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

[Tang Chen]

On 10/21/2015 05:00 PM, Vlastimil Babka wrote:
> On 10/21/2015 04:20 AM, Changsheng Liu wrote:
> > å 2015/10/15 0:18, Vlastimil Babka åé:
> > > On 10/12/2015 08:58 AM, Changsheng Liu wrote:
> > > > From: Changsheng Liu <liuchangcheng@xxxxxxxxxx>
> > > >
> > > > 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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