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

David Hildenbrand
2022-02-03T08:27:22Z

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