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

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Message-ID: <8cd717a3-b149-4eb9-88d2-ea4b9b1112f5@lucifer.local>
Date: Tue, 16 Sep 2025 10:58:37 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: Lance Yang <lance.yang@...ux.dev>, Hugh Dickins <hughd@...gle.com>,
        akpm@...ux-foundation.org, david@...hat.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 10:48:22AM +0100, Kiryl Shutsemau wrote:
> On Tue, Sep 16, 2025 at 10:39:53AM +0100, Lorenzo Stoakes wrote:
> > On Tue, Sep 16, 2025 at 10:29:11AM +0100, Kiryl Shutsemau wrote:
> > > 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.
> >
> > Hm, but we won't be causing minor faults via reclaim right, since they're
> > not on any LRU?
>
> PTEs are still present when we do THP allocation. No reclaim while the
> access is blocked. We only block the access on copy and PTEs->PMD
> collapse.

Right indeed, esp. with compaction being allowed for mlock, I agree with you
that this patch should be dropped :)

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov

Cheers, Lorenzo