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

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Date:	Thu, 24 Apr 2014 09:47:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
Cc:	"Luck, Tony" <tony.luck@...el.com>,
	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

On Thu, Apr 24, 2014 at 10:59:45AM +0800, Jiang Liu wrote:
> On 2014/4/24 1:46, Luck, Tony wrote:
> >>>> 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?
> Hi Tony,
> 	The BIOS always sends CPU hot-addition events before memory
> hot-addition events, so it's hard to change the order.
> 	And we couldn't completely solve this performance penalty because the
> affected code tries to allocate memory for all possible
> CPUs instead of onlined CPUs.

So the BIOS is fucked, news at 11, one would have hoped Intel would have
_some_ say in it, but alas. So how about instead you force memory online
when you online the first CPU, screw whatever the BIOS does or does not?
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