Re: Folio mapcount

[James Houghton <jthoughton@google.com>]

Hi Matthew,

On Mon, Feb 6, 2023 at 12:34 PM Matthew Wilcox <willy@infradead.org> wrote:
> I now think the mapcount has to be defined as "How many VMAs have
> one-or-more pages of this folio mapped".
>
> That means that our future folio_add_file_rmap_range() looks a bit
> like this:
>
> {
>         bool add_mapcount = true;
>
>         if (nr < folio_nr_pages(folio))
>                 add_mapcount = !folio_has_ptes(folio, vma);

Can you elaborate on how folio_has_ptes() might be implemented?

It seems like for a naturally aligned THP, we can see if the PMD is
pmd_present(), otherwise, do we need to check (potentially) all the
PTEs? Does this introduce an ordering requirement, where we have to
update the page table always before/after we call
folio_add_file_rmap_range()?

Thanks!
- James