mm: improve folio_likely_mapped_shared() using the mapcount of large folios

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.

Link: https://lkml.kernel.org/r/20240409192301.907377-6-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Yin Fengwei <fengwei.yin@intel.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Richard Chang <richardycc@google.com>
Cc: Rich Felker <dalias@libc.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm.h

@@ -2184,7 +2184,7 @@ 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
@@ -2201,7 +2201,22 @@ 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