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

David Hildenbrand
2024-04-16T10:47:13Z

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.

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?

--
Cheers,

David / dhildenb