Re: [PATCH mm-hotfixes-unstable v18 00/14] khugepaged: add mTHP collapse support

[Nico Pache]

On Fri, May 22, 2026 at 8:59 AM Nico Pache <npache@xxxxxxxxxx> wrote:
>
> The following series provides khugepaged with the capability to collapse
> anonymous memory regions to mTHPs.
>
> To achieve this we generalize the khugepaged functions to no longer depend
> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual
> pages that are occupied (!none/zero). After the PMD scan is done, we use
> the bitmap to find the optimal mTHP sizes for the PMD range. The
> restriction on max_ptes_none is removed during the scan, to make sure we
> account for the whole PMD range in the bitmap. When no mTHP size is
> enabled, the legacy behavior of khugepaged is maintained.
>
> We currently only support max_ptes_none values of 0 or HPAGE_PMD_NR - 1
> (ie 511). If any other value is specified, the kernel will emit a warning
> and mTHP collapse will default to max_ptes_none=0. If a mTHP collapse is
> attempted, but contains swapped out, or shared pages, we don't perform
> the collapse.
> It is now also possible to collapse to mTHPs without requiring the PMD THP
> size to be enabled. These limitations are to prevent collapse "creep"
> behavior. This prevents constantly promoting mTHPs to the next available
> size, which would occur because a collapse introduces more non-zero pages
> that would satisfy the promotion condition on subsequent scans.
>
> Patch 1-2:   Generalize hugepage_vma_revalidate and alloc_charge_folio
>              for arbitrary orders.
> Patch 3:     Rework max_ptes_* handling into helper functions
> Patch 4:     Generalize __collapse_huge_page_* for mTHP support
> Patch 5:     Require collapse_huge_page to enter/exit with the lock dropped
> Patch 6:     Generalize collapse_huge_page for mTHP collapse
> Patch 7:     Skip collapsing mTHP to smaller orders
> Patch 8-9:   Add per-order mTHP statistics and tracepoints
> Patch 10:    Introduce collapse_allowable_orders helper function
> Patch 11-13: Introduce bitmap and mTHP collapse support, fully enabled
> Patch 14:    Documentation
>
> Testing:
> - Built for x86_64, aarch64, ppc64le, and s390x
> - ran all arches on test suites provided by the kernel-tests project
> - internal testing suites: functional testing and performance testing
> - selftests mm
> - I created a test script that I used to push khugepaged to its limits
>    while monitoring a number of stats and tracepoints. The code is
>    available here[1] (Run in legacy mode for these changes and set mthp
>    sizes to inherit)
>    The summary from my testings was that there was no significant
>    regression noticed through this test. In some cases my changes had
>    better collapse latencies, and was able to scan more pages in the same
>    amount of time/work, but for the most part the results were consistent.
> - redis testing. I did some testing with these changes along with my defer
>   changes (see followup [2] post for more details). We've decided to get
>   the mTHP changes merged first before attempting the defer series.
> - some basic testing on 64k page size.
> - lots of general use.
>
> [1] - https://gitlab.com/npache/khugepaged_mthp_test
> [2] - https://lore.kernel.org/lkml/20250515033857.132535-1-npache@xxxxxxxxxx/
>
> V18 Changes:
> - Added RBs/Acks
> - [patch 02] Guard count_memcg_folio_events with is_pmd_order() to keep
>   THP_COLLAPSE_ALLOC PMD-only (Usama, Lance)
> - [patch 03] Convert C++ comments to C-style; fix "none-page" terminology
>   to "empty PTEs or PTEs mapping the shared zeropage"; drop unnecessary
>   userfaultfd comment; add const to local max_ptes_* variables; fix
>   "repect" typo (Lance, David)
> - [patch 04] collapse_max_ptes_none() now returns 0 instead of -EINVAL for
