linux-kernel - Re: [PATCH v10 00/13] khugepaged: mTHP support

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Message-ID: <f8eab519-edc5-460b-8f7a-83bf7827f502@arm.com>
Date: Fri, 5 Sep 2025 23:35:51 +0530
From: Dev Jain <dev.jain@....com>
To: Nico Pache <npache@...hat.com>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
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Subject: Re: [PATCH v10 00/13] khugepaged: mTHP support


On 19/08/25 7:11 pm, 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 chunks of
> pages (defined by KHUGEPAGED_MTHP_MIN_ORDER) that are utilized. 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.
>
> With the default max_ptes_none=511, the code should keep its most of its
> original behavior. When enabling multiple adjacent (m)THP sizes we need to
> set max_ptes_none<=255. With max_ptes_none > HPAGE_PMD_NR/2 you will
> experience collapse "creep" and constantly promote mTHPs to the next
> available size. This is due the fact that a collapse will introduce at
> least 2x the number of pages, and on a future scan will satisfy the
> promotion condition once again.
>
> Patch 1:     Refactor/rename hpage_collapse
> Patch 2:     Some refactoring to combine madvise_collapse and khugepaged
> Patch 3-5:   Generalize khugepaged functions for arbitrary orders
> Patch 6-8:   The mTHP patches
> Patch 9-10:  Allow khugepaged to operate without PMD enabled
> Patch 11-12: Tracing/stats
> Patch 13:    Documentation
>
>

For the next version, it will be really great if you can mention
the lore links referencing important ideas guiding the evolution
of the algorithm - say, a policy decision we make. (I frequently do
that albeit I think I over-do it :)) I am asking because I am
completely lost on the current discussion going on around
the max_ptes_* scaling (been busy with other stuff).