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

[David Hildenbrand <david@redhat.com>]

We can now read the mapcount of large folios very efficiently. Use it to
improve our handling of partially-mappable folios, falling back
to making a guess only in case the folio is not "obviously mapped shared".

We can now better detect partially-mappable folios where the first page is
not mapped as "mapped shared", reducing "false negatives"; but false
negatives are still possible.

While at it, fixup a wrong comment (false positive vs. false negative)
for KSM folios.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/mm.h | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1862a216af15..daf687f0e8e5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2183,7 +2183,7 @@ static inline size_t folio_size(struct folio *folio)
  *       indicate "mapped shared" (false positive) when two VMAs in the same MM
  *       cover the same file range.
  *    #. For (small) KSM folios, the return value can wrongly indicate "mapped
- *       shared" (false negative), when the folio is mapped multiple times into
+ *       shared" (false positive), when the folio is mapped multiple times into
  *       the same MM.
  *
  * Further, this function only considers current page table mappings that
@@ -2200,7 +2200,22 @@ static inline size_t folio_size(struct folio *folio)
  */
 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;
+
+	/* Let's guess based on the first subpage. */
+	return atomic_read(&folio->_mapcount) > 0;
 }
 
 #ifndef HAVE_ARCH_MAKE_PAGE_ACCESSIBLE
-- 
2.44.0