Re: [PATCH v1 01/36] mm: stop making SPARSEMEM_VMEMMAP user-selectable

On Thu, Aug 28, 2025 at 12:01:05AM +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).

Nice!

And I see SPARSEMEM_VMEMMAP_ENABLE is selected by the arches which support it,
as you say 64-bit (or in other words - modern :)

>
> This is a preparation for not supporting
>
> (1) folio sizes that exceed a single memory section
> (2) CMA allocations of non-contiguous page ranges

Nice. This should simplify things... :)

>
> 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).
>
> Acked-by: Zi Yan <[email protected]>
> Acked-by: Mike Rapoport (Microsoft) <[email protected]>
> Acked-by: SeongJae Park <[email protected]>
> Cc: Huacai Chen <[email protected]>
> Cc: WANG Xuerui <[email protected]>
> Cc: Madhavan Srinivasan <[email protected]>
> Cc: Michael Ellerman <[email protected]>
> Cc: Nicholas Piggin <[email protected]>
> Cc: Christophe Leroy <[email protected]>
> Cc: Paul Walmsley <[email protected]>
> Cc: Palmer Dabbelt <[email protected]>
> Cc: Albert Ou <[email protected]>
> Cc: Alexandre Ghiti <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: Andreas Larsson <[email protected]>
> Signed-off-by: David Hildenbrand <[email protected]>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <[email protected]>

> ---
>  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
>