Re: [PATCH] VMware Balloon driver

From: Jeremy Fitzhardinge
Date: Mon Apr 05 2010 - 18:03:18 EST

On 04/05/2010 02:24 PM, Andrew Morton wrote:
I think I've forgotten what balloon drivers do. Are they as nasty a
hack as I remember believing them to be?

(I haven't looked at Dmitry's patch yet, so this is from the Xen perspective.)

In the simplest form, they just look like a driver which allocates a pile of pages, and the underlying memory gets returned to the hypervisor. When you want the memory back, it reattaches memory to the pageframes and releases the memory back to the kernel. This allows a virtual machine to shrink with respect to its original size.

Going the other way - expanding beyond the memory allocation - is a bit trickier because you need to get some new page structures from somewhere. We don't do this in Xen yet, but I've done some experiments with hotplug memory to implement this. Or a simpler approach is to fake up some reserved E820 ranges to grow into.

A summary of what this code sets out to do, and how it does it would be

Also please explain the applicability of this driver. Will xen use it?
kvm? Out-of-tree code?
The basic idea of the driver is to allow a guest system to give up memory it isn't using so it can be reused by other virtual machines (or the host itself).

Xen and KVM already have equivalents in the kernel. Now that I've had a quick look at Dmitry's patch, it's certainly along the same lines as the Xen code, but it isn't clear to me how much code they could end up sharing. There's a couple of similar-looking loops, but the bulk of the code appears to be VMware specific.

One area that would be very useful as common code would be some kind of policy engine to drive the balloon driver. That is, something that can look at the VM's state and say "we really have a couple hundred MB of excess memory we could happily give back to the host". And - very important - "don't go below X MB, because then we'll die in a flaming swap storm".

At the moment this is driven by vendor-specific tools with heuristics of varying degrees of sophistication (which could be as simple as absolutely manual control). The problem has two sides because there's the decision made by guests on how much memory they can afford to give up, and also on the host side who knows what the system-wide memory pressures are. And it can be affected by hypervisor-specific features, such as whether pages can be transparently shared between domains, demand-faulted from swap, etc.

And Dan Magenheimer is playing with a more fine-grained mechanism where a guest kernel can draw on spare host memory without actually committing that memory to the guest, which allows memory to be reallocated on the fly with more fluidity.

The code implements a user-visible API (in /proc, at least). Please
fully describe the proposed interface(s) in the changelog so we can
review and understand that proposal.

It seems to me that sysfs would be a better match. It would be nice to try and avoid gratuitous differences.


+static bool vmballoon_send_start(struct vmballoon *b)
+ unsigned long status, dummy;
+ STATS_INC(b->stats.start);
+ if (status == VMW_BALLOON_SUCCESS)
+ return true;
+ pr_debug("%s - failed, hv returns %ld\n", __func__, status);
The code refers to something called "hv". I suspect that's stale?

hv = hypervisor

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