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

David Hildenbrand
2025-10-14T12:59:40Z

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.

--
Cheers

David / dhildenb