linux-kernel - Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug

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Date:   Thu, 26 Sep 2019 13:52:58 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

On Thu 26-09-19 07:19:27, Qian Cai wrote:
> 
> 
> > On Sep 26, 2019, at 3:26 AM, Michal Hocko <mhocko@...nel.org> wrote:
> > 
> > OK, this is using for_each_online_cpu but why is this a problem? Have
> > you checked what the code actually does? Let's say that online_pages is
> > racing with cpu hotplug. A new CPU appears/disappears from the online
> > mask while we are iterating it, right? Let's start with cpu offlining
> > case. We have two choices, either the cpu is still visible and we update
> > its local node configuration even though it will disappear shortly which
> > is ok because we are not touching any data that disappears (it's all
> > per-cpu). Case when the cpu is no longer there is not really
> > interesting. For the online case we might miss a cpu but that should be
> > tolerateable because that is not any different from triggering the
> > online independently of the memory hotplug. So there has to be a hook
> > from that code path as well. If there is none then this is buggy
> > irrespective of the locking.
> > 
> > Makes sense?
> 
> This sounds to me requires lots of audits and testing. Also, someone who is more
> familiar with CPU hotplug should review this patch.

Thomas is on the CC list.

> Personally, I am no fun of
> operating on an incorrect CPU mask to begin with, things could go wrong really
> quickly...

Do you have any specific arguments? Just think of cpu and memory
hotplugs being independent operations. There is nothing really
inherently binding them together. If the cpu_online_mask really needs a
special treatment here then I would like to hear about that. Handwaving 
doesn't really helps us.

-- 
Michal Hocko
SUSE Labs