Re: [PATCH v2 18/20] fs/proc/task_mmu: remove per-page mapcount dependency for "mapmax" (CONFIG_NO_PAGE_MAPCOUNT)

David Hildenbrand
2025-02-24T21:04:58Z

On 24.02.25 21:45, Zi Yan wrote:
> On Mon Feb 24, 2025 at 11:56 AM EST, 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 no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.
>>
>> For calculating "mapmax", we now use the average per-page mapcount in
>> a large folio instead of the per-page mapcount.
>>
>> For hugetlb folios and folios that are not partially mapped into MMs,
>> there is no change.
>>
>> Likely, this change will not matter much in practice, and an alternative
>> might be to simple remove this stat with CONFIG_NO_PAGE_MAPCOUNT.
>> However, 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/filesystems/proc.rst | 5 +++++
>> fs/proc/task_mmu.c | 7 ++++++-
>> 2 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
>> index 09f0aed5a08ba..1aa190017f796 100644
>> --- a/Documentation/filesystems/proc.rst
>> +++ b/Documentation/filesystems/proc.rst
>> @@ -686,6 +686,11 @@ Where:
>> node locality page counters (N0 == node0, N1 == node1, ...) and the kernel page
>> size, in KB, that is backing the mapping up.
>>
>> +Note that some kernel configurations do not track the precise number of times
>> +a page part of a larger allocation (e.g., THP) is mapped. In these
>> +configurations, "mapmax" might corresponds to the average number of mappings
>> +per page in such a larger allocation instead.
>> +
>> 1.2 Kernel data
>> ---------------
>>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 80839bbf9657f..d7ee842367f0f 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -2862,7 +2862,12 @@ static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty,
>> unsigned long nr_pages)
>> {
>> struct folio *folio = page_folio(page);
>> - int count = folio_precise_page_mapcount(folio, page);
>> + int count;
>> +
>> + if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
>> + count = folio_precise_page_mapcount(folio, page);
>> + else
>> + count = min_t(int, folio_average_page_mapcount(folio), 1);
>
> s/min/max ?

Indeed, thanks!

>
> Otherwise, count is at most 1. Anyway, if you change
> folio_average_page_mapcount() as I indicated in patch 16, this
> will become count = folio_average_page_mapcount(folio).

No, the average should not be 1 just because a single subpage is mapped.

--
Cheers,

David / dhildenb