Re: [Patch 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

[Michael Bringmann <mwb@linux.vnet.ibm.com>]

On 05/24/2017 06:19 AM, Michael Ellerman wrote:
> Michael Bringmann <mwb@linux.vnet.ibm.com> writes:
> 
>> On 05/23/2017 04:49 PM, Reza Arbab wrote:
>>> On Tue, May 23, 2017 at 03:05:08PM -0500, Michael Bringmann wrote:
>>>> On 05/23/2017 10:52 AM, Reza Arbab wrote:
>>>>> On Tue, May 23, 2017 at 10:15:44AM -0500, Michael Bringmann wrote:
>>>>>> +static void setup_nodes(void)
>>>>>> +{
>>>>>> +    int i, l = 32 /* MAX_NUMNODES */;
>>>>>> +
>>>>>> +    for (i = 0; i < l; i++) {
>>>>>> +        if (!node_possible(i)) {
>>>>>> +            setup_node_data(i, 0, 0);
>>>>>> +            node_set(i, node_possible_map);
>>>>>> +        }
>>>>>> +    }
>>>>>> +}
>>>>>
>>>>> This seems to be a workaround for 3af229f2071f ("powerpc/numa: Reset node_possible_map to only node_online_map").
>>>>
>>>> They may be related, but that commit is not a replacement.  The above patch ensures that
>>>> there are enough of the nodes initialized at startup to allow for memory hot-add into a
>>>> node that was not used at boot.  (See 'setup_node_data' function in 'numa.c'.)  That and
>>>> recording that the node was initialized.
>>>
>>> Is it really necessary to preinitialize these empty nodes using setup_node_data()? When you do memory hotadd into a node that was not used at boot, the node data already gets set up by
>>>
>>> add_memory
>>>  add_memory_resource
>>>    hotadd_new_pgdat
>>>      arch_alloc_nodedata <-- allocs the pg_data_t
>>>      ...
>>>      free_area_init_node <-- sets NODE_DATA(nid)->node_id, etc.
>>>
>>> Removing setup_node_data() from that loop leaves only the call to node_set(). If 3af229f2071f (which reduces node_possible_map) was reverted, you wouldn't need to do that either.
>>
>> With or without 3af229f2071f, we would still need to add something, somewhere to add new
>> bits to the 'node_possible_map'.  That is not being done.
> 
> You mustn't add bits to the possible map after boot.
> 
> That's its purpose, to tell you what nodes could ever *possibly* exist.

The problem that I have been encountering is that the 'possible map' did *not*
show all of the possible nodes.  Rather, it showed only the nodes that were
assigned memory at boot-up.  If more memory were hot-added to the kernel, it
could be assigned into one of the nodes that were skipped at boot.  However,
nothing was updating the 'node_possible_map' correctly in the kernel memory
code.

Reza pointed out a code change in commit 3af229f2071f that has not made it into
the 4.12 checkout i.e. removing the instruction that reduces the node_possible_map.
This may well be a suitable replacement for the code that I have here, and I
will test it here next.

> 
> cheers
> 
> 
Later.

-- 
Michael W. Bringmann
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