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

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Message-ID: <20220203002116.akpk5um5nkpeflve@master>
Date:   Thu, 3 Feb 2022 00:21:16 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Hildenbrand <david@...hat.com>,
        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 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.

>[...]
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me