Re: [PATCH v2] mm/hugetlb: fix validation of unsupported HugeTLB orders

["Oscar Salvador (SUSE)" <osalvador@kernel.org>]

On Wed, Nov 04, 2026 at 03:18:57PM +0800, Muchun Song wrote:
> Commit 7ce6048d3a7d ("loongarch: drop ranges for definition of
> ARCH_FORCE_MAX_ORDER") removed the ARCH_FORCE_MAX_ORDER ranges for
> LoongArch. Besides dropping unreasonable upper limits, this allowed
> values below the architecture defaults.
> 
> With 16KB pages, LoongArch defaults ARCH_FORCE_MAX_ORDER to 11,
> matching HUGETLB_PAGE_ORDER. Forcing it to 10 while enabling HugeTLB
> permits order-11 HugeTLB folios even though MAX_FOLIO_ORDER is 10.
> LoongArch does not select ARCH_HAS_GIGANTIC_PAGE, so this violates the
> generic MM assumption that folios cannot exceed MAX_PAGE_ORDER.
> 
> This invalid configuration is accepted by Kconfig.
> BUILD_BUG_ON_INVALID() does not reject it either because it only checks
> whether the expression is valid and never evaluates the comparison.
> Detection is left to the runtime warning in hugetlb_add_hstate(), after
> earlier boot paths may already have used the invalid configuration.
> 
> When an invalid HugeTLB order can be identified at build time, reject it
> there. Otherwise, reject it at runtime before the mismatch propagates
> further. Use MAYBE_BUILD_BUG_ON() so constant violations fail the build
> while dynamically determined orders are checked at runtime.
> 
> Fixes: 7ce6048d3a7d ("loongarch: drop ranges for definition of ARCH_FORCE_MAX_ORDER")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/sashiko-outbox-192874@kernel.org/
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> Suggested-by: Huacai Chen <chenhuacai@kernel.org>

Acked-by: Oscar Salvador <osalvador@suse.de>

> ---
> v2:
> - Move the order check after hugepages_supported() to avoid a false
>   PowerPC BUG_ON() (suggested by Sashiko)
> ---
>  mm/hugetlb.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 779b1de33c78..b49130eb7ad8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4197,7 +4197,6 @@ 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);
>  
>  	if (!hugepages_supported()) {
>  		if (hugetlb_max_hstate || default_hstate_max_huge_pages)
> @@ -4205,6 +4204,8 @@ static int __init hugetlb_init(void)
>  		return 0;
>  	}
>  
> +	MAYBE_BUILD_BUG_ON(HUGETLB_PAGE_ORDER > MAX_FOLIO_ORDER);
> +
>  	/*
>  	 * Make sure HPAGE_SIZE (HUGETLB_PAGE_ORDER) hstate exists.  Some
>  	 * architectures depend on setup being done here.
> 
> base-commit: 74d73ffcffe03804f97e9fb82301dadf75671282
> -- 
> 2.54.0
> 

-- 
Oscar Salvador
SUSE Labs