Re: [Xen-devel] [PATCH v2] memory-hotplug: add automatic onlining policy for the newly added memory

[David Rientjes <rientjes@xxxxxxxxxx>]

On Tue, 22 Dec 2015, Andrew Morton wrote:

> On Tue, 22 Dec 2015 17:32:30 +0100 Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> 
> wrote:
> 
> > Currently, all newly added memory blocks remain in 'offline' state unless
> > someone onlines them, some linux distributions carry special udev rules
> > like:
> > 
> > SUBSYSTEM=="memory", ACTION=="add", ATTR{state}=="offline", 
> > ATTR{state}="online"
> > 
> > to make this happen automatically. This is not a great solution for virtual
> > machines where memory hotplug is being used to address high memory pressure
> > situations as such onlining is slow and a userspace process doing this
> > (udev) has a chance of being killed by the OOM killer as it will probably
> > require to allocate some memory.
> > 
> > Introduce default policy for the newly added memory blocks in
> > /sys/devices/system/memory/hotplug_autoonline file with two possible
> > values: "offline" which preserves the current behavior and "online" which
> > causes all newly added memory blocks to go online as soon as they're added.
> > The default is "online" when MEMORY_HOTPLUG_AUTOONLINE kernel config option
> > is selected.
> 
> I think the default should be "offline" so vendors can ship kernels
> which have CONFIG_MEMORY_HOTPLUG_AUTOONLINE=y while being
> back-compatible with previous kernels.
> 

But isn't the premise of the changelog that this is currently being 
handled by the distribution?  Perhaps I don't understand why this patch 
can't end up just introducing a sysfs tunable that is always present and 
can be set by initscripts of that distribution.

I'd also suggest that hotplug_autoonline be renamed to auto_online_block.

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