linux-kernel - Re: [PATCH] acpi: drop support for force

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Date:   Tue, 11 Apr 2017 00:15:42 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Joey Lee <jlee@...e.com>, Jiri Kosina <jkosina@...e.cz>,
        Kani Toshimitsu <toshi.kani@....com>,
        Len Brown <lenb@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-acpi@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH] acpi: drop support for force_remove

On Monday, April 10, 2017 07:13:44 PM Michal Hocko wrote:
> On Mon 03-04-17 09:40:23, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > /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.
> > 
> > Reviewed-by: Lee, Chun-Yi <jlee@...e.com>
> > Signed-off-by: Michal Hocko <mhocko@...e.com>
> 
> ping on this?

/shrug/

I'll apply it if nobody has any problems with it.

Thanks,
Rafael