Re: [PATCH RFC 01/35] mm: stop making SPARSEMEM_VMEMMAP user-selectable

[SeongJae Park <sj@kernel.org>]

On Thu, 21 Aug 2025 22:06:27 +0200 David Hildenbrand <david@redhat.com> 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: SeongJae Park <sj@kernel.org>


Thanks,
SJ

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