mm: clarify folio_likely_mapped_shared() documentation for KSM folios
Subsystems (2)
PATCH 1/1 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 <[email protected]>
Signed-off-by David Hildenbrand <[email protected]>
--- include/linux/mm.h | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-)
include/linux/mm.h +8 -6
@@ -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).
Signature · 1 line
2.45.2