linux-kernel - Re: memory hotplug and force

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Message-ID: <alpine.LSU.2.20.1703301046570.31814@cbobk.fhfr.pm>
Date:   Thu, 30 Mar 2017 10:47:52 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
cc:     Michal Hocko <mhocko@...nel.org>, Toshi Kani <toshi.kani@...com>,
        joeyli <jlee@...e.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org
Subject: Re: memory hotplug and force_remove

On Tue, 28 Mar 2017, Rafael J. Wysocki wrote:

> > > > we have been chasing the following BUG() triggering during the memory
> > > > hotremove (remove_memory):
> > > > 	ret = walk_memory_range(PFN_DOWN(start), PFN_UP(start + size - 1), NULL,
> > > > 				check_memblock_offlined_cb);
> > > > 	if (ret)
> > > > 		BUG();
> > > > 
> > > > and it took a while to learn that the issue is caused by
> > > > /sys/firmware/acpi/hotplug/force_remove being enabled. I was really
> > > > surprised to see such an option because at least for the memory hotplug
> > > > it cannot work at all. Memory hotplug fails when the memory is still
> > > > in use. Even if we do not BUG() here enforcing the hotplug operation
> > > > will lead to problematic behavior later like crash or a silent memory
> > > > corruption if the memory gets onlined back and reused by somebody else.
> > > > 
> > > > I am wondering what was the motivation for introducing this behavior and
> > > > whether there is a way to disallow it for memory hotplug. Or maybe drop
> > > > it completely. What would break in such a case?
> > > 
> > > Honestly, I don't remember from the top of my head and I haven't looked at
> > > that code for several months.
> > > 
> > > I need some time to recall that.
> > 
> > Did you have any chance to look into this?
> 
> Well, yes.
> 
> It looks like that was added for some people who depended on the old behavior
> at that time.
> 
> I guess we can try to drop it and see what happpens. :-)

I'd agree with that; at the same time, udev rule should be submitted to 
systemd folks though. I don't think there is anything existing in this 
area yet (neither do distros ship their own udev rules for this AFAIK).

Thanks,

-- 
Jiri Kosina
SUSE Labs