The patch titled
Subject: mm: clarify folio_likely_mapped_shared() documentation for KSM folios
has been added to the -mm mm-unstable branch. Its filename is
mm-clarify-folio_likely_mapped_shared-documentation-for-ksm-folios.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-clarify-folio_likely_mapped_shared-documentation-for-ksm-folios.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm: clarify folio_likely_mapped_shared() documentation for KSM folios
Date: Wed, 31 Jul 2024 18:07:58 +0200
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@xxxxxxxxxx
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mm.h | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)
--- a/include/linux/mm.h~mm-clarify-folio_likely_mapped_shared-documentation-for-ksm-folios
+++ a/include/linux/mm.h
@@ -2132,14 +2132,19 @@ static inline size_t folio_size(const st
* 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 st
* #. 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).
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-turn-use_split_pte_ptlocks-use_split_pte_ptlocks-into-kconfig-options.patch
mm-hugetlb-enforce-that-pmd-pt-sharing-has-split-pmd-pt-locks.patch
powerpc-8xx-document-and-enforce-that-split-pt-locks-are-not-used.patch
mm-simplify-arch_make_folio_accessible.patch
mm-gup-convert-to-arch_make_folio_accessible.patch
s390-uv-drop-arch_make_page_accessible.patch
mm-hugetlb-remove-hugetlb_follow_page_mask-leftover.patch
mm-rmap-cleanup-partially-mapped-handling-in-__folio_remove_rmap.patch
mm-clarify-folio_likely_mapped_shared-documentation-for-ksm-folios.patch
