :)

Cheers,
-- Nico

>
> Note: I have not yet enabled large order collapse for file folios (file folios
> currently only support PMD-sized large folio collapse). Although file large order
> collapse would be more straightforward to implement after shmem mTHP collapse
> support is added (requiring some changes to file_thp_enabled()), I think this
> still need some discussion on whether it is necessary to support other large
> orders collapse for file folios.
>
> Comments are welcome. Thanks.
>
> Changes from RFC v1:
> https://lore.kernel.org/all/cover.1755677674.git.baolin.wang@xxxxxxxxxxxxxxxxx/
>  - Rebase on the new code, and update to use the new functions.
>  - Add more test cases.
>
> [1] https://lore.kernel.org/all/20260605161422.213817-1-npache@xxxxxxxxxx/
>
> Baolin Wang (11):
>   mm: khugepaged: add max_ptes_none check in collapse_file()
>   mm: khugepaged: generalize collapse_file() for shmem mTHP support
>   mm: khugepaged: add an order check for PMD-sized THP statistics
>   mm: khugepaged: add shmem mTHP collapse support
>   mm: shmem: run khugepaged for all shmem mTHP orders
>   mm: khugepaged: allow khugepaged to check all shmem mTHP-sized 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-sized hugepage check helpers
>   selftests: mm: add mTHP collapse test cases
>
>  include/linux/shmem_fs.h                      |   4 +-
>  mm/khugepaged.c                               | 174 ++++++++++++----
>  mm/shmem.c                                    |  10 +-
>  .../selftests/mm/folio_split_race_test.c      |   2 +-
>  tools/testing/selftests/mm/khugepaged.c       | 195 +++++++++++++-----
>  .../testing/selftests/mm/prctl_thp_disable.c  |   2 +-
>  tools/testing/selftests/mm/run_vmtests.sh     |   4 +
>  tools/testing/selftests/mm/soft-dirty.c       |   2 +-
>  .../selftests/mm/split_huge_page_test.c       | 139 +------------
>  tools/testing/selftests/mm/uffd-common.c      |   4 +-
>  tools/testing/selftests/mm/vm_util.c          | 184 ++++++++++++++++-
>  tools/testing/selftests/mm/vm_util.h          |   8 +-
>  12 files changed, 492 insertions(+), 236 deletions(-)
>
> --
> 2.47.3
>