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

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Message-ID: <aKiID8i6dYrlVi5T@kernel.org>
Date: Fri, 22 Aug 2025 18:09:03 +0300
From: Mike Rapoport <rppt@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Huacai Chen <chenhuacai@...nel.org>,
	WANG Xuerui <kernel@...0n.name>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
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	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	"David S. Miller" <davem@...emloft.net>,
	Andreas Larsson <andreas@...sler.com>,
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	Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH RFC 01/35] mm: stop making SPARSEMEM_VMEMMAP
 user-selectable

On Thu, Aug 21, 2025 at 10:06:27PM +0200, David Hildenbrand wrote:
> 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>

Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>

> ---
>  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
> 

-- 
Sincerely yours,
Mike.