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 05:00 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 ?

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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