Re: [PATCH v2 1/4] mm/memory: introduce is_huge_zero_pfn() and use it in vm_normal_page_pmd()

[Shivank Garg <shivankg@amd.com>]

On 7/8/2025 12:20 AM, Luiz Capitulino wrote:
> From: David Hildenbrand <david@redhat.com>
> 
> Let's avoid working with the PMD when not required. If
> vm_normal_page_pmd() would be called on something that is not a present
> pmd, it would already be a bug (pfn possibly garbage).
> 
> While at it, let's support passing in any pfn covered by the huge zero
> folio by masking off PFN bits -- which should be rather cheap.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>  include/linux/huge_mm.h | 12 +++++++++++-
>  mm/memory.c             |  2 +-
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2f190c90192d..59e93fba15f4 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -486,9 +486,14 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
>  	return READ_ONCE(huge_zero_folio) == folio;
>  }
>  
> +static inline bool is_huge_zero_pfn(unsigned long pfn)
> +{
> +	return READ_ONCE(huge_zero_pfn) == (pfn & ~(HPAGE_PMD_NR - 1));
> +}
> +
>  static inline bool is_huge_zero_pmd(pmd_t pmd)
>  {
> -	return pmd_present(pmd) && READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd);
> +	return pmd_present(pmd) && is_huge_zero_pfn(pmd_pfn(pmd));
>  }
>  
>  struct folio *mm_get_huge_zero_folio(struct mm_struct *mm);
> @@ -636,6 +641,11 @@ static inline bool is_huge_zero_folio(const struct folio *folio)
>  	return false;
>  }
>  
> +static inline bool is_huge_zero_pfn(unsigned long pfn)
> +{
> +	return false;
> +}
> +
>  static inline bool is_huge_zero_pmd(pmd_t pmd)
>  {
>  	return false;
> diff --git a/mm/memory.c b/mm/memory.c
> index b0cda5aab398..3a765553bacb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -687,7 +687,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>  
>  	if (pmd_devmap(pmd))
>  		return NULL;
> -	if (is_huge_zero_pmd(pmd))
> +	if (is_huge_zero_pfn(pfn))
>  		return NULL;
>  	if (unlikely(pfn > highest_memmap_pfn))
>  		return NULL;

Reviewed-by: Shivank Garg <shivankg@amd.com>