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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Sat, Aug 08, 2026 at 05:49:48PM -0700, Suren Baghdasaryan wrote:
> On Thu, Aug 6, 2026 at 1:23 PM Lorenzo Stoakes (ARM) <ljs@kernel.org> 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

Ack. I ran these patches through claude several times and it didn't get this ;)
the machines are not taking over just yet...

Andrew - could you change this in-place? Thanks!

> > 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?

Sure will do! It is very confusing stuff (and I was duly confused also at
first).

To save a respin (unless there's more stuff I need to address) I will probably
reply to the relevant patch with a proposed new comment.

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

--
Cheers, Lorenzo