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

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Message-Id: <20090717095745.1d3039b1.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 17 Jul 2009 09:57:45 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Miao Xie <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>,
	Yasunori Goto <y-goto@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [BUG] set_mempolicy(MPOL_INTERLEAV) cause kernel panic

On Fri, 17 Jul 2009 09:04:46 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> > On Wed, 15 Jul 2009, Lee Schermerhorn wrote:
> > 
> > > Interestingly, on ia64, the top cpuset mems_allowed gets set to all
> > > possible nodes, while on x86_64, it gets set to on-line nodes [or nodes
> > > with memory].  Maybe this is a to support hot-plug?
> > > 
> > 
> > numactl --interleave=all simply passes a nodemask with all bits set, so if 
> > cpuset_current_mems_allowed includes offline nodes from node_possible_map, 
> > then mpol_set_nodemask() doesn't mask them off.
> > 
> > Seems like we could handle this strictly in mempolicies without worrying 
> > about top_cpuset like in the following?
> 
> This patch seems band-aid patch. it will change memory-hotplug behavior.
> Please imazine following scenario:
> 
> 1. numactl interleave=all process-A
> 2. memory hot-add
> 
> before 2.6.30:
> 		-> process-A can use hot-added memory
> 
> your proposal patch:
> 		-> process-A can't use hot-added memory
> 

IMHO, the application itseld should be notifed to change its mempolicy by
hot-plug script on the host. While an application uses interleave, a new node
hot-added is just a noise. I think "How pages are interleaved" should not be
changed implicitly. Then, checking at set_mempolicy() seems sane. If notified,
application can do page migration and rebuild his mapping in ideal way.

BUT I don't linke init->mem_allowed contains N_POSSIBLE...it should be initialized
to N_HIGH_MEMORY, IMHO.

Thanks,
-Kame


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