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

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Message-ID: <YfubhocKKKWgKlEq@kernel.org>
Date:   Thu, 3 Feb 2022 11:08:22 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     David Hildenbrand <david@...hat.com>
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 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 ;-)
 
> or (because there is also /sys/devices/system/node/online  )
> 
> "so that the node won't be indicated as online to user space, for
> example, via register_one_node()."
> 
> [1] https://lkml.kernel.org/r/20220128151540.164759-1-david@redhat.com
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.