Re: [RFC PATCH 3/3] powerpc/kho: exclude THP and HUGETLB_PAGE

[Pratyush Yadav <pratyush@kernel.org>]

On Fri, Aug 21 2026, Sourabh Jain wrote:

> Enabling CONFIG_KEXEC_HANDOVER together with CONFIG_TRANSPARENT_HUGEPAGE
> or CONFIG_HUGETLB_PAGE on powerpc fails to build:
>
>     kernel/liveupdate/kexec_handover.c:49:39: error: expression in static assertion is not constant
>        49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
>     ./include/linux/build_bug.h:80:56: note: in definition of macro '__static_assert'
>        80 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
>           |                                                        ^~~~
>     kernel/liveupdate/kexec_handover.c:49:1: note: in expansion of macro 'static_assert'
>        49 | static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
>
> CMA_MIN_ALIGNMENT_BYTES is derived from pageblock_order, which on
> powerpc is not a compile-time constant when THP or HUGETLB_PAGE is
> enabled -- powerpc can determine huge page size at runtime in that
> configuration, so pageblock_order becomes a variable rather than a
> constant expression. KHO's static_assert() on
> SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES then fails to
> compile, since it requires both sides to be constant expressions.
>
> Until CMA_MIN_ALIGNMENT_BYTES (or KHO's use of it) is made to handle
> the non-constant pageblock_order case on powerpc, exclude
> TRANSPARENT_HUGEPAGE and HUGETLB_PAGE from ARCH_SUPPORTS_KEXEC_HANDOVER
> so the two can't be enabled together and hit this build failure.

This should be a part of patch 2 I think. It is not a good idea to break
the build in one patch and fix it in the next.

That said, this is a debug check of sorts since SCRATCH_ALIGNMENT_BYTES
is of MAX_ORDER_NR_PAGES, and pageblocks _should_ always be smaller of
equal to this.

Anyway, I don't think this is something you should gate by a config. I
think you should either remove the static_assert(), or turn it into a
runtime check.

+Cc Michal.

>
> Cc: Aditya Gupta <adityag@linux.ibm.com>
> Cc: Alexander Graf <graf@amazon.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Baoquan He <baoquan.he@linux.dev>
> Cc: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
> Cc: Hari Bathini <hbathini@linux.ibm.com>
> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
> Cc: Pratyush Yadav <pratyush@kernel.org>
> Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
> Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
> Cc: kexec@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>  arch/powerpc/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 61350d3e7a19..f68b9fc883ba 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -720,6 +720,7 @@ config ARCH_SUPPORTS_KEXEC_HANDOVER
>  	def_bool y
>  	depends on PPC64
>  	depends on !CRASH_DUMP
> +	depends on !TRANSPARENT_HUGEPAGE && !HUGETLB_PAGE
>  
>  config ARCH_SUPPORTS_CRASH_HOTPLUG
>  	def_bool y

-- 
Regards,
Pratyush Yadav