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

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Message-ID: <b92419d3-c8ae-47b0-b9f2-0c0c46057cf9@linux.dev>
Date: Mon, 8 Sep 2025 19:32:52 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: akpm@...ux-foundation.org, david@...hat.com, 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 2025/9/8 18:38, 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?

Good catch!

Surprisingly, my test shows that KSM is indeed able to replace a zeroed
mlocked THP with ZERO_PAGE(). And MADV_MERGEABLE allows a VMA to be marked
as mergeable even if it's already locked(VM_LOCKED) ...