[PATCH v13 mm-new 00/16] khugepaged: mTHP support

Nico Pache
2025-12-01T17:47:33Z

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: Refactor/rename hpage_collapse
Patch 2: Refactoring to combine madvise_collapse and khugepaged
Patch 3-8: Generalize khugepaged functions for arbitrary orders and
introduce some helper functions
Patch 9: skip collapsing mTHP to smaller orders
Patch 10-11: Add per-order mTHP statistics and tracepoints
Patch 12: Introduce collapse_allowable_orders
Patch 13-15: Introduce bitmap and mTHP collapse support, fully enabled
Patch 16: 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.

V13 Changes:
- Lots of minor nits, cleanups, comments, and renames
- Bitmap function simplification and more helpers (Wei, Lorenzo)
- Max_ptes_none (0 or 511) restriction
- commit description expansion
- list all reachable enum values in mthp_collapse()
- Fix ppc64 compile error due to using HPAGE_PMD_ORDER (replace with
ilog2(MAX_PTRS_PER_PTE))

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://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 (14):
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: introduce is_mthp_order helper
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 | 80 ++-
include/linux/huge_mm.h | 5 +
include/trace/events/huge_memory.h | 34 +-
mm/huge_memory.c | 11 +
mm/khugepaged.c | 696 +++++++++++++++------
mm/mremap.c | 2 +-
6 files changed, 618 insertions(+), 210 deletions(-)

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2.51.1