mm: clarify folio_likely_mapped_shared() documentation for KSM folios

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.

Link: https://lkml.kernel.org/r/20240731160758.808925-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bd219ac9c0264133f777e68775116605ecab023f..2f6c08b53e4fb2f03e6bd66dd7cdf6d0ecb5ae50 100644 (file)
--- 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).