On Tue, Feb 07, 2023 at 02:56:52PM -0800, James Houghton wrote:
> Hi Matthew,
>
> On Mon, Feb 6, 2023 at 12:34 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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?
... oh. First I called it folio_has_ptes() and then I went looking
for inspiration on how to implement it, and I found page_mapped_in_vma()
and so I thought folio_mapped_in_vma() would be a better name, but
I forgot to change it here. Sorry.
> 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()?
Actually, you _mustn't_ update the page table first, or it will see
the PTEs and say "Ah, the folio is already mapped, I should not
increment mapcount". So we keep the calling order as it is now;
folio_add_X_rmap() followed by a call to set_ptes().
