linux-kernel - Re: [PATCH V7] mm: memory hot-add: memory can not be added to movable zone defaultly

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Date:	Wed, 21 Oct 2015 11:00:19 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Changsheng Liu <liuchangsheng@...pur.com>,
	akpm@...ux-foundation.org, isimatu.yasuaki@...fujitsu.com,
	yasu.isimatu@...il.com, tangchen@...fujitsu.com
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	wangnan0@...wei.com, dave.hansen@...el.com, yinghai@...nel.org,
	toshi.kani@...com, qiuxishi@...wei.com, wunan@...pur.com,
	yanxiaofeng@...pur.com, fandd@...pur.com,
	Changsheng Liu <liuchangcheng@...pur.com>
Subject: Re: [PATCH V7] mm: memory hot-add: memory can not be added to movable
 zone defaultly

On 10/21/2015 04:20 AM, Changsheng Liu wrote:
>
>
> 在 2015/10/15 0:18, Vlastimil Babka 写道:
>> On 10/12/2015 08:58 AM, Changsheng Liu wrote:
>>> From: Changsheng Liu <liuchangcheng@...pur.com>
>>>
>>> After the user config CONFIG_MOVABLE_NODE,
>>> When the memory is hot added, should_add_memory_movable() return 0
>>> because all zones including ZONE_MOVABLE are empty,
>>> so the memory that was hot added will be assigned to ZONE_NORMAL
>>> and ZONE_NORMAL will be created firstly.
>>> But we want the whole node to be added to ZONE_MOVABLE by default.
>>>
>>> So we change should_add_memory_movable(): if the user config
>>> CONFIG_MOVABLE_NODE and sysctl parameter hotadd_memory_as_movable is 1
>>> and the ZONE_NORMAL is empty or the pfn of the hot-added memory
>>> is after the end of the ZONE_NORMAL it will always return 1
>>> and then the whole node will be added to ZONE_MOVABLE by default.
>>> If we want the node to be assigned to ZONE_NORMAL,
>>> we can do it as follows:
>>> "echo online_kernel > /sys/devices/system/memory/memoryXXX/state"
>>>
>>> By the patch, the behavious of kernel is changed by sysctl,
>>> user can automatically create movable memory
>>> by only the following udev rule:
>>> SUBSYSTEM=="memory", ACTION=="add",
>>> ATTR{state}=="offline", ATTR{state}="online"
>       I'm sorry for replying you so late due to the busy business trip.
>> So just to be clear, we are adding a new sysctl, because the existing
>> movable_node kernel option, which is checked by movable_node_is_enabled(), and
>> does the same thing for non-hot-added-memory (?) cannot be reused for hot-added
>> memory, as that would be a potentially surprising behavior change? Correct? Then
>> this should be mentioned in the changelog too, and wherever "movable_node" is
>> documented should also mention the new sysctl. Personally, I would expect
>> movable_node to affect hot-added memory as well, and would be surprised that it
>> doesn't...
>       I think it can let the user decides when to use this feature.
>       The user can enable the feature when making the hot_added memory
> of a node movable and
>       make the feature disable to assign the hot_added memory of the next
> node to ZONE_NORMAL .

So you mean sysctl is more flexible than boot option. OK, but wasn't 
such flexibility already provided by "echo online_kernel" vs "echo 
online_movable"? It doesn't sound like a strong reason for a new sysctl? 
Not doing surprising behavior change maybe does...
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