Re: [PATCH v1 1/2] kho: check scratch vs CMA alignment at runtime

[Mike Rapoport <rppt@kernel.org>]

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 <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: linux-kernel@vger.kernel.org
> Cc: linuxppc-dev@lists.ozlabs.org
> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
> ---
>  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.