[PATCH mm-unstable v15 00/13] khugepaged: mTHP support

Nico Pache
2026-02-26T03:18:12Z

This series depends on [1], which provides some cleanup and prereqs.
Some of those patches used to belong to the V14 of this series.

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 no mTHP collapse will be attempted. 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 limitiations 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 1-5: Generalize khugepaged functions for arbitrary orders and
introduce some helper functions
Patch 6: Skip collapsing mTHP to smaller orders
Patch 7-8: Add per-order mTHP statistics and tracepoints
Patch 9: Introduce collapse_allowable_orders
Patch 10-12: Introduce bitmap and mTHP collapse support, fully enabled
Patch 13: 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[2] (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 [4] 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.

V15 changes:
- Split the series into two [1] to ease review, and keep this series
fully khugepaged related (David, Lorenzo)
- Refactored collapse_max_ptes_none to remove the full_scan boolean arg
moving the logic to the caller (Lorenzo)
- added /*bool=*/ comments to ambiguous function arguments (Lorenzo)
- A few changes that were requested in v14 were done in [1], such as
introducing map_anon_folio_pte_(no)pf, defining the
COLLAPSE_MAX_PTES_LIMIT macro, and the fixup of the writeback retry
logic. These changes were noted in the v1 of the cleanup series [1].
Some of these requested changes are leveraged in this series
(is_pmd_order, DEFINE usage, and map_anon_folio_pte_(no)pf).

V14: https://lore.kernel.org/lkml/20260122192841.128719-1-npache@redhat.com/
V13: https://lore.kernel.org/lkml/20251201174627.23295-1-npache@redhat.com/
V12: https://lore.kernel.org/lkml/20251022183717.70829-1-npache@redhat.com/
V11: https://lore.kernel.org/lkml/20250912032810.197475-1-npache@redhat.com/
V10: https://lore.kernel.org/lkml/20250819134205.622806-1-npache@redhat.com/
V9 : https://lore.kernel.org/lkml/20250714003207.113275-1-npache@redhat.com/
V8 : https://lore.kernel.org/lkml/20250702055742.102808-1-npache@redhat.com/
V7 : https://lore.kernel.org/lkml/20250515032226.128900-1-npache@redhat.com/
V6 : https://lore.kernel.org/lkml/20250515030312.125567-1-npache@redhat.com/
V5 : https://lore.kernel.org/lkml/20250428181218.85925-1-npache@redhat.com/
V4 : https://lore.kernel.org/lkml/20250417000238.74567-1-npache@redhat.com/
V3 : https://lore.kernel.org/lkml/20250414220557.35388-1-npache@redhat.com/
V2 : https://lore.kernel.org/lkml/20250211003028.213461-1-npache@redhat.com/
V1 : https://lore.kernel.org/lkml/20250108233128.14484-1-npache@redhat.com/

A big thanks to everyone that has reviewed, tested, and participated in
the development process. Its been a great experience working with all of
you on this endeavour.

[1] - https://lore.kernel.org/lkml/20260226012929.169479-1-npache@redhat.com/
[2] - https://gitlab.com/npache/khugepaged_mthp_test
[3] - https://lore.kernel.org/lkml/20260212021835.17755-1-npache@redhat.com/
[4] - https://lore.kernel.org/lkml/20250515033857.132535-1-npache@redhat.com/

Baolin Wang (1):
mm/khugepaged: run khugepaged for all orders

Dev Jain (1):
mm/khugepaged: generalize alloc_charge_folio()

Nico Pache (11):
mm/khugepaged: generalize hugepage_vma_revalidate for mTHP support
mm/khugepaged: generalize __collapse_huge_page_* for mTHP support
mm/khugepaged: introduce collapse_max_ptes_none helper function
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 | 80 +++-
include/linux/huge_mm.h | 5 +
include/trace/events/huge_memory.h | 34 +-
mm/huge_memory.c | 11 +
mm/khugepaged.c | 519 +++++++++++++++++----
5 files changed, 522 insertions(+), 127 deletions(-)

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2.53.0