Re: [PATCH v3 20/20] mm: stop maintaining the per-page mapcount of large folios (CONFIG_NO_PAGE_MAPCOUNT)

David Hildenbrand
2025-10-20T13:53:11Z

On 15.10.25 02:45, Wei Yang wrote:
> On Tue, Oct 14, 2025 at 04:38:38PM +0200, David Hildenbrand wrote:
>> On 14.10.25 16:32, Matthew Wilcox wrote:
>>> On Tue, Oct 14, 2025 at 02:59:30PM +0200, David Hildenbrand wrote:
>>>>> As commit 349994cf61e6 mentioned, we don't support partially mapped PUD-sized
>>>>> folio yet.
>>>>
>>>> We do support partially mapped PUD-sized folios I think, but not anonymous
>>>> PUD-sized folios.
>>>
>>> I don't think so? The only mechanism I know of to allocate PUD-sized
>>> chunks of memory is hugetlb, and that doesn't permit partial mappings.
>>
>> Greetings from the latest DAX rework :)
>
> After a re-think, do you think it's better to align the behavior between
> CONFIG_NO_PAGE_MAPCOUNT and CONFIG_PAGE_MAPCOUNT?
>
> It looks we treat a PUD-sized folio partially_mapped if CONFIG_NO_PAGE_MAPCOUNT,
> but !partially_mapped if CONFIG_PAGE_MAPCOUNT, if my understanding is correct.

I'd just leave it alone unless there is a problem right now.

--
Cheers

David / dhildenb