linux-kernel - Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated

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Date:   Wed, 25 Jan 2023 22:29:31 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        linux-mm@...ck.org
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated_mapcount()

On 25.01.23 11:24, David Hildenbrand wrote:
> On 25.01.23 11:20, David Hildenbrand wrote:
>> On 24.01.23 02:22, Vishal Moola (Oracle) wrote:
>>> folio_estimated_mapcount() takes in a folio and calls page_mapcount() on
>>> the first page of that folio.
>>>
>>> This is necessary for folio conversions where we only care about either the
>>> entire_mapcount of a large folio, or the mapcount of a not large folio.
>>>
>>> This is in contrast to folio_mapcount() which calculates the total
>>> number of the times a folio and its subpages are mapped.
>>>
>>> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
>>> ---
>>>     include/linux/mm.h | 5 +++++
>>>     1 file changed, 5 insertions(+)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index c9db257f09b3..543c360f7ecc 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -875,6 +875,11 @@ static inline int page_mapcount(struct page *page)
>>>     	return mapcount;
>>>     }
>>>     
>>> +static inline int folio_estimated_mapcount(struct folio *folio)
>>> +{
>>> +	return page_mapcount(folio_page(folio, 0));
>>> +}
>>> +
>>>     int folio_total_mapcount(struct folio *folio);
>>>     
>>>     /**
>>
>> I'm sorry, but "estimated" as absolutely unclear semantics. You could
>> have a THP mapped into 9999 processes using THP and the estimation would
>> be "0".
> 
> ... or would it be 9999 ? What about a PMD-mapped THP? What about a
> partially unmapped THP?
> 
> What are we estimating?

Thinking about mapcounts again, might not have been my smartest moment.

What we return here is the precise number of times the first subpage is 
mapped (via the large folio and directly). That's supposed to be an 
estimate for the number of times any subpage part of the folio is mapped.

I really don't know a better name, but folio_estimated_mapcount() does 
not feel completely right to me and triggere dmy confusion in the first 
place ... hm ...

-- 
Thanks,

David / dhildenb