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

[sashiko-bot@kernel.org]

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
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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?

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)?

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Sashiko AI review · https://sashiko.dev/#/patchset/20261103023831.1805473-1-songmuchun@bytedance.com?part=1