linux-kernel - Re: [PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs

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Message-ID: <e0f9754b-ef24-4821-8b41-acb8a1d842f3@redhat.com>
Date: Wed, 3 Sep 2025 08:52:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org, kas@...nel.org,
 willy@...radead.org, hughd@...gle.com
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, baohua@...nel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: Enable khugepaged to operate on non-writable VMAs

On 03.09.25 07:46, Dev Jain wrote:
> Currently khugepaged does not collapse a region which does not have a
> single writable page. This is wasteful since non-writable VMAs mapped by
> the application won't benefit from THP collapse. Therefore, remove this
> restriction and allow khugepaged to collapse a VMA with arbitrary
> protections.
> 
> Along with this, currently MADV_COLLAPSE does not perform a collapse on a
> non-writable VMA, and this restriction is nowhere to be found on the
> manpage - the restriction itself sounds wrong to me since the user knows
> the protection of the memory it has mapped, so collapsing read-only
> memory via madvise() should be a choice of the user which shouldn't
> be overriden by the kernel.
> 
> On an arm64 machine, an average of 5% improvement is seen on some mmtests
> benchmarks, particularly hackbench, with a maximum improvement of 12%.
> 
> Signed-off-by: Dev Jain <dev.jain@....com>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb