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

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Message-ID: <ol447ofo44vwtyfwg3zrdtcdlkfzzmx4rre6qhyotmwvecnec4@usa3nonuk2sn>
Date: Tue, 16 Sep 2025 10:29:11 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: Lance Yang <lance.yang@...ux.dev>
Cc: Hugh Dickins <hughd@...gle.com>, 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()

On Tue, Sep 16, 2025 at 02:21:26PM +0800, Lance Yang wrote:
> 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 ;)

Generally, we allow minor page faults into mlocked VMAs and avoid major.
This is minor page fault territory in my view.

Also it is very similar to what compaction does and we allow compaction
of mlocked VMA by default, unless sysctl vm.compact_unevictable_allowed
is set to zero.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov