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

Wei Yang
2025-10-15T00:45:46Z

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.

>
>--
>Cheers
>
>David / dhildenb

--
Wei Yang
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