For KSM folios, the function actually does what it is supposed to do:
even having multiple mappings inside the same MM is considered
"sharing", as there is no real relationship between these KSM page
mappings -- in contrast to mapping the same file range twice and having the
same pagecache page mapped twice.
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
include/linux/mm.h | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index a890a1731c14..58ca060f5dce 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2132,14 +2132,19 @@ static inline size_t folio_size(const struct folio *folio)
* MM ("mapped shared"), or if the folio is only mapped into a single MM
* ("mapped exclusively").
*
+ * For KSM folios, this function also returns "mapped shared" when a folio is
+ * mapped multiple times into the same MM, because the individual page mappings
+ * are independent.
+ *
* As precise information is not easily available for all folios, this function
* estimates the number of MMs ("sharers") that are currently mapping a folio
* using the number of times the first page of the folio is currently mapped
* into page tables.
*
- * For small anonymous folios (except KSM folios) and anonymous hugetlb folios,
- * the return value will be exactly correct, because they can only be mapped
- * at most once into an MM, and they cannot be partially mapped.
+ * For small anonymous folios and anonymous hugetlb folios, the return
+ * value will be exactly correct: non-KSM folios can only be mapped at most once
+ * into an MM, and they cannot be partially mapped. KSM folios are
+ * considered shared even if mapped multiple times into the same MM.
*
* For other folios, the result can be fuzzy:
* #. For partially-mappable large folios (THP), the return value can wrongly
@@ -2148,9 +2153,6 @@ static inline size_t folio_size(const struct folio *folio)
* #. For pagecache folios (including hugetlb), the return value can wrongly
* 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 positive), when the folio is mapped multiple times into
- * the same MM.
*
* Further, this function only considers current page table mappings that
* are tracked using the folio mapcount(s).
--
2.45.2
