linux-kernel - Re: [PATCH v3 20/20] mm: stop maintaining the per-page mapcount of large folios (CONFIG_NO_PAGE

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Message-ID: <d16132cd-81c5-4655-a788-0797553238bf@redhat.com>
Date: Mon, 20 Oct 2025 15:53:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, cgroups@...r.kernel.org, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>,
 Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Jonathan Corbet <corbet@....net>, Andy Lutomirski <luto@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Muchun Song <muchun.song@...ux.dev>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v3 20/20] mm: stop maintaining the per-page mapcount of
 large folios (CONFIG_NO_PAGE_MAPCOUNT)

On 15.10.25 02:45, Wei Yang wrote:
> On Tue, Oct 14, 2025 at 04:38:38PM +0200, David Hildenbrand wrote:
>> On 14.10.25 16:32, Matthew Wilcox wrote:
>>> On Tue, Oct 14, 2025 at 02:59:30PM +0200, David Hildenbrand wrote:
>>>>> As commit 349994cf61e6 mentioned, we don't support partially mapped PUD-sized
>>>>> folio yet.
>>>>
>>>> We do support partially mapped PUD-sized folios I think, but not anonymous
>>>> PUD-sized folios.
>>>
>>> I don't think so?  The only mechanism I know of to allocate PUD-sized
>>> chunks of memory is hugetlb, and that doesn't permit partial mappings.
>>
>> Greetings from the latest DAX rework :)
> 
> After a re-think, do you think it's better to align the behavior between
> CONFIG_NO_PAGE_MAPCOUNT and CONFIG_PAGE_MAPCOUNT?
> 
> It looks we treat a PUD-sized folio partially_mapped if CONFIG_NO_PAGE_MAPCOUNT,
> but !partially_mapped if CONFIG_PAGE_MAPCOUNT, if my understanding is correct.

I'd just leave it alone unless there is a problem right now.

-- 
Cheers

David / dhildenb