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: <20251014122335.dpyk5advbkioojnm@master>
Date: Tue, 14 Oct 2025 12:23:35 +0000
From: Wei Yang <richard.weiyang@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: 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>,
	"Matthew Wilcox (Oracle)" <willy@...radead.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 Mon, Mar 03, 2025 at 05:30:13PM +0100, David Hildenbrand wrote:
[...]
>@@ -1678,6 +1726,22 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> 		break;
> 	case RMAP_LEVEL_PMD:
> 	case RMAP_LEVEL_PUD:
>+		if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
>+			last = atomic_add_negative(-1, &folio->_entire_mapcount);
>+			if (level == RMAP_LEVEL_PMD && last)
>+				nr_pmdmapped = folio_large_nr_pages(folio);
>+			nr = folio_dec_return_large_mapcount(folio, vma);
>+			if (!nr) {
>+				/* Now completely unmapped. */
>+				nr = folio_large_nr_pages(folio);
>+			} else {
>+				partially_mapped = last &&
>+						   nr < folio_large_nr_pages(folio);

Hi, David

Do you think this is better to be?

	partially_mapped = last && nr < nr_pmdmapped;

As commit 349994cf61e6 mentioned, we don't support partially mapped PUD-sized
folio yet.

Not sure what I missed here.

>+				nr = 0;
>+			}
>+			break;
>+		}
>+
> 		folio_dec_large_mapcount(folio, vma);
> 		last = atomic_add_negative(-1, &folio->_entire_mapcount);
> 		if (last) {
>-- 
>2.48.1
>

-- 
Wei Yang
Help you, Help me