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

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Date:   Thu, 3 Feb 2022 09:39:32 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Wei Yang <richard.weiyang@...il.com>, 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 03-02-22 09:23:20, 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
>  */

Yes, I like this comment more than mine. Andrew could you replace it in
your tree or should I repost the patch?

Thanks!
-- 
Michal Hocko
SUSE Labs