linux-kernel - Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with max_ptes

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Message-ID: <ebb2a888-a111-4c27-925e-58bfa8e07852@redhat.com>
Date: Fri, 5 Sep 2025 17:36:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Dev Jain <dev.jain@....com>,
 Barry Song <baohua@...nel.org>, Usama Arif <usamaarif642@...il.com>
Subject: Re: [PATCH v1] mm/huge_memory: fix shrinking of all-zero THPs with
 max_ptes_none default

On 05.09.25 17:30, Lorenzo Stoakes wrote:
> On Fri, Sep 05, 2025 at 04:11:37PM +0200, David Hildenbrand wrote:
>> We added an early exit in thp_underused(), probably to avoid scanning
>> pages when there is no chance for success.
>>
>> However, assume we have max_ptes_none = 511 (default).
>>
>> Nothing should stop us from freeing all pages part of a THP that
> 
> Freeing 'all pages which are part of a THP' rather?

I'm German, I don't know what I'm doing. :D

> 
>> is completely zero (512) and khugepaged will for sure not try to
> 
> that is -> that are?

the THP is zero?

> 
>> instantiate a THP in that case (512 shared zeropages).
> 
> But if you write faulted they're not the zero page? And how are they shared? I
> mean be being dumb here.

The shrinker will replace zeroed pages by the shared zeropages.

> 
>>
>> This can just trivially happen if someone writes a single 0 byte into a
>> PMD area, or of course, when data ends up being zero later.
>>
>> So let's remove that early exit.
>>
>> Do we want to CC stable? Hm, not sure. Probably not urgent.
> 
> Surely this is worth having?

Alrighty, let me cc stable.

> 
>>
>> Note that, as default, the THP shrinker is active
>> (/sys/kernel/mm/transparent_hugepage/shrink_underused = 1), and all
>> THPs are added to the deferred split lists. However, with the
>> max_ptes_none default we would never scan them. We would not do that. If
> 
> Nit but 'we would not do that' is kinda duplicative here :)

Yeah, fixed it already while rewriting: this was meant to be "would now".

Thanks!

-- 
Cheers

David / dhildenb