On 03/10/26 13:29, Mike Rapoport wrote:
> 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?
Yeah, that's a good idea. We can have a dynamic check for cases
where a static check is not possible.
Thanks for the suggestion.
- Sourabh Jain
> 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 at linux.ibm.com>
>> Cc: Alexander Graf <graf at amazon.com>
>> Cc: Andrew Morton <akpm at linux-foundation.org>
>> Cc: Baoquan He <baoquan.he at linux.dev>
>> Cc: Christophe Leroy (CS GROUP) <chleroy at kernel.org>
>> Cc: Hari Bathini <hbathini at linux.ibm.com>
>> Cc: Madhavan Srinivasan <maddy at linux.ibm.com>
>> Cc: Mahesh Salgaonkar <mahesh at linux.ibm.com>
>> Cc: Michael Ellerman <mpe at ellerman.id.au>
>> Cc: Mike Rapoport <rppt at kernel.org>
>> Cc: Nicholas Piggin <npiggin at gmail.com>
>> Cc: Pasha Tatashin <pasha.tatashin at soleen.com>
>> Cc: Pratyush Yadav <pratyush at kernel.org>
>> Cc: Ritesh Harjani (IBM) <ritesh.list at gmail.com>
>> Cc: Shivang Upadhyay <shivangu at linux.ibm.com>
>> Cc: Shrikanth Hegde <sshegde at linux.ibm.com>
>> Cc: linux-kernel at vger.kernel.org
>> Cc: linuxppc-dev at lists.ozlabs.org
>> Signed-off-by: Sourabh Jain <sourabhjain at 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
>>