Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

[David Hildenbrand]

On 10.08.23 05:25, Matthew Wilcox wrote:
> On Wed, Aug 09, 2023 at 05:23:46PM -0400, Peter Xu wrote:
> > Hi, David,
> >
> > Some pure questions below..
> >
> > On Wed, Aug 09, 2023 at 10:32:56AM +0200, David Hildenbrand wrote:
> > > Let's track the total mapcount for all large folios in the first subpage.
> > >
> > > The total mapcount is what we actually want to know in folio_mapcount()
> > > and it is also sufficient for implementing folio_mapped(). This also
> > > gets rid of any "raceiness" concerns as expressed in
> > > folio_total_mapcount().
> >
> > Any more information for that "raciness" described here?
>
> UTSL.
>
>          /*
>           * Add all the PTE mappings of those pages mapped by PTE.
>           * Limit the loop to folio_nr_pages_mapped()?
>           * Perhaps: given all the raciness, that may be a good or a bad idea.
>           */

Yes, that comment from Hugh primarily discusses how we could possibly 
optimize the loop, and if relying on folio_nr_pages_mapped() to reduce 
the iterations would be racy. As far as I can see, there are cases where 
"it would be certainly a bad idea" :)


In the other comment in that function, it's also made clear what the 
traditional behavior with PMD-mappable THP was "In the common case, 
avoid the loop when no pages mapped by PTE", which will no longer hold 
with sub-PMD THP.

--
Cheers,

David / dhildenb