linux-kernel - Re: [PATCH 2/2] mm, memory_hotplug: do not assume ZONE

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Date:   Fri, 23 Jun 2017 09:37:22 +0800
From:   Wei Yang <richard.weiyang@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>,
        Yasuaki Ishimatsu <yasu.isimatu@...il.com>,
        qiuxishi@...wei.com, Kani Toshimitsu <toshi.kani@....com>,
        slaoub@...il.com, Joonsoo Kim <js1304@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm, memory_hotplug: do not assume ZONE_NORMAL is
 default kernel zone

On Thu, Jun 22, 2017 at 08:16:57PM +0200, Michal Hocko wrote:
>> 
>> Hmm... a corner case jumped into my mind which may invalidate this
>> calculation.
>> 
>> The case is:
>> 
>> 
>>        Zone:         | DMA   | DMA32      | NORMAL       |
>>                      v       v            v              v
>>        
>>        Phy mem:      [           ]     [                  ]
>>        
>>                      ^           ^     ^                  ^
>>        Node:         |   Node0   |     |      Node1       |
>>                              A   B     C  D
>> 
>> 
>> The key point is
>> 1. There is a hole between Node0 and Node1
>> 2. The hole sits in a non-normal zone
>> 
>> Let's mark the boundary as A, B, C, D. Then we would have
>> node0->zone[dma21] = [A, B]
>> node1->zone[dma32] = [C, D]
>> 
>> If we want to hotplug a range in [B, C] on node0, it looks not that bad. While
>> if we want to hotplug a range in [B, C] on node1, it will introduce the
>> overlapped zone. Because the range [B, C] intersects none of the existing
>> zones on node1.
>> 
>> Do you think this is possible?
>
>Yes, it is possible. I would be much more more surprised if it was real
>as well. Fixing that would require to use arch_zone_{lowest,highest}_possible_pfn
>which is not available after init section disappears and I am not even
>sure we should care. I would rather wait for a real life example of such
>a configuration to fix it.

Yep, not easy to fix, so wait for real case.

Or possible to add a line in commit log?

>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me

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