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

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Date:   Wed, 24 May 2017 21:19:42 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        Reza Arbab <arbab@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Bharata B Rao <bharata@...ux.vnet.ib>,
        Shailendra Singh <shailendras@...dia.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linuxppc-dev@...ts.ozlabs.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [Patch 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

Michael Bringmann <mwb@...ux.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.

cheers