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

From: Wei Yang <richard.weiyang@gmail.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Wei Yang <richard.weiyang@gmail.com>,
	Mike Rapoport <rppt@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	David Hildenbrand <david@redhat.com>,
	Alexey Makhalov <amakhalov@vmware.com>,
	Dennis Zhou <dennis@kernel.org>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	Oscar Salvador <osalvador@suse.de>, Tejun Heo <tj@kernel.org>,
	Christoph Lameter <cl@linux.com>, Nico Pache <npache@redhat.com>,
	Rafael Aquini <raquini@redhat.com>
Subject: Re: [PATCH 2/6] mm: handle uninitialized numa nodes gracefully
Date: Thu, 3 Feb 2022 00:21:16 +0000	[thread overview]
Message-ID: <20220203002116.akpk5um5nkpeflve@master> (raw)
In-Reply-To: <YfkDXYhGjAvAKKeU@dhcp22.suse.cz>

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