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

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Message-ID: <zpmk35ljrb7qlfel32bpiomcj3y3ugyz6p35x43oryjigizxvm@jrmzg7ao6omq>
Date: Wed, 3 Sep 2025 10:03:54 +0100
From: Kiryl Shutsemau <kas@...nel.org>
To: Dev Jain <dev.jain@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, willy@...radead.org, 
	hughd@...gle.com, 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 Wed, Sep 03, 2025 at 11:16:34AM +0530, 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>

Reviewed-by: Kiryl Shutsemau <kas@...nel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov