Re: [PATCH v4 01/20] mm/vma: introduce VMA anon page offset field and add helpers

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

On Sat, Aug 08, 2026 at 05:51:10PM -0700, Suren Baghdasaryan wrote:
> On Thu, Aug 6, 2026 at 1:22 PM Lorenzo Stoakes (ARM) <ljs@kernel.org> wrote:
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 87feaa5a2b78..df78847f5f07 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -4393,6 +4393,65 @@ static inline pgoff_t vma_last_pgoff(const struct vm_area_struct *vma)
> >         return vma_end_pgoff(vma) - 1;
> >  }
> >
> > +/**
> > + * vma_start_anon_pgoff() - Get the anonymous page offset of the start of @vma
> > + * @vma: The VMA whose anonymous page offset is required.
> > + *
> > + * If unfaulted, then this is vma->vm_start >> PAGE_SHIFT, if faulted then the
> > + * anonymous page offset at the time of first fault.
> > + *
> > + * If the VMA is anonymous, this returns the same value as vma_start_pgoff().
> > + *
> > + * This value is used for tracking MAP_PRIVATE file-backed mappings by their
> > + * anonymous page offset.
>
> I assume this function should not be used with shared file-backed
> mappings, right? If so, maybe add a comment like the one you have for
> linear_anon_page_index(): "It is not valid to call this function for
> shared file-backed mappings."?

No that's not the case, it is valid to access this for any VMA though it's only
meaningful for MAP_PRIVATE and anonymous VMAs (though in the latter case pgoff
== anon pgoff).

The code keeps the anon pgoff values consistent even for shared mappings because
- hey - we have the field anyway and it's easiest and safest to just keep it the
same.

One alternative would be to have code that checks the flags and zeroes the field
otherwise , but then you have problems like - early on initialisation now
there's an ordering requirement which can easily go wrong.

Another alternative is to just leave it stale, but then that seems objectively
worse and again requires branching code on update and set.

All-in-all it's easier to keep this working the same for any type of mapping,
it's just useless to do anything with the anon pgoff for a shared mapping :)

--
Cheers, Lorenzo