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

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Date: Tue, 16 Apr 2024 12:53:57 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, cgroups@...r.kernel.org, chris@...kel.net,
 corbet@....net, dalias@...c.org, fengwei.yin@...el.com,
 glaubitz@...sik.fu-berlin.de, hughd@...gle.com, jcmvbkbc@...il.com,
 linmiaohe@...wei.com, linux-doc@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-sh@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, muchun.song@...ux.dev,
 naoya.horiguchi@....com, peterx@...hat.com, richardycc@...gle.com,
 ryan.roberts@....com, shy828301@...il.com, willy@...radead.org,
 ysato@...rs.sourceforge.jp, ziy@...dia.com
Subject: Re: [PATCH v1 05/18] mm: improve folio_likely_mapped_shared() using
 the mapcount of large folios

On 16.04.24 12:52, Lance Yang wrote:
> On Tue, Apr 16, 2024 at 6:47 PM David Hildenbrand <david@...hat.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