[PATCH v12 mm-new 00/15] khugepaged: mTHP support

Nico Pache
2025-10-22T18:38:31Z

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 do
binary recursion on 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. When no mTHP size is enabled, the
legacy behavior of khugepaged is maintained. max_ptes_none will be scaled
by the attempted collapse order to determine how full a mTHP must be to be
eligible for the collapse to occur. 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.

When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on
4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for
mTHP collapses 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: Refactor/rename hpage_collapse
Patch 2: Refactoring to combine madvise_collapse and khugepaged
Patch 3-7: Generalize khugepaged functions for arbitrary orders
Patch 8: skip collapsing mTHP to smaller orders
Patch 9-10: Add per-order mTHP statistics and tracepoints
Patch 11: Introduce collapse_allowable_orders
Patch 12-14: Introduce bitmap and mTHP collapse support
Patch 15: Documentation

---------
Testing
---------
- Built for x86_64, aarch64, ppc64le, and s390x
- 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 tested 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.

V12 Changes:
- variable cleanups & minor nits
- Documentation updates to be less specific about the internal workings
- Added Review by Baolin (thanks!)
- Improved readability of the collapse_max_ptes_none function
- Improved collapse_scan_pmd max_ptes_none logic using the helper

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

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

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

Nico Pache (13):
khugepaged: rename hpage_collapse_* to collapse_*
introduce collapse_single_pmd to unify khugepaged and madvise_collapse
khugepaged: generalize hugepage_vma_revalidate for mTHP support
khugepaged: generalize __collapse_huge_page_* for mTHP support
khugepaged: introduce collapse_max_ptes_none helper function
khugepaged: generalize collapse_huge_page for mTHP collapse
khugepaged: skip collapsing mTHP to smaller orders
khugepaged: add per-order mTHP collapse failure statistics
khugepaged: improve tracepoints for mTHP orders
khugepaged: introduce collapse_allowable_orders helper function
khugepaged: Introduce mTHP collapse support
khugepaged: avoid unnecessary mTHP collapse attempts
Documentation: mm: update the admin guide for mTHP collapse

Documentation/admin-guide/mm/transhuge.rst | 84 ++-
include/linux/huge_mm.h | 5 +
include/linux/khugepaged.h | 2 +
include/trace/events/huge_memory.h | 34 +-
mm/huge_memory.c | 11 +
mm/khugepaged.c | 614 +++++++++++++++------
mm/mremap.c | 2 +-
7 files changed, 539 insertions(+), 213 deletions(-)

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2.51.0