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

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Date:   Thu, 3 Feb 2022 09:23:20 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     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>,
        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 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
 */

-- 
Sincerely yours,
Mike.