>   unsupported values; remove SCAN_INVALID_PTES_NONE; change return type
>   from int to unsigned int and propagate to all callers; add comment above
>   __collapse_huge_page_swapin explaining mTHP swap bail-out (David,
>   Lorenzo, Lance, Wei Yang, Usama)
> - [patch 05] Rewrite collapse_huge_page lock comment to David's suggested
>   wording (David)
> - [patch 11] Propagate unsigned int return type for max_ptes_none; remove
>   the now-unnecessary negative return check (consequence of patch 04);
>   Add optimization to the next_order goto that will prevent unnecessary
>   iterations if there are no lower orders enabled (Vernon); update locking
>   comment; pass VMA to mthp_collapse to improve uffd-armed detection, and
>   prevent unnecessary work. (Wei)
> - [patch 14] Update documentation to reflect fallback-to-0 behavior
>
> V17: https://lore.kernel.org/all/20260511185817.686831-1-npache@xxxxxxxxxx
> V16: https://lore.kernel.org/all/20260419185750.260784-1-npache@xxxxxxxxxx
> V15: https://lore.kernel.org/all/20260226031741.230674-1-npache@xxxxxxxxxx
> V14: https://lore.kernel.org/all/20260122192841.128719-1-npache@xxxxxxxxxx
> V13: https://lore.kernel.org/all/20251201174627.23295-1-npache@xxxxxxxxxx
> V12: https://lore.kernel.org/all/20251022183717.70829-1-npache@xxxxxxxxxx
> V11: https://lore.kernel.org/all/20250912032810.197475-1-npache@xxxxxxxxxx
> V10: https://lore.kernel.org/all/20250819134205.622806-1-npache@xxxxxxxxxx
> V9 : https://lore.kernel.org/all/20250714003207.113275-1-npache@xxxxxxxxxx
> V8 : https://lore.kernel.org/all/20250702055742.102808-1-npache@xxxxxxxxxx
> V7 : https://lore.kernel.org/all/20250515032226.128900-1-npache@xxxxxxxxxx
> V6 : https://lore.kernel.org/all/20250515030312.125567-1-npache@xxxxxxxxxx
> V5 : https://lore.kernel.org/all/20250428181218.85925-1-npache@xxxxxxxxxx
> V4 : https://lore.kernel.org/all/20250417000238.74567-1-npache@xxxxxxxxxx
> V3 : https://lore.kernel.org/all/20250414220557.35388-1-npache@xxxxxxxxxx
> V2 : https://lore.kernel.org/all/20250211003028.213461-1-npache@xxxxxxxxxx
> V1 : https://lore.kernel.org/all/20250108233128.14484-1-npache@xxxxxxxxxx
>
> Baolin Wang (1):
>   mm/khugepaged: run khugepaged for all orders
>
> Dev Jain (1):
>   mm/khugepaged: generalize alloc_charge_folio()
>
> Nico Pache (12):
>   mm/khugepaged: generalize hugepage_vma_revalidate for mTHP support
>   mm/khugepaged: rework max_ptes_* handling with helper functions
>   mm/khugepaged: generalize __collapse_huge_page_* for mTHP support
>   mm/khugepaged: require collapse_huge_page to enter/exit with the lock
>     dropped
>   mm/khugepaged: generalize collapse_huge_page for mTHP collapse
>   mm/khugepaged: skip collapsing mTHP to smaller orders
>   mm/khugepaged: add per-order mTHP collapse failure statistics
>   mm/khugepaged: improve tracepoints for mTHP orders
>   mm/khugepaged: introduce collapse_allowable_orders helper function
>   mm/khugepaged: Introduce mTHP collapse support
>   mm/khugepaged: avoid unnecessary mTHP collapse attempts
>   Documentation: mm: update the admin guide for mTHP collapse
>
>  Documentation/admin-guide/mm/transhuge.rst |  72 ++-
>  include/linux/huge_mm.h                    |   5 +
>  include/trace/events/huge_memory.h         |  34 +-
>  mm/huge_memory.c                           |  11 +
>  mm/khugepaged.c                            | 634 ++++++++++++++++-----
>  5 files changed, 584 insertions(+), 172 deletions(-)
>
>
> base-commit: 6c8cb505a5634594b3ea159fd1c71bce2acf3346

Whoops I manually changed the coverletter subject to reflect that this
in on mm-hotfixes-unstable but never updated the others...

Hopefully that is ok. Just a small mistake. Base commit is referenced here.

-- Nico


> --
> 2.54.0
>