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Date: Tue, 2 Mar 2010 18:39:20 -0800 (PST)
From: David Rientjes <rientjes@google.com>
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To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
cc: Christoph Lameter <cl@linux-foundation.org>,
       Pekka Enberg <penberg@cs.helsinki.fi>, Andi Kleen <andi@firstfloor.org>,
       Nick Piggin <npiggin@suse.de>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, haicheng.li@intel.com
Subject: Re: [patch] slab: add memory hotplug support
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On Wed, 3 Mar 2010, KAMEZAWA Hiroyuki wrote:

> At node hot-add
> 
>  * pgdat is allocated from other node (because we have no memory for "nid")
>  * memmap for the first section (and possiby others) will be allocated from
>    other nodes.
>  * Once a section for the node is onlined, any memory can be allocated localy.
> 

Correct, and the struct kmem_list3 is also alloacted from other nodes with 
my patch.

>    (Allocating memory from local node requires some new implementation as
>     bootmem allocater, we didn't that.)
> 
>  Before this patch, slab's control layer is allocated by cpuhotplug.
>  So, at least keeping this order,
>     memory online -> cpu online
>  slab's control layer is allocated from local node.
> 
>  When node-hotadd is done in this order
>     cpu online -> memory online
>  kmalloc_node() will allocate memory from other node via fallback.
> 
>  After this patch, slab's control layer is allocated by memory hotplug.
>  Then, in any order, slab's control will be allocated via fallback routine.
> 

Again, this addresses memory hotplug that requires a new node to be 
onlined that do not have corresponding cpus that are being onlined.  On 
x86, these represent ACPI_SRAT_MEM_HOT_PLUGGABLE regions that are onlined 
either by the acpi hotplug or done manually with CONFIG_ARCH_MEMORY_PROBE.  
On other architectures such as powerpc, this is done in different ways.

All of this is spelled out in the changelog for the patch.