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

[Sourabh Jain]

On 21/08/26 17:04, Pratyush Yadav wrote:
> 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.

Agree. Since this is an RFC patch series, I kept it separate so that 
this issue could
be reviewed independently from patch 2. Going forward, I will merge them.


> 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.

Agreed. I will evaluate both options and decide which approach makes 
more sense.

Thanks for the review Pratyush.

- Sourabh Jain


> +Cc Michal.
>
> > Cc: Aditya Gupta <adityag@xxxxxxxxxxxxx>
> > Cc: Alexander Graf <graf@xxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Baoquan He <baoquan.he@xxxxxxxxx>
> > Cc: Christophe Leroy (CS GROUP) <chleroy@xxxxxxxxxx>
> > Cc: Hari Bathini <hbathini@xxxxxxxxxxxxx>
> > Cc: Madhavan Srinivasan <maddy@xxxxxxxxxxxxx>
> > Cc: Mahesh Salgaonkar <mahesh@xxxxxxxxxxxxx>
> > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> > Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> > Cc: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > Cc: Pratyush Yadav <pratyush@xxxxxxxxxx>
> > Cc: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> > Cc: Shivang Upadhyay <shivangu@xxxxxxxxxxxxx>
> > Cc: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > Cc: kexec@xxxxxxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>
> > ---
> >   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