linux-kernel - Re: memory hotplug and force

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Message-ID: <2735706.OR0SQDpVy6@aspire.rjw.lan>
Date:   Mon, 20 Mar 2017 22:24:42 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Toshi Kani <toshi.kani@...com>, Jiri Kosina <jkosina@...e.cz>,
        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 Monday, March 20, 2017 03:29:39 PM Michal Hocko wrote:
> Hi Rafael,

Hi,

> 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.

Thanks,
Rafael