linux-kernel - Re: [PATCH v3 6/6] mm: document semantics of ZONE

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Message-ID: <65deeb21-63fe-d1c1-bb87-74a08035f79a@redhat.com>
Date:   Tue, 4 Aug 2020 11:55:10 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Baoquan He <bhe@...hat.com>
Subject: Re: [PATCH v3 6/6] mm: document semantics of ZONE_MOVABLE

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@...ux-foundation.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Michael S. Tsirkin <mst@...hat.com>
>> Cc: Mike Kravetz <mike.kravetz@...cle.com>
>> Cc: Mike Rapoport <rppt@...nel.org>
>> Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
>> Cc: Baoquan He <bhe@...hat.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Several nits below, othersize
> 
> Acked-by: Mike Rapoport <rppt@...ux.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