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

[Lance Yang]

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 :(

Thanks,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>