Re: [PATCH mm-unstable v2 1/6] mm: Add folio_estimated_mapcount()

[David Hildenbrand]

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@xxxxxxxxx>
> > ---
> >    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?

--
Thanks,

David / dhildenb