linux-kernel - [PATCH v2 15/20] mm: CONFIG_NO_PAGE_MAPCOUNT to prepare for not maintain per-page mapcounts in large folios

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Message-ID: <20250224165603.1434404-16-david@redhat.com>
Date: Mon, 24 Feb 2025 17:55:57 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-doc@...r.kernel.org,
	cgroups@...r.kernel.org,
	linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	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: [PATCH v2 15/20] mm: CONFIG_NO_PAGE_MAPCOUNT to prepare for not maintain per-page mapcounts in large folios

We're close to the finishing line: let's introduce a new
CONFIG_NO_PAGE_MAPCOUNT config option where we will incrementally remove
any dependencies on per-page mapcounts in large folios. Once that's
done, we'll stop maintaining the per-page mapcounts with this
config option enabled.

CONFIG_NO_PAGE_MAPCOUNT will be EXPERIMENTAL for now, as we'll have to
learn about some of the real world impact of some of the implications.

As writing "!CONFIG_NO_PAGE_MAPCOUNT" is really nasty, let's introduce
a helper config option "CONFIG_PAGE_MAPCOUNT" that expresses the
negation.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/Kconfig | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index af72fe92f96b0..64bb80a281dd9 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -889,8 +889,26 @@ config READ_ONLY_THP_FOR_FS
 	  support of file THPs will be developed in the next few release
 	  cycles.
 
+config NO_PAGE_MAPCOUNT
+	bool "No per-page mapcount (EXPERIMENTAL)"
+	help
+	  Do not maintain per-page mapcounts for pages part of larger
+	  allocations, such as transparent huge pages.
+
+	  When this config option is enabled, some interfaces that relied on
+	  this information will rely on less-precise per-allocation information
+	  instead: for example, using the average per-page mapcount in such
+	  a large allocation instead of the per-page mapcount.
+
+	  EXPERIMENTAL because the severity of some of the implications first
+	  have to be understood properly.
+
 endif # TRANSPARENT_HUGEPAGE
 
+# simple helper to make the code a bit easier to read
+config PAGE_MAPCOUNT
+	def_bool !NO_PAGE_MAPCOUNT
+
 #
 # The architecture supports pgtable leaves that is larger than PAGE_SIZE
 #
-- 
2.48.1