Re: [PATCH part1 v6 4/6] x86/mem-hotplug: Support initialize page tables in bottom-up

[Zhang Yanfei <zhangyanfei.yes@gmail.com>]

Hello Peter,

On 10/10/2013 05:14 AM, H. Peter Anvin wrote:
> On 10/09/2013 02:11 PM, Tejun Heo wrote:
>> Hello, Toshi.
>>
>> On Wed, Oct 09, 2013 at 02:58:31PM -0600, Toshi Kani wrote:
>>> Let's not assume that memory hotplug is always a niche feature for huge
>>> & special systems.  It may be a niche to begin with, but it could be
>>> supported on VMs, which allows anyone to use.  Vasilis has been working
>>> on KVM to support memory hotplug.
>>
>> I'm not saying hotplug will always be niche.  I'm saying the approach
>> we're currently taking is.  It seems fairly inflexible to hang the
>> whole thing on NUMA nodes.  What does the planned kvm support do?
>> Splitting SRAT nodes so that it can do both actual NUMA node
>> distribution and hotplug granuliarity?  IIRC I asked a couple times
>> what the long term plan was for this feature and there doesn't seem to
>> be any road map for this thing to become a full solution.  Unless I
>> misunderstood, this is more of "let's put out the fire as there
>> already are (or gonna be) machines which can do it" kinda thing, which
>> is fine too.  My point is that it doesn't make a lot of sense to
>> change boot sequence invasively to accomodate that.
>>
> 
> I would also argue that in the VM scenario -- and arguable even in the
> hardware scenario -- the right thing is to not expose the flexible
> memory in the e820/EFI tables, and instead have it hotadded (possibly
> *immediately* so) on boot.  This avoids both the boot time funnies as
> well as the scaling issues with metadata.
> 

So in this kind of scenario, hotpluggable memory will not be detected
at boot time, and admin should not use this movable_node boot option
and the kernel will act as before, using top-down allocation always.

-- 
Thanks.
Zhang Yanfei

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