linux-kernel - Re: [PATCH v2 1/1] mm: skip mlocked THPs that are underused early in deferred_split

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Message-ID: <5j6i2o6umqwxabdfncbrdytmvdma4yrraxe6hu4csckcniduya@sm3mlablwbad>
Date: Mon, 8 Sep 2025 12:44:29 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: David Hildenbrand <david@...hat.com>
Cc: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org, 
	Liam.Howlett@...cle.com, baohua@...nel.org, baolin.wang@...ux.alibaba.com, 
	dev.jain@....com, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	lorenzo.stoakes@...cle.com, npache@...hat.com, ryan.roberts@....com, usamaarif642@...il.com, 
	ziy@...dia.com
Subject: Re: [PATCH v2 1/1] mm: skip mlocked THPs that are underused early in
 deferred_split_scan()

On Mon, Sep 08, 2025 at 01:32:05PM +0200, David Hildenbrand wrote:
> On 08.09.25 12:38, Kiryl Shutsemau wrote:
> > On Mon, Sep 08, 2025 at 05:07:41PM +0800, Lance Yang wrote:
> > > From: Lance Yang <lance.yang@...ux.dev>
> > > 
> > > When we stumble over a fully-mapped mlocked THP in the deferred shrinker,
> > > it does not make sense to try to detect whether it is underused, because
> > > try_to_map_unused_to_zeropage(), called while splitting the folio, will not
> > > actually replace any zeroed pages by the shared zeropage.
> > 
> > It makes me think, does KSM follows the same logic as
> > try_to_map_unused_to_zeropage()?
> > 
> > I cannot immediately find what prevents KSM from replacing zeroed mlocked
> > folio with ZERO_PAGE().
> > 
> > Hm?
> 
> I assume if you're using mlock and at the same time enable KSM for a
> process/VMA, you're doing something wrong.
> 
> In contrast, THP is supposed to be transparent (yeah, I know ...).

Yeah, I guess it is user error.

Maybe we should make ksm_compatible() return false for VM_LOCKED?
KSM breaks mlock() contract.

But it can be risky if someone already relies on this broken behaviour.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov