linux-kernel - RE: [Bugfix] sched: fix possible invalid memory access caused by CPU hot-addition

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Date:	Wed, 23 Apr 2014 17:46:26 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Jiang Liu <jiang.liu@...ux.intel.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	"Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [Bugfix] sched: fix possible invalid memory access caused by
 CPU hot-addition

> > > 1) Handle CPU hot-addition event
> > > 1.a) gather platform specific information
> > > 1.b) associate hot-added CPU with a node
> > > 1.c) create CPU device
> > > 2) User online hot-added CPUs through sysfs:
> > > 2.a)	cpu_up()
> > > 2.b)		->try_online_node()
> > > 2.c)			->hotadd_new_pgdat()
> > > 2.d)			->node_set_online()
> > > 
> > > So between 1.b and 2.c, kmalloc_node(nid) may cause invalid
> > > memory access without the node_online(nid) check.
> > 
> > Any why was all this not in the Changelog?
>
> Also, do explain what kind of hardware you needed to trigger this. This
> code has been like this for a good while.

With your proposed fix in place the allocations will succeed - but they
will be done from other nodes ... and this cpu will have to do a remote
NUMA access for the rest of time.

It would be better to switch the order above - add the memory first,
then add the cpus.  Is that possible?

-Tony
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