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

[Muchun Song]

> On Nov 3, 2026, at 16:01, Qi Zheng <qi.zheng@xxxxxxxxx> wrote:
> 
> From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
> 
> A 32-bit x86 kernel can run on a 64-bit-capable CPU advertising GBPAGES.

I don't think anyone would do that. That said, it is genuinely an issue.

> 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@xxxxxxxxxxxxxxx
> Cc: x86@xxxxxxxxxx
> Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

Acked-by: Muchun Song <muchun.song@xxxxxxxxx>

Thanks.