Re: [PATCH v2 1/4] x86: hugetlb: disable default CMA reservations on 32-bit kernel

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

On Thu, Nov 05, 2026 at 12:07:43PM +0800, Qi Zheng wrote:
> From: Qi Zheng <zhengqi.arch@bytedance.com>
> 
> A 32-bit x86 kernel can run on a 64-bit-capable CPU advertising GBPAGES.
> arch_hugetlb_cma_order() checks only this feature and returns a nonzero
> order, although gigantic HugeTLB support on x86 requires CONFIG_X86_64.
> Specifying hugetlb_cma= can therefore reserve a CMA area that cannot
> serve the intended gigantic HugeTLB allocations.
> 
> On non-PAE kernels, the folded PUD_SHIFT yields a 4 MiB order, which is
> equal to MAX_PAGE_ORDER. With CONFIG_DEBUG_VM enabled, this triggers the
> VM_WARN_ON(order <= MAX_PAGE_ORDER) check in hugetlb_cma_reserve(),
> warning that hugetlb_cma is attempting to reserve a non-gigantic order.
> 
> Require CONFIG_X86_64 as well as GBPAGES when selecting the default CMA
> order to avoid these ineffective reservations.
> 
> Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepages using cma")
> Cc: stable@vger.kernel.org
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> Acked-by: Muchun Song <muchun.song@linux.dev>
> Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

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

> ---
>  arch/x86/mm/hugetlbpage.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/hugetlbpage.c b/arch/x86/mm/hugetlbpage.c
> index 3b26621c91286..70726a99ede7b 100644
> --- a/arch/x86/mm/hugetlbpage.c
> +++ b/arch/x86/mm/hugetlbpage.c
> @@ -45,7 +45,7 @@ arch_initcall(gigantic_pages_init);
>  
>  unsigned int __init arch_hugetlb_cma_order(void)
>  {
> -	if (boot_cpu_has(X86_FEATURE_GBPAGES))
> +	if (IS_ENABLED(CONFIG_X86_64) && boot_cpu_has(X86_FEATURE_GBPAGES))
>  		return PUD_SHIFT - PAGE_SHIFT;
>  
>  	return 0;
> -- 
> 2.54.0
> 

-- 
Oscar Salvador
SUSE Labs