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

[Vlastimil Babka]

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...
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