linux-kernel - [PATCH RFC 01/35] mm: stop making SPARSEMEM

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Message-ID: <20250821200701.1329277-2-david@redhat.com>
Date: Thu, 21 Aug 2025 22:06:27 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: David Hildenbrand <david@...hat.com>,
	Huacai Chen <chenhuacai@...nel.org>,
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Subject: [PATCH RFC 01/35] mm: stop making SPARSEMEM_VMEMMAP user-selectable

In an ideal world, we wouldn't have to deal with SPARSEMEM without
SPARSEMEM_VMEMMAP, but in particular for 32bit SPARSEMEM_VMEMMAP is
considered too costly and consequently not supported.

However, if an architecture does support SPARSEMEM with
SPARSEMEM_VMEMMAP, let's forbid the user to disable VMEMMAP: just
like we already do for arm64, s390 and x86.

So if SPARSEMEM_VMEMMAP is supported, don't allow to use SPARSEMEM without
SPARSEMEM_VMEMMAP.

This implies that the option to not use SPARSEMEM_VMEMMAP will now be
gone for loongarch, powerpc, riscv and sparc. All architectures only
enable SPARSEMEM_VMEMMAP with 64bit support, so there should not really
be a big downside to using the VMEMMAP (quite the contrary).

This is a preparation for not supporting

(1) folio sizes that exceed a single memory section
(2) CMA allocations of non-contiguous page ranges

in SPARSEMEM without SPARSEMEM_VMEMMAP configs, whereby we
want to limit possible impact as much as possible (e.g., gigantic hugetlb
page allocations suddenly fails).

Cc: Huacai Chen <chenhuacai@...nel.org>
Cc: WANG Xuerui <kernel@...0n.name>
Cc: Madhavan Srinivasan <maddy@...ux.ibm.com>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Nicholas Piggin <npiggin@...il.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Paul Walmsley <paul.walmsley@...ive.com>
Cc: Palmer Dabbelt <palmer@...belt.com>
Cc: Albert Ou <aou@...s.berkeley.edu>
Cc: Alexandre Ghiti <alex@...ti.fr>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Andreas Larsson <andreas@...sler.com>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 mm/Kconfig | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/Kconfig b/mm/Kconfig
index 4108bcd967848..330d0e698ef96 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -439,9 +439,8 @@ config SPARSEMEM_VMEMMAP_ENABLE
 	bool
 
 config SPARSEMEM_VMEMMAP
-	bool "Sparse Memory virtual memmap"
+	def_bool y
 	depends on SPARSEMEM && SPARSEMEM_VMEMMAP_ENABLE
-	default y
 	help
 	  SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise
 	  pfn_to_page and page_to_pfn operations.  This is the most
-- 
2.50.1