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: <20251015004543.md5x4cjtkyjzpf4b@master>
Date: Wed, 15 Oct 2025 00:45:43 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Wei Yang <richard.weiyang@...il.com>, 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 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.

>
>-- 
>Cheers
>
>David / dhildenb

-- 
Wei Yang
Help you, Help me