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

Wei Yang
2025-10-14T13:31:35Z

On Tue, Oct 14, 2025 at 02:59:30PM +0200, David Hildenbrand wrote:
>On 14.10.25 14:23, Wei Yang wrote:
>> On Mon, Mar 03, 2025 at 05:30:13PM +0100, David Hildenbrand wrote:
>> [...]
>> > @@ -1678,6 +1726,22 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>> > break;
>> > case RMAP_LEVEL_PMD:
>> > case RMAP_LEVEL_PUD:
>> > + if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
>> > + last = atomic_add_negative(-1, &folio->_entire_mapcount);
>> > + if (level == RMAP_LEVEL_PMD && last)
>> > + nr_pmdmapped = folio_large_nr_pages(folio);
>> > + nr = folio_dec_return_large_mapcount(folio, vma);
>> > + if (!nr) {
>> > + /* Now completely unmapped. */
>> > + nr = folio_large_nr_pages(folio);
>> > + } else {
>> > + partially_mapped = last &&
>> > + nr < folio_large_nr_pages(folio);
>>
>> Hi, David
>
>Hi!
>
>>
>> Do you think this is better to be?
>>
>> partially_mapped = last && nr < nr_pmdmapped;
>
>I see what you mean, it would be similar to the CONFIG_PAGE_MAPCOUNT case
>below.
>
>But probably it could then be
>
> partially_mapped = nr < nr_pmdmapped;
>
>because nr_pmdmapped is only set when "last = true".
>
>I'm not sure if there is a good reason to change it at this point though.
>Smells like a micro-optimization for PUD, which we probably shouldn't worry
>about.
>
>>
>> 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.
>
>So consequently the partially_mapped variable will never really be used later
>on, because the folio_test_anon() will never hit in the PUD case.
>

Ok, folio_test_anon() takes care of it. We won't add it to defer list by
accident.

>--
>Cheers
>
>David / dhildenb

--
Wei Yang
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