Messages in this thread | | | Date | Sun, 4 Aug 2013 15:30:36 +0100 | From | Stefano Stabellini <> | Subject | Re: [PATCH v3 1/2] xen/balloon: set a mapping for ballooned out pages |
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On Mon, 29 Jul 2013, Konrad Rzeszutek Wilk wrote: > 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?
OK, I'll add an early_initcall.
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