Re: [PATCH RESEND part2 v2 0/8] Arrange hotpluggable memory as ZONE_MOVABLE

[Ingo Molnar <mingo@kernel.org>]

* Zhang Yanfei <zhangyanfei.yes@gmail.com> wrote:

> Hello Andrew
> 
> On 12/04/2013 07:48 AM, Andrew Morton wrote:
> > On Tue, 03 Dec 2013 10:19:44 +0800 Zhang Yanfei <zhangyanfei@cn.fujitsu.com> wrote:
> > 
> >> The current Linux cannot migrate pages used by the kerenl because
> >> of the kernel direct mapping. In Linux kernel space, va = pa + PAGE_OFFSET.
> >> When the pa is changed, we cannot simply update the pagetable and
> >> keep the va unmodified. So the kernel pages are not migratable.
> >>
> >> There are also some other issues will cause the kernel pages not migratable.
> >> For example, the physical address may be cached somewhere and will be used.
> >> It is not to update all the caches.
> >>
> >> When doing memory hotplug in Linux, we first migrate all the pages in one
> >> memory device somewhere else, and then remove the device. But if pages are
> >> used by the kernel, they are not migratable. As a result, memory used by
> >> the kernel cannot be hot-removed.
> >>
> >> Modifying the kernel direct mapping mechanism is too difficult to do. And
> >> it may cause the kernel performance down and unstable. So we use the following
> >> way to do memory hotplug.
> >>
> >>
> >> [What we are doing]
> >>
> >> In Linux, memory in one numa node is divided into several zones. One of the
> >> zones is ZONE_MOVABLE, which the kernel won't use.
> >>
> >> In order to implement memory hotplug in Linux, we are going to arrange all
> >> hotpluggable memory in ZONE_MOVABLE so that the kernel won't use these memory.
> > 
> > How does the user enable this?  I didn't spot a Kconfig variable which
> > enables it.  Is there a boot option?
> 
> Yeah, there is a Kconfig variable "MOVABLE_NODE" and a boot option "movable_node"
> 
> mm/Kconfig
> 
> config MOVABLE_NODE

Some bikeshedding: I suspect 'movable nodes' is the right idiom to use 
here, unless the feature is restricted to a single node only.

So the option should be 'CONFIG_MOVABLE_NODES=y' and 
'movable_nodes=...'.

Thanks,

	Ingo

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