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

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Message-Id: <20170523214922.bns675oqzqj4pkhc@arbab-laptop.localdomain>
Date:   Tue, 23 May 2017 16:49:22 -0500
From:   Reza Arbab <arbab@...ux.vnet.ibm.com>
To:     Michael Bringmann <mwb@...ux.vnet.ibm.com>
Cc:     Balbir Singh <bsingharora@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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
Subject: Re: [Patch 2/2]: powerpc/hotplug/mm: Fix hot-add memory node assoc

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.

>I didn't see where any part of commit 3af229f2071f would touch the 'node_possible_map'
>which is needed by 'numa.c' and 'workqueue.c'.  The nodemask created and updated by
>'mem_cgroup_may_update_nodemask()' does not appear to be the same mask.

Are you sure you're looking at 3af229f2071f? It only adds one line of 
code; the reduction of node_possible_map.

-- 
Reza Arbab