Re: [PATCH] mm/hugetlb: fix validation of unsupported HugeTLB orders

[Muchun Song <muchun.song@linux.dev>]

> On Nov 3, 2026, at 16:13, sashiko-bot@kernel.org wrote:
> 
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] mm/hugetlb: boot BUG_ON() in hugetlb_init() when HPAGE_SHIFT is 0 on powerpc
> --
> 
> commit 83959b12982c256d20ca0de51021b3beae201569
> Author: Muchun Song <muchun.song@linux.dev>
> 
> mm/hugetlb: fix validation of unsupported HugeTLB orders
> 
> This commit replaces BUILD_BUG_ON_INVALID() with MAYBE_BUILD_BUG_ON() in
> hugetlb_init() so that configurations where HUGETLB_PAGE_ORDER exceeds
> MAX_FOLIO_ORDER are rejected at build time when constant or via BUG_ON() at
> runtime otherwise.
> 
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index ff9dbfbc7a961..e21ac667694bd 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -4181,7 +4181,7 @@ static int __init hugetlb_init(void)
>> 
>> 	BUILD_BUG_ON(sizeof_field(struct page, private) * BITS_PER_BYTE <
>> 			__NR_HPAGEFLAGS);
>> -	BUILD_BUG_ON_INVALID(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER);
>> +	MAYBE_BUILD_BUG_ON(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER);
> 
> [Severity: High]
> Will placing MAYBE_BUILD_BUG_ON() before the hugepages_supported() check in
> hugetlb_init() trigger a boot BUG_ON() on architectures like PowerPC
> Book3S-64 when huge pages are unsupported at runtime?

Yeah, it's easy to fix, but I'll wait another day before updating it—just
in case others have feedback.

Thanks,
Muchun

> 
> On CONFIG_PPC_BOOK3S_64, arch/powerpc/include/asm/page.h defines HPAGE_SHIFT
> as a runtime unsigned int variable:
> 
>    extern unsigned int hpage_shift;
>    #define HPAGE_SHIFT hpage_shift
>    ...
>    #define HUGETLB_PAGE_ORDER (HPAGE_SHIFT - PAGE_SHIFT)
> 
> Because hpage_shift is not a compile-time constant, MAYBE_BUILD_BUG_ON()
> expands to a runtime BUG_ON(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER).
> 
> When huge pages are not supported by the platform or are disabled at boot
> (for example, when hugetlbpage_init_defaultsize() in
> arch/powerpc/mm/book3s64/hugetlbpage.c finds no supported huge page sizes),
> hpage_shift remains 0, which hugepages_supported() checks:
> 
>    static inline bool hugepages_supported(void)
>    {
>            if (hugetlb_disabled)
>                    return false;
> 
>            return HPAGE_SHIFT != 0;
>    }
> 
> Evaluating HUGETLB_PAGE_ORDER before checking hugepages_supported() computes
> (0U - PAGE_SHIFT) in unsigned int arithmetic, wrapping around to a large
> unsigned value that exceeds MAX_FOLIO_ORDER and hits BUG_ON().
> 
> Should MAYBE_BUILD_BUG_ON(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER) be moved
> after the if (!hugepages_supported()) block in hugetlb_init(), similar to how
> hugepage_init() in mm/huge_memory.c checks has_transparent_hugepage() before
> evaluating MAYBE_BUILD_BUG_ON(HPAGE_PMD_ORDER > MAX_PAGE_ORDER)?
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20261103023831.1805473-1-songmuchun@bytedance.com?part=1