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

Yin, Fengwei
2024-04-19T13:47:47Z

On 4/19/2024 5:19 PM, David Hildenbrand wrote:
> 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.
Right. I missed this part. Thanks for clarification.

Regards
Yin, Fengwei

>
> 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!
>