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

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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





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