From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Michal Hocko <mhocko@kernel.org>
Cc: joeyli <jlee@suse.com>, Kani Toshimitsu <toshi.kani@hpe.com>,
        Jiri Kosina <jkosina@suse.cz>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>, linux-api@vger.kernel.org
Subject: Re: memory hotplug and force_remove
Date: Sat, 01 Apr 2017 00:35:16 +0200
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On Friday, March 31, 2017 02:02:36 PM Michal Hocko wrote:
> On Fri 31-03-17 19:55:30, Joey Lee wrote:
> > On Fri, Mar 31, 2017 at 12:55:05PM +0200, Michal Hocko wrote:
> > > On Fri 31-03-17 18:49:05, Joey Lee wrote:
> > > > Hi Michal,
> > > > 
> > > > On Fri, Mar 31, 2017 at 10:30:17AM +0200, Michal Hocko wrote:
> > > [...]
> > > > > @@ -241,11 +232,10 @@ static int acpi_scan_try_to_offline(struct acpi_device *device)
> > > > >  		acpi_walk_namespace(ACPI_TYPE_ANY, handle, ACPI_UINT32_MAX,
> > > > >  				    NULL, acpi_bus_offline, (void *)true,
> > > > >  				    (void **)&errdev);
> > > > > -		if (!errdev || acpi_force_hot_remove)
> > > > > +		if (!errdev)
> > > > >  			acpi_bus_offline(handle, 0, (void *)true,
> > > > >  					 (void **)&errdev);
> > > > > -
> > > > > -		if (errdev && !acpi_force_hot_remove) {
> > > > > +		else {
> > > >               ^^^^^^^^^^^^^
> > > > Here should still checks the parent's errdev state then rollback
> > > > parent/children to online state:
> > > > 
> > > > -		if (errdev && !acpi_force_hot_remove) {
> > > > +		if (errdev) {
> > > 
> > > You are right, I have missed that acpi_bus_offline modifies errdev.
> > > Thanks for spotting that! Updated patch is below.
> > > ---
> > > >From 8df0abd29988ffb52b6df52407b96d6015861bb7 Mon Sep 17 00:00:00 2001
> > > From: Michal Hocko <mhocko@suse.com>
> > > Date: Fri, 31 Mar 2017 10:08:41 +0200
> > > Subject: [PATCH] acpi: drop support for force_remove
> > > 
> > > /sys/firmware/acpi/hotplug/force_remove was presumably added to support
> > > auto offlining in the past. This is, however, inherently dangerous for
> > > some hotplugable resources like memory. The memory offlining fails when
> > > the memory is still in use and cannot be dropped or migrated. If we
> > > ignore the failure we are basically allowing for subtle memory
> > > corruption or a crash.
> > > 
> > > We have actually noticed the later while hitting BUG() 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();
> > > 
> > > it took us quite non-trivial time realize that the customer had
> > > force_remove enabled. Even if the BUG was removed here and we could
> > > propagate the error up the call chain it wouldn't help at all because
> > > then we would hit a crash or a memory corruption later and harder to
> > > debug. So force_remove is unfixable for the memory hotremove. We haven't
> > > checked other hotplugable resources to be prone to a similar problems.
> > > 
> > > Remove the force_remove functionality because it is not fixable currently.
> > > Keep the sysfs file and report an error if somebody tries to enable it.
> > > Encourage users to report about the missing functionality and work with
> > > them with an alternative solution.
> > > 
> > > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > 
> > This patch is good to me. Please feel free to add:
> > 
> > Reviewed-by: Lee, Chun-Yi <jlee@suse.com>
> 
> Thanks for the review Joey!

Can you please resend it with a CC to linux-acpi to give the people on that list
a chance to speak up if they have any concerns?

Thanks,
Rafael