Re: [PATCH v4 03/20] mm: introduce linear_anon_page_index()

On Thu, Aug 6, 2026 at 1:23 PM Lorenzo Stoakes (ARM) <[email protected]> wrote:
>
> This function provides the anonymous equivalent of linear_page_index(),
> instead offsetting based on the anonymous page offset of the VMA.
>
> It is valid only for anonymous or MAP_PRIVATE file-backed mappings, in
> other words CoW mappings.
>
> For pure anon VMAs, this will be equal to linear_page_index().
>
> Assert that both of these invariants are true In linear_anon_page_index()

s/In/in

> and implement the algorithm in __linear_anon_page_index().
>
> Note that MAP_PRIVATE-/dev/zero mappings will satisfy vma_is_anonymous()
> but not fulfill this invariant, so when asserting this we check
> vma->vm_file to account for this.
>
> We do not update callsites yet, so no functional change intended.
>
> Also const-ify vma_is_anonymous() to make it compatible with the
> const-ified linear_anon_page_index().
>
> While we're here, update linear_page_index() to be more succinct.
>
> VMA userland tests are also updated accordingly.
>
> Reviewed-by: Gregory Price (Meta) <[email protected]>
> Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
> ---
>  include/linux/mm.h              |  2 +-
>  include/linux/pagemap.h         | 40 +++++++++++++++++++++++++++++++++++++---
>  tools/testing/vma/include/dup.h | 25 ++++++++++++++++++++++++-
>  3 files changed, 62 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a3368c542947..da36a6cc907c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1556,7 +1556,7 @@ static inline void vma_desc_set_anonymous(struct vm_area_desc *desc)
>         desc->vm_ops = NULL;
>  }
>
> -static inline bool vma_is_anonymous(struct vm_area_struct *vma)
> +static inline bool vma_is_anonymous(const struct vm_area_struct *vma)
>  {
>         return !vma->vm_ops;
>  }
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index c6fc783aaee5..0adfa6605653 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -1094,10 +1094,44 @@ static inline pgoff_t linear_page_delta(const struct vm_area_struct *vma,
>  static inline pgoff_t linear_page_index(const struct vm_area_struct *vma,
>                                         const unsigned long address)
>  {
> -       pgoff_t pgoff;
> +       return linear_page_delta(vma, address) + vma_start_pgoff(vma);
> +}
> +
> +static inline pgoff_t __linear_anon_page_index(const struct vm_area_struct *vma,
> +               const unsigned long address)
> +{
> +       return linear_page_delta(vma, address) + vma_start_anon_pgoff(vma);
> +}
> +
> +/**
> + * linear_anon_page_index() - Determine the absolute anonymous page offset of
> + * @address within @vma.
> + * @vma: An anonymous or MAP_PRIVATE file-backed VMA in which @address resides.
> + * @address: The address whose absolute page offset is required.
> + *
> + * This returns the anonymous page offset of @address, which is the page offset
> + * the address possessed at the time the VMA was first faulted.
> + *
> + * For anonymous mappings, this returns the same value as linear_page_index().
> + *
> + * For MAP_PRIVATE file-backed mappings, this returns the anonymous page offset
> + * of @address, which is the page offset the address possessed at the time the
> + * VMA was first faulted.
> + *
> + * It is not valid to call this function for shared file-backed mappings.
> + *
> + * Returns: The absolute anonymous page offset of @address within @vma.
> + */
> +static inline pgoff_t linear_anon_page_index(const struct vm_area_struct *vma,
> +               const unsigned long address)
> +{
> +       const pgoff_t pgoff = __linear_anon_page_index(vma, address);
> +
> +       VM_WARN_ON_ONCE(!vma_is_cow_mapping(vma));

As discussed off the list, this assertion works even for read-only
anon mappings because, as you said, "there's no such thing as a pure
anon mapping that is !VMA_MAYWRITE_BIT, there is no way to achieve
that."
It took me some time to see that after reading your discussion with
David at [1] and [2], and I don't think it's very intuitive. Could you
please add a comment explaining that? Maybe add a comment at
vma_is_cow_mapping() definition since you are using the same assertion
in another patch of this series?

[1] https://lore.kernel.org/linux-mm/anBjpdMicqaGADr-@lucifer/
[2] https://lore.kernel.org/linux-mm/anLyhaI6kW-EWk13@lucifer/



> +       /* Account for MAP_PRIVATE-/dev/zero which is only semi-anonymous. */
> +       if (vma_is_anonymous(vma) && !vma->vm_file)
> +               VM_WARN_ON_ONCE(pgoff != linear_page_index(vma, address));
>
> -       pgoff = linear_page_delta(vma, address);
> -       pgoff += vma_start_pgoff(vma);
>         return pgoff;
>  }
>
> diff --git a/tools/testing/vma/include/dup.h b/tools/testing/vma/include/dup.h
> index 40ad83936b28..4c58487b764e 100644
> --- a/tools/testing/vma/include/dup.h
> +++ b/tools/testing/vma/include/dup.h
> @@ -1428,7 +1428,7 @@ static inline void vma_iter_set(struct vma_iterator *vmi, unsigned long addr)
>         mas_set(&vmi->mas, addr);
>  }
>
> -static inline bool vma_is_anonymous(struct vm_area_struct *vma)
> +static inline bool vma_is_anonymous(const struct vm_area_struct *vma)
>  {
>         return !vma->vm_ops;
>  }
> @@ -1621,3 +1621,26 @@ static inline pgprot_t vma_get_page_prot(const struct vm_area_struct *vma)
>  {
>         return vma_flags_to_page_prot(vma->flags);
>  }
> +
> +static inline pgoff_t __linear_anon_page_index(const struct vm_area_struct *vma,
> +                                              const unsigned long address)
> +{
> +       pgoff_t pgoff;
> +
> +       pgoff = linear_page_delta(vma, address);
> +       pgoff += vma_start_anon_pgoff(vma);
> +       return pgoff;
> +}
> +
> +static inline pgoff_t linear_anon_page_index(const struct vm_area_struct *vma,
> +               const unsigned long address)
> +{
> +       const pgoff_t pgoff = __linear_anon_page_index(vma, address);
> +
> +       VM_WARN_ON_ONCE(!vma_is_cow_mapping(vma));
> +       /* Account for MAP_PRIVATE-/dev/zero which is only semi-anonymous. */
> +       if (vma_is_anonymous(vma) && !vma->vm_file)
> +               VM_WARN_ON_ONCE(pgoff != linear_page_index(vma, address));
> +
> +       return pgoff;
> +}
>
> --
> 2.55.0
>