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

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
back-compatible with previous kernels.

> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2537,6 +2537,8 @@ bytes respectively. Such letter suffixes can also be 
> entirely omitted.
>                       shutdown the other cpus.  Instead use the REBOOT_VECTOR
>                       irq.
> +     nomemhp_autoonline      Don't automatically online newly added memory.
> +

This wasn't mentioned in the changelog.  Why do we need a boot
parameter as well as the sysfs knob?

> +     bool "Automatically online hot-added memory"
> +     depends on MEMORY_HOTPLUG_SPARSE
> +     help
> +       When memory is hot-added, it is not at ready-to-use state, a special

"When memory is hot-added it is not in a ready-to-use state.  A special"

> +       userspace action is required to online the newly added blocks. With
> +       this option enabled, the kernel will try to online all newly added
> +       memory automatically.
> +
> ...

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