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

David Hildenbrand
2024-04-09T19:24:40Z

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