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

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Message-ID: <d93d1f3c-7a83-41b6-8166-be545f6270f5@redhat.com>
Date: Fri, 5 Sep 2025 17:11:48 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 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 16:11, 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
> is completely zero (512) and khugepaged will for sure not try to
> instantiate a THP in that case (512 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.
> 
> 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
> that's not desirable, we should just disable the shrinker as default,
> also not adding all THPs to the deferred split lists.
> 
> Easy to reproduce:
> 

Just a note that I will send a v2 with an updated patch description 
after the discussions here settled.

-- 
Cheers

David / dhildenb