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

David Hildenbrand
2020-08-04T09:55:22Z

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

?

Thanks!

--
Thanks,

David / dhildenb