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

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 12/10/26 20:00, Pratyush Yadav wrote:
> On Mon, Sep 28 2026, 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.
>>
>> 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)) {
> Nit: do we need both the WARN_ON_ONCE() and the pr_err()? Can't we drop
> the WARN and keep the pr_err only?

Yes pr_err is good enough. I will remove WARN_ON_ONCE()...

>
> LGTM otherwise.

Thanks for the review.

- Sourabh Jain

>
>> +		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 */