linux-kernel - Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in khugepaged_scan_mm

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Message-ID: <a0ec4014-384b-4c04-bf0b-777c989eabcb@linux.dev>
Date: Tue, 16 Sep 2025 14:21:26 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Hugh Dickins <hughd@...gle.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
 npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
 baohua@...nel.org, ioworker0@...il.com, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in
 khugepaged_scan_mm_slot()

Hi Hugh,

Thanks for taking a look and for raising this important point!

On 2025/9/16 13:32, Hugh Dickins wrote:
> On Sun, 14 Sep 2025, Lance Yang wrote:
> 
>> From: Lance Yang <lance.yang@...ux.dev>
>>
>> Let's skip unsuitable VMAs early in the khugepaged scan; specifically,
>> mlocked VMAs should not be touched.
> 
> Why?  That's a change in behaviour, isn't it?
> 
> I'm aware that hugepage collapse on an mlocked VMA can insert a fault
> latency, not universally welcome; but I've not seen discussion, let
> alone agreement, that current behaviour should be changed.
> Somewhere in yet-to-be-read mail?  Please give us a link.
> 
> Hugh

You're right, this is indeed a change in behaviour. But it's specifically
for khugepaged.

Users of mlock() expect low and predictable latency. THP collapse is a
heavy operation that introduces exactly the kind of unpredictable delays
they want to avoid. It has to unmap PTEs, copy data from the small folios
to a new THP, and then remap the THP back to the PMD ;)

IMO, that change is acceptable because THP is generally transparent to
users, and khugepaged does not guarantee when THP collapse or split will
happen.

Well, we don't have a discussion on that, just something I noticed.

Thanks,
Lance