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Date: Mon, 28 Aug 2006 23:19:17 -0700
From: Andrew Morton <akpm@osdl.org>
To: Paul Jackson <pj@sgi.com>
Cc: nathanl@austin.ibm.com, Simon.Derr@bull.net, linux-kernel@vger.kernel.org,
       ntl@pobox.com, y-goto@jp.fujitsu.com, Anton Blanchard <anton@samba.org>,
       Dave Hansen <haveblue@us.ibm.com>, kamezawa.hiroyu@jp.fujitsu.com
Subject: Re: [PATCH] cpuset: hotunplug cpus and mems in all cpusets
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On Mon, 28 Aug 2006 23:08:24 -0700
Paul Jackson <pj@sgi.com> wrote:

> The cpuset code handling hot unplug of CPUs or Memory Nodes
> was incorrect - it could remove a CPU or Node from the top
> cpuset, while leaving it still in some child cpusets.
> 
> One basic rule of cpusets is that each cpusets cpus and mems
> are subsets of its parents.  The cpuset hot unplug code
> violated this rule.
> 
> So the cpuset hotunplug handler must walk down the tree,
> removing any removed CPU or Node from all cpusets.
> 
> However, it is not allowed to make a cpusets cpus or mems
> become empty.  They can only transition from empty to non-empty,
> not back.
> 
> So if the last CPU or Node would be removed from a cpuset by
> the above walk, we scan back up the cpuset hierarchy, finding
> the nearest ancestor that still has something online, and copy
> its CPU or Memory placement.

Did you consider failing the hotremove request instead?