Re: [PATCH v2 17/20] fs/proc/task_mmu: remove per-page mapcount dependency for PM_MMAP_EXCLUSIVE (CONFIG_NO_PAGE_MAPCOUNT)

[David Hildenbrand <david@redhat.com>]

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 no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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@redhat.com>
> ---
>   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