linux-kernel - [PATCH v1] mm: clarify folio_likely_mapped

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Message-ID: <20240731160758.808925-1-david@redhat.com>
Date: Wed, 31 Jul 2024 18:07:58 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v1] 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.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 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