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

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Message-ID: <15ad305f-a147-4a9c-35e5-bb0a868499ad@redhat.com>
Date:   Thu, 3 Feb 2022 10:11:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        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 10:08, Mike Rapoport wrote:
> On Thu, Feb 03, 2022 at 09:27:16AM +0100, David Hildenbrand wrote:
>> 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.
> 
> Heh, this will be your responsibility to update the comment here when you
> post non-RFC version ;-)

I'm usually sending patches against Linus' tree. And I'll post non-RFC
most probably today (after testing on aarch64) ;)

So I'd appreciate if we could just phrase it more generically, as I
tried. But of course, we can try making my life harder ;)

-- 
Thanks,

David / dhildenb