Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios

[David Hildenbrand]

On 16.04.24 12:52, Lance Yang wrote:
> On Tue, Apr 16, 2024 at 6:47 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 16.04.24 12:40, Lance Yang wrote:
> > > Hey David,
> > >
> > > Maybe I spotted a bug below.
> >
> > Thanks for the review!
> >
> > > [...]
> > >    static inline bool folio_likely_mapped_shared(struct folio *folio)
> > >    {
> > > -     return page_mapcount(folio_page(folio, 0)) > 1;
> > > +     int mapcount = folio_mapcount(folio);
> > > +
> > > +     /* Only partially-mappable folios require more care. */
> > > +     if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
> > > +             return mapcount > 1;
> > > +
> > > +     /* A single mapping implies "mapped exclusively". */
> > > +     if (mapcount <= 1)
> > > +             return false;
> > > +
> > > +     /* If any page is mapped more than once we treat it "mapped shared". */
> > > +     if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
> > > +             return true;
> > >
> > > bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
> > > function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).
> >
> > If it's exclusively mapped, then folio_mapcount(folio)==1. In which case
> > the previous statement:
> >
> > if (mapcount <= 1)
> >          return false;
> >
> > Catches it.
>
> You're right!
>
> > IOW, once we reach this point we now that folio_mapcount(folio) > 1, and
> > there must be something else besides the entire mapping ("more than once").
> >
> >
> > Or did I not address your concern?
>
> Sorry, my mistake :(

No worries, thanks for the review and thinking this through!

--
Cheers,

David / dhildenb