Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using the mapcount of large folios

David Hildenbrand
2024-04-19T09:19:58Z

On 19.04.24 04:29, Yin, Fengwei wrote:
>
>
> On 4/10/2024 3:22 AM, David Hildenbrand wrote:
>> @@ -2200,7 +2200,22 @@ static inline size_t folio_size(struct folio *folio)
>> */
>> static inline bool folio_likely_mapped_shared(struct folio *folio)
>> {
>> - return page_mapcount(folio_page(folio, 0)) > 1;
>> + int mapcount = folio_mapcount(folio);
>> +
>> + /* Only partially-mappable folios require more care. */
>> + if (!folio_test_large(folio) || unlikely(folio_test_hugetlb(folio)))
>> + return mapcount > 1;
> My understanding is that mapcount > folio_nr_pages(folio) can cover
> order 0 folio. And also folio_entire_mapcount() can cover hugetlb (I am
> not 100% sure for this one). I am wondering whether we can drop above
> two lines? Thanks.

folio_entire_mapcount() does not apply to small folios, so we must not
call that for small folios.

Regarding hugetlb, subpage mapcounts are completely unused, except
subpage 0 mapcount, which is now *always* negative (storing a page type)
-- so there is no trusting on that value at all.

So in the end, it all looked cleanest when only special-casing on
partially-mappable folios where we know the entire mapcount exists and
we know that subapge mapcount 0 actually stores something reasonable
(not a type).

Thanks!

--
Cheers,

David / dhildenb