linux-kernel - Re: [PATCH 2/6] mm: handle uninitialized numa nodes gracefully

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Date:   Thu, 3 Feb 2022 09:27:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>,
        Wei Yang <richard.weiyang@...il.com>
Cc:     Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Makhalov <amakhalov@...are.com>,
        Dennis Zhou <dennis@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Oscar Salvador <osalvador@...e.de>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Nico Pache <npache@...hat.com>,
        Rafael Aquini <raquini@...hat.com>
Subject: Re: [PATCH 2/6] mm: handle uninitialized numa nodes gracefully

On 03.02.22 08:23, Mike Rapoport wrote:
> On Thu, Feb 03, 2022 at 12:21:16AM +0000, Wei Yang wrote:
>> On Tue, Feb 01, 2022 at 10:54:37AM +0100, Michal Hocko wrote:
>>> On Tue 01-02-22 02:41:19, Wei Yang wrote:
>>>> On Thu, Jan 27, 2022 at 03:47:40PM +0100, Michal Hocko wrote:
>>>> [...]
>>>>>>> +			/*
>>>>>>> +			 * not marking this node online because we do not want to
>>>>>>> +			 * confuse userspace by sysfs files/directories for node
>>>>>>> +			 * without any memory attached to it (see topology_init)
>>>>>>> +			 * The pgdat will get fully initialized when a memory is
>>>>>>> +			 * hotpluged into it by hotadd_init_pgdat
>>>>>>> +			 */
>>>>
>>>> Hmm... which following step would mark the node online? On x86, the node is
>>>> onlined in alloc_node_date(). This is not onlined here.
>>>
>>> The comment tries to explain that this happens during the memory
>>> hotplug. Or maybe I have missed your question?
>>>
>>
>> I am not sure for others, while the comment confused me a little.
>>
>> Currently in kernel, there are two situations for node online:
>>
>>   * during memory hotplug
>>   * during sys-init
>>
>> For memory hotplug, we allocate pgdat and online node. And current hot-add
>> process has already put them in two steps:
>>
>>   1. __try_online_node()
>>   2. node_set_online()
>>
>> So emphasize "not online" node here, confuse me a little. It is a natural
>> thing to not online node until it has memory.
>>
>> But from another point of view, the comment here is reasonable. During
>> sys-init, we online node at the same time when creating pgdat. And even for
>> memory-less node on x86, we online them too.
>>
>> Well, this is all about the comment. I have tried to grab may head but not
>> come up with a better idea.
>>
>> Or maybe this is just my personal feeling, don't bother if no-one else feel
>> like this.
> 
> I shuffled the words a bit, maybe this sounds better not only to me :)
> 
> /*
>  * We do not want to confuse userspace by sysfs files/directories for node
>  * without any memory attached to it, so this node is not marked as
>  * N_MEMORY and not marked online so that topology_init() won't create
>  * sysfs hierarchy for this node.  The pgdat will get fully initialized by
>  * hotadd_init_pgdat() when memory is hotpluged into this node
>  */
> 

Note that the topology_init() part might change soon [1] so maybe we
want to rephrase that to "so that no sysfs hierarchy will be created via
register_one_node() for this node." right away.

or (because there is also /sys/devices/system/node/online  )

"so that the node won't be indicated as online to user space, for
example, via register_one_node()."

[1] https://lkml.kernel.org/r/20220128151540.164759-1-david@redhat.com

-- 
Thanks,

David / dhildenb