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

Lorenzo Stoakes
2026-01-08T16:12:45Z

One nit on this series - you sent it on the 1st day of the merge window :)

We unfortunately have a suboptimal approach to the merge window in mm in my
view. Unfortunately it seems unlikely to change any time soon, so I would
simply politely ask that in future esp. with big series whether you could
send a respin within a cycle? :)

Sorry for the longer delay here btw, I've been on leave after that!
Unfortunate timing.

Note that I am scaling down my review somewhat at the moment as I am simply
disinclined to deal with the merge-by-default insanity mm engages in any
longer, but since I promised to review this I will do so.

Cheers, Lorenzo

On Mon, Dec 01, 2025 at 10:46:11AM -0700, Nico Pache 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 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(-)
>
> --
> 2.51.1
>