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

[Suren Baghdasaryan]

On Mon, Aug 10, 2026 at 1:40 AM Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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.

Sounds good!

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