Re: [PATCH v3 6/6] mm: document semantics of ZONE_MOVABLE

Mike Rapoport
2020-08-04T10:03:43Z

On Tue, Aug 04, 2020 at 11:55:10AM +0200, David Hildenbrand wrote:
> On 04.08.20 11:33, Mike Rapoport wrote:
> > On Tue, Aug 04, 2020 at 09:24:08AM +0200, David Hildenbrand wrote:
> >> Let's document what ZONE_MOVABLE means, how it's used, and which special
> >> cases we have regarding unmovable pages (memory offlining vs. migration /
> >> allocations).
> >>
> >> Cc: Andrew Morton <akpm@linux-foundation.org>
> >> Cc: Michal Hocko <mhocko@suse.com>
> >> Cc: Michael S. Tsirkin <mst@redhat.com>
> >> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> >> Cc: Mike Rapoport <rppt@kernel.org>
> >> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
> >> Cc: Baoquan He <bhe@redhat.com>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >
> > Several nits below, othersize
> >
> > Acked-by: Mike Rapoport <rppt@linux.ibm.com>
> >
> >> ---
> >> include/linux/mmzone.h | 34 ++++++++++++++++++++++++++++++++++
> >> 1 file changed, 34 insertions(+)
> >>
> >> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> >> index f6f884970511d..600d449e7d9e9 100644
> >> --- a/include/linux/mmzone.h
> >> +++ b/include/linux/mmzone.h
> >> @@ -372,6 +372,40 @@ enum zone_type {
> >> */
> >> ZONE_HIGHMEM,
> >> #endif
> >> + /*
> >> + * ZONE_MOVABLE is similar to ZONE_NORMAL, except that it *primarily*
> >> + * only contains movable pages. Main use cases are to make memory
> >
> > "Primarily only" sounds awkward. Maybe
> >
> > ... except that it only contains movable pages with few exceptional
> > cases described below.
> >
> > And then
> >
> > Main use cases for ZONE_MOVABLE are ...
>
> Ack!
>
> >
> >> + * offlining more likely to succeed, and to locally limit unmovable
> >> + * allocations - e.g., to increase the number of THP/huge pages.
> >> + * Notable special cases are:
> >> + *
> >> + * 1. Pinned pages: (Long-term) pinning of movable pages might
> >
> > ^long, capital L looked out of place for me
>
> Ack!
>
> >
> >> + * essentially turn such pages unmovable. Memory offlining might
> >> + * retry a long time.
> >> + * 2. memblock allocations: kernelcore/movablecore setups might create
> >> + * situations where ZONE_MOVABLE contains unmovable allocations
> >> + * after boot. Memory offlining and allocations fail early.
> >> + * 3. Memory holes: Such pages cannot be allocated. Applies only to
> >> + * boot memory, not hotplugged memory. Memory offlining and
> >> + * allocations fail early.
> >
> > I would clarify where page struct for abscent memory come from
>
> Something like:
>
> Memory holes: We might have a memmap for memory holes, for example, if

^w ;-)

> we have sections that are only partially System RAM. Such pages cannot
> be ...

How about

... sections that are only partially populated

?

> ?
>
> Thanks!
>
> --
> Thanks,
>
> David / dhildenb
>

--
Sincerely yours,
Mike.