linux-kernel - Re: [BUG] set_mempolicy(MPOL_INTERLEAV) cause kernel panic

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Date:	Mon, 27 Jul 2009 17:38:30 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	miaox@...fujitsu.com, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Paul Menage <menage@...gle.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, y-goto@...fujitsu.com,
	Pekka Enberg <penberg@...helsinki.fi>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] set_mempolicy(MPOL_INTERLEAV) cause kernel panic

On Tue, 28 Jul 2009, KAMEZAWA Hiroyuki wrote:

> Because we dont' update, task->mems_allowed need to be initilaized as
> N_POSSIBLE_NODES. At usual thinking,  it should be N_HIGH_MEMORY or
> N_ONLINE_NODES, as my patch does.
> 

On MEM_OFFLINE, cpusets calls scan_for_empty_cpusets() which will 
intersect each system cpuset's mems_allowed with N_HIGH_MEMORY.  It then 
calls update_tasks_nodemask() which will update task->mems_allowed for 
each task assigned to those cpusets.  This has a callback into the 
mempolicy code to rebind the policy with the new mems.

So there's no apparent issue with memory hotplug in dealing with cpuset 
mems, although I suggested that this be done for MEM_GOING_OFFLINE instead 
of waiting until the mem is actually offline.

The problem originally reported here doesn't appear to have anything to do 
with hotplug, it looks like it is the result of Lee's observation that 
ia64 defaults top_cpuset's mems to N_POSSIBLE, which _should_ have been 
updated by cpuset_init_smp().  So it makes me believe that N_HIGH_MEMORY 
isn't actually ready by the time do_basic_setup() is called to be useful.
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