Commit 17d5f38b authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

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: default avatarDavid Hildenbrand <david@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 6654d289

include/linux/mm.h

+8 −6
Original line number Diff line number Diff line
@@ -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).