Re: [PATCH v3 1/2] xen/balloon: set a mapping for ballooned out pages

[Konrad Rzeszutek Wilk]

On Thu, Jul 25, 2013 at 04:31:07AM +0100, Ian Campbell wrote:
> On Wed, 2013-07-24 at 10:58 -0400, Konrad Rzeszutek Wilk wrote:
> > On Wed, Jul 24, 2013 at 12:05:05PM +0100, Stefano Stabellini wrote:
> > > On Tue, 23 Jul 2013, Konrad Rzeszutek Wilk wrote:
> > > > On Tue, Jul 23, 2013 at 07:00:09PM +0100, Ian Campbell wrote:
> > > > > On Tue, 2013-07-23 at 18:27 +0100, Stefano Stabellini wrote:
> > > > > > +static int __cpuinit balloon_cpu_notify(struct notifier_block *self,
> > > > > > +				    unsigned long action, void *hcpu)
> > > > > > +{
> > > > > > +	int cpu = (long)hcpu;
> > > > > > +	switch (action) {
> > > > > > +	case CPU_UP_PREPARE:
> > > > > > +		if (per_cpu(balloon_scratch_page, cpu) != NULL)
> > > > > > +			break;
> > > > > 
> > > > > Thinking about this a bit more -- do we know what happens to the per-cpu
> > > > > area for a CPU which is unplugged and then reintroduced? Is it preserved
> > > > > or is it reset?
> > > > > 
> > > > > If it is reset then this gets more complicated :-( We might be able to
> > > > > use the core mm page reference count, so that when the last reference is
> > > > > removed the page is automatically reclaimed. We can obviously take a
> > > > > reference whenever we add a mapping of the trade page, but I'm not sure
> > > > > we are always on the path which removes such mappings... Even then you
> > > > > could waste pages for some potentially large amount of time each time
> > > > > you replug a VCPU.
> > > > > 
> > > > > Urg, I really hope the per-cpu area is preserved!
> > > > 
> > > > It is. During bootup time you see this:
> > > > 
> > > > [    0.000000] smpboot: Allowing 128 CPUs, 96 hotplug CPU
> > > > [    0.000000] setup_percpu: NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:128 nr_node_ids:1
> > > > 
> > > > which means that all of the per_CPU are shrunk down to 128 (from 
> > > > CONFIG_NR_CPUS=512 was built with) and stays for the lifetime of the kernel.
> > > > 
> > > > You might have to clear it when the vCPU comes back up though - otherwise you
> > > > will have garbage.
> > > 
> > > I don't see anything in the hotplug code that would modify the value of
> > > the per_cpu area of offline cpus.
> > 
> > You might have never onlined the CPUs and the kernel is built with DEBUG options
> > which poison the page.
> > 
> > Anyhow, doing a memset seems like a prudent thing to do? Perhaps when
> > built with CONFG_DEBUG_XENFS you add poison values to it?
> 
> The point is that the patches need for the per-cpu areas to *not* be
> reinitialised over a vcpu unplug+plug, otherwise we will leak the
> original page when we allocate the new one on plug.

OK.
> 
> We can't just free the page on vcpu unplug because it might still be in
> use.

I am still worried about before-the-cpu-is-up-the-per-cpu-has-garbage case.
We could add code in the boot-before-smp (so when there is only one CPU) to
do:

for_each_possible(cpu)
	memset(__per_cpu(some_memory),0,sizeof...);

and then I think it satisfies your concerns and mine?
> 
> Ian.
> 
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