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

[Mike Rapoport]

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@xxxxxxxxxxxxxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxx>
> >> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> >> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> >> Cc: Mike Rapoport <rppt@xxxxxxxxxx>
> >> Cc: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
> >> Cc: Baoquan He <bhe@xxxxxxxxxx>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > 
> > Several nits below, othersize
> > 
> > Acked-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > 
> >> ---
> >>  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.