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: <aO5fCT62gZZw9-wQ@casper.infradead.org>
Date: Tue, 14 Oct 2025 15:32:41 +0100
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: 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 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.