On 24.02.25 17:55, David Hildenbrand wrote:
Let's implement an alternative when per-page mapcounts in large folios are
no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.
PM_MMAP_EXCLUSIVE will now be set if folio_likely_mapped_shared() is
true -- when the folio is considered "mapped shared", including when
it once was "mapped shared" but no longer is, as documented.
This might result in and under-indication of "exclusively mapped", which
is considered better than over-indicating it: under-estimating the USS
(Unique Set Size) is better than over-estimating it.
As an alternative, we could simply remove that flag with
CONFIG_NO_PAGE_MAPCOUNT completely, but there might be value to it. So,
let's keep it like that and document the behavior.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
Documentation/admin-guide/mm/pagemap.rst | 9 +++++++++
fs/proc/task_mmu.c | 11 +++++++++--
2 files changed, 18 insertions(+), 2 deletions(-)
diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
index 49590306c61a0..131c86574c39a 100644
--- a/Documentation/admin-guide/mm/pagemap.rst
+++ b/Documentation/admin-guide/mm/pagemap.rst
@@ -37,6 +37,15 @@ There are four components to pagemap:
precisely which pages are mapped (or in swap) and comparing mapped
pages between processes.
+ Note that in some kernel configurations, all pages part of a larger
+ allocation (e.g., THP) might be considered "mapped shared" if the large
+ allocation is considered "mapped shared": if not all pages are exclusive to
+ the same process. Further, some kernel configurations might consider larger
+ allocations "mapped shared", if they were at one point considered
+ "mapped shared", even if they would now be considered "exclusively mapped".
+ Consequently, in these kernel configurations, bit 56 might be set although
+ the page is actually "exclusively mapped"
I rewrote this yet another time to maybe make it clearer ...
+ Traditionally, bit 56 indicates that a page is mapped exactly once and bit
+ 56 is clear when a page is mapped multiple times, even when mapped in the
+ same process multiple times. In some kernel configurations, the semantics
+ for pages part of a larger allocation (e.g., THP) differ: bit 56 is set if
+ all pages part of the corresponding large allocation are *certainly* mapped
+ in the same process, even if the page is mapped multiple times in that
+ process. Bit 56 is clear when any page page of the larger allocation
+ is *maybe* mapped in a different process. In some cases, a large allocation
+ might be treated as "maybe mapped by multiple processes" even though this
+ is no longer the case.
(talking about "process" is not completely correct, it's actually "MMs"; but
that might add more confusion here)
--
Cheers,
David / dhildenb
