Re: [PATCH v1 1/4] mm: move vma_kernel_pagesize() from hugetlb to mm.h

[Lorenzo Stoakes (Oracle)]

On Fri, Mar 06, 2026 at 11:15:57AM +0100, David Hildenbrand (Arm) wrote:
> In the past, only hugetlb had special "vma_kernel_pagesize()"
> requirements, so it provided its own implementation.
>
> In commit 05ea88608d4e ("mm, hugetlbfs: introduce ->pagesize() to
> vm_operations_struct") we generalized that approach by providing a
> vm_ops->pagesize() callback to be used by device-dax.
>
> Once device-dax started using that callback in commit c1d53b92b95c
> ("device-dax: implement ->pagesize() for smaps to report MMUPageSize")
> it was missed that CONFIG_DEV_DAX does not depend on hugetlb support.
>
> So building a kernel with CONFIG_DEV_DAX but without CONFIG_HUGETLBFS
> would not pick up that value.
>
> Fix it by moving vma_kernel_pagesize() to mm.h, providing only a single
> implementation. While at it, improve the kerneldoc a bit.
>
> Ideally, we'd move vma_mmu_pagesize() as well to the header. However,
> its __weak symbol might be overwritten by a PPC variant in hugetlb code.
> So let's leave it in there for now, as it really only matters for some
> hugetlb oddities.
>
> This was found by code inspection.
>
> Fixes: c1d53b92b95c ("device-dax: implement ->pagesize() for smaps to report MMUPageSize")
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

LGTM, but you need to fix up VMA tests, I attach a patch below to do this. Will
this resolved:

Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@xxxxxxxxxx>

> ---
>  include/linux/hugetlb.h |  7 -------
>  include/linux/mm.h      | 20 ++++++++++++++++++++
>  mm/hugetlb.c            | 17 -----------------
>  3 files changed, 20 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 65910437be1c..44c1848a2c21 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -777,8 +777,6 @@ static inline unsigned long huge_page_size(const struct hstate *h)
>  	return (unsigned long)PAGE_SIZE << h->order;
>  }
>
> -extern unsigned long vma_kernel_pagesize(struct vm_area_struct *vma);
> -
>  extern unsigned long vma_mmu_pagesize(struct vm_area_struct *vma);
>
>  static inline unsigned long huge_page_mask(struct hstate *h)
> @@ -1177,11 +1175,6 @@ static inline unsigned long huge_page_mask(struct hstate *h)
>  	return PAGE_MASK;
>  }
>
> -static inline unsigned long vma_kernel_pagesize(struct vm_area_struct *vma)
> -{
> -	return PAGE_SIZE;
> -}
> -
>  static inline unsigned long vma_mmu_pagesize(struct vm_area_struct *vma)
>  {
>  	return PAGE_SIZE;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 44e04a42fe77..227809790f1a 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1307,6 +1307,26 @@ static inline bool vma_is_shared_maywrite(const struct vm_area_struct *vma)
>  	return is_shared_maywrite(&vma->flags);
>  }
>
> +/**
> + * vma_kernel_pagesize - Default page size granularity for this VMA.
> + * @vma: The user mapping.
> + *
> + * The kernel page size specifies in which granularity VMA modifications
> + * can be performed. Folios in this VMA will be aligned to, and at least
> + * the size of the number of bytes returned by this function.
> + *
> + * The default kernel page size is not affected by Transparent Huge Pages
> + * being in effect.
> + *
> + * Return: The default page size granularity for this VMA.
> + */
> +static inline unsigned long vma_kernel_pagesize(struct vm_area_struct *vma)
> +{
> +	if (unlikely(vma->vm_ops && vma->vm_ops->pagesize))
> +		return vma->vm_ops->pagesize(vma);
> +	return PAGE_SIZE;
> +}
> +
>  static inline
>  struct vm_area_struct *vma_find(struct vma_iterator *vmi, unsigned long max)
>  {
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 1d41fa3dd43e..66eadfa9e958 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1017,23 +1017,6 @@ static pgoff_t vma_hugecache_offset(struct hstate *h,
>  			(vma->vm_pgoff >> huge_page_order(h));
>  }
>
> -/**
> - * vma_kernel_pagesize - Page size granularity for this VMA.
> - * @vma: The user mapping.
> - *
> - * Folios in this VMA will be aligned to, and at least the size of the
> - * number of bytes returned by this function.
> - *
> - * Return: The default size of the folios allocated when backing a VMA.
> - */
> -unsigned long vma_kernel_pagesize(struct vm_area_struct *vma)
> -{
> -	if (vma->vm_ops && vma->vm_ops->pagesize)
> -		return vma->vm_ops->pagesize(vma);
> -	return PAGE_SIZE;
> -}
> -EXPORT_SYMBOL_GPL(vma_kernel_pagesize);
> -
>  /*
>   * Return the page size being used by the MMU to back a VMA. In the majority
>   * of cases, the page size used by the kernel matches the MMU size. On
> --
> 2.43.0
>

----8<----
This breaks the VMA tests when patch 2/4 removes the references in other
headers. So this patch should also update them, I enclose a simple fix for
convenience: