kevin/linux
1
Fork 0
Code

mm: clarify folio_likely_mapped_shared() documentation for KSM folios

Browse Source 
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>
This commit is contained in:
David Hildenbrand 2024-07-31 18:07:58 +02:00 committed by Andrew Morton
parent 6654d28995
commit 17d5f38b33
1 changed files with 8 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
include/linux/mm.h
View File

@ -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).