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Date:	Fri, 25 Jul 2014 15:19:19 +0800
From:	Zhang Zhen <zhenzhang.zhang@...wei.com>
To:	Dave Hansen <dave.hansen@...el.com>
CC:	<mingo@...hat.com>, Yinghai Lu <yinghai@...nel.org>,
	<mgorman@...e.de>, <akpm@...ux-foundation.org>,
	<zhangyanfei@...fujitsu.com>, <wangnan0@...wei.com>,
	Linux MM <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memory-hotplug: add sysfs zone_index attribute

On 2014/7/25 10:39, Zhang Zhen wrote:
> On 2014/7/25 1:59, Dave Hansen wrote:
>> On 07/24/2014 12:41 AM, Zhang Zhen wrote:
>>> Currently memory-hotplug has two limits:
>>> 1. If the memory block is in ZONE_NORMAL, you can change it to
>>> ZONE_MOVABLE, but this memory block must be adjacent to ZONE_MOVABLE.
>>> 2. If the memory block is in ZONE_MOVABLE, you can change it to
>>> ZONE_NORMAL, but this memory block must be adjacent to ZONE_NORMAL.
>>>
>>> Without this patch, we don't know which zone a memory block is in.
>>> So we don't know which memory block is adjacent to ZONE_MOVABLE or
>>> ZONE_NORMAL.
>>>
>>> On the other hand, with this patch, we can easy to know newly added
>>> memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA, for x86_32,
>>> ZONE_HIGHMEM).
>>
>> A section can contain more than one zone.  This interface will lie about
>> such sections, which is quite unfortunate.

Hi Dave,

You are right, i only considered the memory block added after machine booted.
For a x86_64 machine booted with "mem=400M" and with 2GiB memory installed.
Sample output of the sysfs files:
# cat block_size_bytes
8000000
# cat memory0/zone_index
DMA

Here memory0 cantain DMA_ZONE and DMA32_ZONE.
>>
> 1. In arch_add_memory(), x86_64 add the new pages of the new memory block default to
> ZONE_NORMAL (for powerpc, ZONE_DMA, for x86_32, ZONE_HIGHMEM).
> 
> 2. In __offline_pages(), test_pages_in_a_zone() guaranteed the pages of a memory block
> we try to offline are in the same zone. If a section contains more than one zone,
> the memory block can not be offlined.
> 
> Based on the above two points, i think the pages of a memory block are in one zone, and the sections
> of a memory block are in one zone.
> 
> Could you please explain in detail what is the case a section can contain more than one zone ?
> 
> Thanks for your comments!
> 
>> I'd really much rather see an interface that has a section itself
>> enumerate to which zones it may be changed.  The way you have it now,
>> any user has to know the rules that you've laid out above.  If the
>> kernel changed those restrictions, we'd have to teach every application
>> about the change in restrictions.
>>
Here you are right too, we should add an interface to show which zones a memory block may
be changed to. So user doesn't need to know the rules above.
I will send a new version.

Thank you very much !

> 
> This interface is designed to show which zone a memory block is in. If the kernel changed those
> restrictions, this interface doesn't need to change.
> For a x86_64 machine booted with "mem=400M" and with 2GiB memory installed.
> Sample output of the sysfs files:
> # cat block_size_bytes
> 8000000
> # cat memory0/zone_index
> DMA
> # cat memory1/zone_index
> DMA32
> # cat memory2/zone_index
> DMA32
> # cat memory3/zone_index
> DMA32
> # echo 0x20000000 > probe
> # cat memory4/zone_index
> Normal
> # echo online > memory4/state
> # cat memory4/zone_index
> Normal
> 
> # echo offline > memory4/state
> # echo online_movable > memory4/state
> # cat memory4/zone_index
> Movable
> 
> Thanks!
> 
> Best regards!
>>
>>
>>
>> .
>>
> 
> 
> --
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> 
> 


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