Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock

From: Qian Cai <cai@lca.pw>
To: Michal Hocko <mhocko@kernel.org>
Cc: David Hildenbrand <david@redhat.com>,
	linux-kernel@vger.kernel.org,  linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Oscar Salvador <osalvador@suse.de>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock
Date: Thu, 26 Sep 2019 09:02:26 -0400	[thread overview]
Message-ID: <1569502946.5576.237.camel@lca.pw> (raw)
In-Reply-To: <20190926115258.GH20255@dhcp22.suse.cz>

On Thu, 2019-09-26 at 13:52 +0200, Michal Hocko wrote:
> On Thu 26-09-19 07:19:27, Qian Cai wrote:
> > 
> > 
> > > On Sep 26, 2019, at 3:26 AM, Michal Hocko <mhocko@kernel.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.

That is why I said it needs CPU hotplug experts to confirm that things including
if CPU masks are tolerate to this kind of "abuse", or in-depth analysis of each 
calls sites that access CPU masks in both online_pages() and offline_pages() as
well as ideally, more testing data in those areas.

However, many kernel commits were merged with the expectations that people are
going to deal with the aftermath, so I am not going to insist.