Hi,
On Mon, Sep 28, 2026 at 02:02:25PM +0530, Sourabh Jain wrote:
> KHO requires the scratch alignment to be at least CMA_MIN_ALIGNMENT_BYTES,
> and this is currently enforced with a static_assert().
>
> That assertion breaks the build on powerpc once CONFIG_KEXEC_HANDOVER is
> enabled together with CONFIG_TRANSPARENT_HUGEPAGE or CONFIG_HUGETLB_PAGE:
>
> 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 depends on pageblock_order, which is not a
> compile-time constant with certain configs, such as
> CONFIG_HUGETLB_PAGE_SIZE_VARIABLE on powerpc. The next patch in this
> series enables KHO on powerpc, so the alignment must be checked at
> runtime instead.
>
> Replace the static_assert() with a runtime check at the start of
> kho_reserve_scratch(), before any scratch memory is reserved. If the
> scratch alignment is smaller than the CMA minimum alignment, warn once
> and disable KHO rather than continue with a misaligned reservation.
Can we keep static_assert() for everything !powerpc please?
Like something along these lines:
#ifndef CONFIG_HUGETLB_PAGE_SIZE_VARIABLE
#define check_scratch_alignment static_assert(...)
#else
static inline bool check_scratch_alignment()
{
...
}
#endif
> Cc: Aditya Gupta <[email protected]>
> Cc: Alexander Graf <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Baoquan He <[email protected]>
> Cc: Christophe Leroy (CS GROUP) <[email protected]>
> Cc: Hari Bathini <[email protected]>
> Cc: Madhavan Srinivasan <[email protected]>
> Cc: Mahesh Salgaonkar <[email protected]>
> Cc: Michael Ellerman <[email protected]>
> Cc: Mike Rapoport <[email protected]>
> Cc: Nicholas Piggin <[email protected]>
> Cc: Pasha Tatashin <[email protected]>
> Cc: Pratyush Yadav <[email protected]>
> Cc: Ritesh Harjani (IBM) <[email protected]>
> Cc: Shivang Upadhyay <[email protected]>
> Cc: Shrikanth Hegde <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Sourabh Jain <[email protected]>
> ---
> kernel/liveupdate/kexec_handover.c | 23 +++++++++++++++--------
> 1 file changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/liveupdate/kexec_handover.c
> b/kernel/liveupdate/kexec_handover.c
> index 7c4d86daf86d..dc76dd6b74da 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -39,15 +39,7 @@
> #include "../kexec_internal.h"
> #include "kexec_handover_internal.h"
>
> -/*
> - * This is the minimal alignment required by deferred struct page init.
> - * deferred_init_memmap_chunk frees memory to the buddy allocator, which
> looks
> - * at the neighboring pages (up to MAX_PAGE_ORDER) to merge them.
> - * If KHO scratch is not aligned to that value, buddy can access
> uninitialized
> - * struct pages, which can cause a crash.
> - */
> #define SCRATCH_ALIGNMENT_BYTES (PAGE_SIZE * MAX_ORDER_NR_PAGES)
> -static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
>
> /* The magic token for preserved pages */
> #define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
> @@ -836,6 +828,21 @@ static void __init kho_reserve_scratch(void)
> if (!kho_enable)
> return;
>
> + /*
> + * This is the minimal alignment required by deferred struct page init.
> + * deferred_init_memmap_chunk frees memory to the buddy allocator,
> which looks
> + * at the neighboring pages (up to MAX_PAGE_ORDER) to merge them.
> + * If KHO scratch is not aligned to that value, buddy can access
> uninitialized
> + * struct pages, which can cause a crash.
> + */
> + if (WARN_ON_ONCE(SCRATCH_ALIGNMENT_BYTES < CMA_MIN_ALIGNMENT_BYTES)) {
> + pr_err("scratch alignment %lu < CMA minimum %lu, disabling\n",
> + (unsigned long)SCRATCH_ALIGNMENT_BYTES,
> + (unsigned long)CMA_MIN_ALIGNMENT_BYTES);
> + kho_enable = false;
> + return;
> + }
> +
> scratch_size_update();
>
> /* FIXME: deal with node hot-plug/remove */
> --
> 2.55.0
>
--
Sincerely yours,
Mike.
