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Date:   Tue, 2 Nov 2021 10:40:43 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Alexey Makhalov <amakhalov@vmware.com>
Cc:     David Hildenbrand <david@redhat.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Oscar Salvador <OSalvador@suse.com>
Subject: Re: [PATCH] mm: fix panic in __alloc_pages
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On Tue 02-11-21 10:04:23, Michal Hocko wrote:
[...]
> > Is there any reason why try_online_node() resides in cpu_up() and not in add_cpu()?
> > I think it would be correct to online node during the CPU hot add to align with
> > memory hot add.
> 
> I am not familiar with cpu hotplug, but this doesn't seem to be anything
> new so how come this became problem only now?

Just looked at the cpu hotplug part. I do not see add_cpu to add much
here. Here is what I can see in the current Linus tree
add_cpu
  device_online() # cpu device - cpu_sys_devices with cpu_subsys bus
    dev->bus->online -> cpu_subsys_online
      cpu_device_up
        cpu_up
	  try_online_node

So we should be bringing up the node during add_cpu. Unless something
fails on the way - e.g. cpu_possible check or something similar.

-- 
Michal Hocko
SUSE Labs