linux-kernel - Re: [PATCH v6 00/15] memory-hotplug: hot-remove physical memory

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Date:	Thu, 31 Jan 2013 00:19:22 -0600
From:	Simon Jeons <simon.jeons@...il.com>
To:	Tang Chen <tangchen@...fujitsu.com>
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com,
	len.brown@...el.com, benh@...nel.crashing.org, paulus@...ba.org,
	cl@...ux.com, minchan.kim@...il.com,
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	yinghai@...nel.org, glommer@...allels.com, x86@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-acpi@...r.kernel.org,
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Subject: Re: [PATCH v6 00/15] memory-hotplug: hot-remove physical memory

Hi Tang,
On Thu, 2013-01-31 at 11:31 +0800, Tang Chen wrote:
> Hi Simon,
> 
> Please see below. :)
> 
> On 01/31/2013 09:22 AM, Simon Jeons wrote:
> >
> > Sorry, I still confuse. :(
> > update node_states[N_NORMAL_MEMORY] to node_states[N_MEMORY] or
> > node_states[N_NORMAL_MEMOR] present 0...ZONE_MOVABLE?
> >
> > node_states is what? node_states[N_NORMAL_MEMOR] or
> > node_states[N_MEMORY]?
> 
> Are you asking what node_states[] is ?
> 
> node_states[] is an array of nodemask,
> 
>      extern nodemask_t node_states[NR_NODE_STATES];
> 
> For example, node_states[N_NORMAL_MEMOR] represents which nodes have 
> normal memory.
> If N_MEMORY == N_HIGH_MEMORY == N_NORMAL_MEMORY, node_states[N_MEMORY] is
> node_states[N_NORMAL_MEMOR]. So it represents which nodes have 0 ... 
> ZONE_MOVABLE.
> 

Sorry, how can nodes_state[N_NORMAL_MEMORY] represents a node have 0 ...
*ZONE_MOVABLE*, the comment of enum nodes_states said that
N_NORMAL_MEMORY just means the node has regular memory.  

> 
> > Why check !z1->wait_table in function move_pfn_range_left and function
> > __add_zone? I think zone->wait_table is initialized in
> > free_area_init_core, which will be called during system initialization
> > and hotadd_new_pgdat path.
> 
> I think,
> 
> free_area_init_core(), in the for loop,
>   |--> size = zone_spanned_pages_in_node();
>   |--> if (!size)
>                continue;  ----------------  If zone is empty, we jump 
> out the for loop.
>   |--> init_currently_empty_zone()
> 
> So, if the zone is empty, wait_table is not initialized.
> 
> In move_pfn_range_left(z1, z2), we move pages from z2 to z1. But z1 
> could be empty.
> So we need to check it and initialize z1->wait_table because we are 
> moving pages into it.

thanks.

> 
> 
> > There is a zone populated check in function online_pages. But zone is
> > populated in free_area_init_core which will be called during system
> > initialization and hotadd_new_pgdat path. Why still need this check?
> >
> 
> Because we could also rebuild zone list when we offline pages.
> 
> __offline_pages()
>   |--> zone->present_pages -= offlined_pages;
>   |--> if (!populated_zone(zone)) {
>                build_all_zonelists(NULL, NULL);
>        }
> 
> If the zone is empty, and other zones on the same node is not empty, the 
> node
> won't be offlined, and next time we online pages of this zone, the pgdat 
> won't
> be initialized again, and we need to check populated_zone(zone) when 
> onlining
> pages.

thanks.

> 
> Thanks. :)
> 
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