Re: [PATCH mm-unstable v19 00/14] khugepaged: add mTHP collapse support

[Lorenzo Stoakes <ljs@kernel.org>]

On Fri, Jun 05, 2026 at 08:07:02PM +0200, David Hildenbrand (Arm) wrote:
> On 6/5/26 18:14, 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 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_possible_orders helper functions
> > Patch 11-13: Introduce bitmap and mTHP collapse support, fully enabled
> > Patch 14:    Documentation
> >
>
> Went through it and didn't find any blockers. Let's wait for Lorenzo's assessment.
>
> If he also doesn't find anything major, I think we can move forward with merging
> it and handle smaller things as follow-ups.

All LGTM :)

Unless sashiko tells us about something utterly broken (trivial things meh,
existing things meh), or we see some massive breakage in testing, I
think... *drum roll*

We're good to take this this cycle :)

Thanks Nico for your patience during this process and obviously David and Lance
and all the other reviews for taking part.

We got there in the end :)

>
> --
> Cheers,
>
> David

Cheers, Lorenzo