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@kernel.org>
> Cc: WANG Xuerui <kernel@xen0n.name>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Paul Walmsley <paul.walmsley@sifive.com>
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Cc: Albert Ou <aou@eecs.berkeley.edu>
> Cc: Alexandre Ghiti <alex@ghiti.fr>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Andreas Larsson <andreas@gaisler.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.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.