Re: [PATCH 1/1] mm/huge_memory: disallow raw PMD mappings of the huge zero page

[David Hildenbrand (Arm)]

On 9/17/26 14:10, Lance Yang wrote:
> vm_mixed_zeropage_allowed() validates zeropage insertion into
> VM_MIXEDMAP VMAs. No in-tree user needs to insert the huge zero page
> through vmf_insert_pfn_pmd().

I think we want a clear description why we would want to forbid it. Is it
related to vm_mixed_ok/vm_mixed_zeropage_allowed?

Once again I am confused about which interfaces are actually valid for
VM_MIXEDMAP.>
> Return VM_FAULT_SIGBUS if vmf_insert_pfn_pmd() is asked to map it.
> 
> Link: https://lore.kernel.org/linux-mm/f76beaf8-351a-4b22-b362-a945a5e1af6a@xxxxxxxxxx/
> Suggested-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> ---
>  mm/huge_memory.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bfd972b997f9..c02f21ce4f4c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1728,6 +1728,9 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, unsigned long pfn,
>  						(VM_PFNMAP|VM_MIXEDMAP));
>  	BUG_ON((vma->vm_flags & VM_PFNMAP) && vma_is_cow_mapping(vma));
>  
> +	if (unlikely(is_huge_zero_pfn(pfn)))
> +		return VM_FAULT_SIGBUS;
> +
>  	pfnmap_setup_cachemode_pfn(pfn, &pgprot);
>  
>  	return insert_pmd(vma, addr, vmf->pmd, fop, pgprot, write);

-- 
Cheers,

David