Re: Folio mapcount

[Matthew Wilcox <willy@infradead.org>]

On Mon, Feb 06, 2023 at 02:55:09PM -0800, Yang Shi wrote:
> > I've been thinking about this a lot more, and I have changed my
> > mind.  It works fine to answer the question "Is any page in this
> > folio mapped", but it's now hard to answer the question "I have it
> > mapped, does anybody else?"  That question is asked, for example,
> > in madvise_cold_or_pageout_pte_range().
> >
> > With this definition, if the mapcount is 1, it's definitely only mapped
> > by us.  If it's more than 2, it's definitely mapped by somebody else (*).
> > If it's 2, maybe we have the folio mapped twice, and maybe we have it
> > mapped once and somebody else has it mapped once, so we have to consult
> > the rmap to find out.  Not fun times.
> >
> > (*) If we support folios larger than PMD size, then the answer is more
> > complex.
> >
> > I now think the mapcount has to be defined as "How many VMAs have
> > one-or-more pages of this folio mapped".
> 
> IIRC it may be still possible the folio's mapcount is two, but it is
> only mapped by us (for example, two VMAs from the same process).

That's true, but it's also true for single-page folios that are mapped
by two VMAs from the same process.  I don't want to change the definition
that much!