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

David Hildenbrand
2020-08-04T10:04:31Z

On 04.08.20 12:03, Mike Rapoport wrote:
> 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
>
> ?

Yeah, shorter. Thanks!

--
Thanks,

David / dhildenb