linux-kernel - Re: [PATCH v7 00/12] khugepaged: mTHP support

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Message-ID: <a956379a-7472-4972-bf3d-fe2b2b6715b5@linux.alibaba.com>
Date: Wed, 28 May 2025 20:39:10 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.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
Cc: david@...hat.com, ziy@...dia.com, lorenzo.stoakes@...cle.com,
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 mathieu.desnoyers@...icios.com, akpm@...ux-foundation.org,
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 wangkefeng.wang@...wei.com, usamaarif642@...il.com, sunnanyong@...wei.com,
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Subject: Re: [PATCH v7 00/12] khugepaged: mTHP support



On 2025/5/15 11:22, Nico Pache wrote:
> The following series provides khugepaged and madvise collapse 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 keep track of chunks of pages
> (defined by KHUGEPAGED_MTHP_MIN_ORDER) that are utilized. This info is
> tracked using a bitmap. 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 THP must be to be
> eligible. If a mTHP collapse is attempted, but contains swapped out, or
> shared pages, we dont perform the collapse.
> 
> With the default max_ptes_none=511, the code should keep its most of its
> original behavior. To exercise mTHP collapse 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 it will introduce at least 2x
> the number of pages, and on a future scan will satisfy that 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 6-9:   The mTHP patches
> Patch 10-11: Tracing/stats
> Patch 12:    Documentation

When I tested 64K mTHP collapse and disabled PMD-sized THP, I found that 
khugepaged couldn't scan and collapse 64K mTHP. I send out two fix 
patches[1], and with these patches applied, 64K mTHP collapse works 
well. I hope my two patches can be folded into your next version series 
if you think there are no issues. Thanks.

[1] 
https://lore.kernel.org/all/ac9ed6d71b439611f9c94b3506a8ce975d4636e9.1748435162.git.baolin.wang@linux.alibaba.com/