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

David Hildenbrand
2024-04-16T10:54:04Z

On 16.04.24 12:52, Lance Yang wrote:
> On Tue, Apr 16, 2024 at 6:47 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 16.04.24 12:40, Lance Yang wrote:
>>> Hey David,
>>>
>>> Maybe I spotted a bug below.
>>
>> Thanks for the review!
>>
>>>
>>> [...]
>>> 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;
>>> +
>>> + /* A single mapping implies "mapped exclusively". */
>>> + if (mapcount <= 1)
>>> + return false;
>>> +
>>> + /* If any page is mapped more than once we treat it "mapped shared". */
>>> + if (folio_entire_mapcount(folio) || mapcount > folio_nr_pages(folio))
>>> + return true;
>>>
>>> bug: if a PMD-mapped THP is exclusively mapped, the folio_entire_mapcount()
>>> function will return 1 (atomic_read(&folio->_entire_mapcount) + 1).
>>
>> If it's exclusively mapped, then folio_mapcount(folio)==1. In which case
>> the previous statement:
>>
>> if (mapcount <= 1)
>> return false;
>>
>> Catches it.
>
> You're right!
>
>>
>> IOW, once we reach this point we now that folio_mapcount(folio) > 1, and
>> there must be something else besides the entire mapping ("more than once").
>>
>>
>> Or did I not address your concern?
>
> Sorry, my mistake :(

No worries, thanks for the review and thinking this through!

--
Cheers,

David / dhildenb