linux-kernel - Re: powerpc,numa: Memory hotplug to memory-less nodes ?

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Message-ID: <558A9EF0.3010305@linux.vnet.ibm.com>
Date:	Wed, 24 Jun 2015 07:13:36 -0500
From:	Nathan Fontenot <nfont@...ux.vnet.ibm.com>
To:	Bharata B Rao <bharata.rao@...il.com>,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@....ibm.com>
Subject: Re: powerpc,numa: Memory hotplug to memory-less nodes ?

On 06/23/2015 11:01 PM, Bharata B Rao wrote:
> So will it be correct to say that memory hotplug to memory-less node
> isn't supported by PowerPC kernel ? Should I enforce the same in QEMU
> for PowerKVM ?
>

I'm not sure if that is correct. It appears that we initialize all online
nodes, even those without spanned_pages, at boot time. This occurs
in setup_node_data() called from initmem_init().

Looking at this I would think that we could add memory to any online node
even if it does not have any spanned_pages. I think an interesting test
we be to check for the node being online instead of checking to see if
it has any memory.

-Nathan

> On Mon, Jun 22, 2015 at 10:18 AM, Bharata B Rao <bharata.rao@...il.com> wrote:
>> Hi,
>>
>> While developing memory hotplug support in QEMU for PoweKVM, I
>> realized that guest kernel has specific checks to prevent hot addition
>> of memory to a memory-less node.
>>
>> I am referring to arch/powerpc/mm/numa.c:hot_add_scn_to_nid() which
>> has explicit checks to ensure that it returns a nid that has some some
>> memory (NODE_DATA(nid)->node_spanned_pages) even when user wants to
>> hotplug to a node that currently has zero memory.
>>
>> Is this limitation by design ?
>>
>> Regards,
>> Bharata.
>> --
>> http://raobharata.wordpress.com/
> 
> 
> 

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