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

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Message-ID: <ae9e3d36-4186-44e4-ba1e-30f9f73acc18@kernel.org>
Date: Thu, 8 Jan 2026 17:09:49 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Nico Pache <npache@...hat.com>, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v13 mm-new 00/16] khugepaged: mTHP support

On 12/1/25 18:46, 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

These two patches could get queued already separately, right?

-- 
Cheers

David