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

[Tang Chen <tangchen@cn.fujitsu.com>]

On 10/21/2015 05:18 PM, Tang Chen wrote:
>
> On 10/21/2015 05:00 PM, Vlastimil Babka wrote:
>> 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@inspur.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...
>> .
>>
> Hi Vlastimil,
>
> The current kernel will add hot-added memory to ZONE_NORMAL by 
> default. If users use a udev rule as below:
>
> SUBSYSTEM=="memory", ACTION=="add", ATTR{state}=="offline", 
> ATTR{state}="online"
>
> it will online the memory as normal memory, which will not be 
> hotpluggable.
>
> Please refer to: https://lkml.org/lkml/2015/10/9/58
>
> I think this is the root motivation of the patch.
>
> But BTW, I'm quite familiar with udev rules, but can something like 
> this work ?

sorry, I'm NOT quite familiar with udev rules, ......

>
> SUBSYSTEM=="memory", ACTION=="add", ATTR{state}=="offline", 
> ATTR{state}="online_movable"
>
> I'm not sure. I added Ishimatu in.
>
> For now, I think, if the above rule works, we don't need this patch. 
> If not, maybe we should just change the kernel behavior to make the 
> hot-added memory be added to ZONE_MOVABLE by default.
>
> I don't have objection. But a sysctl doesn't sound necessary.
>
> Thanks.
>
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