linux-kernel - Re: [PATCH v11 00/15] khugepaged: mTHP support

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Message-ID: <ppzgohmkll7dbf2aiwhw7f4spf6kxjtwwe3djkx26pwy4ekrnd@mgeantq5sn2z>
Date: Fri, 12 Sep 2025 13:19:05 +0100
From: Kiryl Shutsemau <kas@...nel.org>
To: Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support

On Thu, Sep 11, 2025 at 09:27:55PM -0600, 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 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.

Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count
all-zeros 4k as none_or_zero? It mirrors the logic of shrinker.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov