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

[Lance Yang]

Hey David,

Maybe I spotted a bug below.

[...]
 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).

IIUC, when mapping a PMD entry for the entire THP, folio->_entire_mapcount
increments from -1 to 0.

Thanks,
Lance

+
+	/* Let's guess based on the first subpage. */
+	return atomic_read(&folio->_mapcount) > 0;
 }
[...]