Re: [PATCH] Add a page cache-backed balloon device driver.

On Wed, Jun 27, 2012 at 08:48:55AM -0700, Frank Swiderski wrote:

On Tue, Jun 26, 2012 at 7:56 PM, Rusty Russell [off-list ref] wrote:

quoted

On Wed, 27 Jun 2012 00:41:06 +0300, "Michael S. Tsirkin" [off-list ref] wrote:

quoted

On Tue, Jun 26, 2012 at 01:32:58PM -0700, Frank Swiderski wrote:

quoted

This implementation of a virtio balloon driver uses the page cache to
"store" pages that have been released to the host.  The communication
(outside of target counts) is one way--the guest notifies the host when
it adds a page to the page cache, allowing the host to madvise(2) with
MADV_DONTNEED.  Reclaim in the guest is therefore automatic and implicit
(via the regular page reclaim).  This means that inflating the balloon
is similar to the existing balloon mechanism, but the deflate is
different--it re-uses existing Linux kernel functionality to
automatically reclaim.

Signed-off-by: Frank Swiderski <redacted>

I'm pondering this:

Should it really be a separate driver/device ID?
If it behaves the same from host POV, maybe it
should be up to the guest how to inflate/deflate
the balloon internally?

Well, it shouldn't steal ID 10, either way :)  Either use a completely
bogus number, or ask for an id.

But AFAICT this should be a an alternate driver of for the same device:
it's not really a separate device, is it?

Cheers,
Rusty.

Apologies, Rusty.  Asking for an ID is in the virtio spec, and I
completely neglected that step.  Though as you and others have pointed
out, this probably fits better as a different driver for the same
device.  Since it changes whether or not the deflate operation is
necessary, it also seems that how this should look is different
behavior based on a feature bit in the device.

If that sounds reasonable, then what I'll do with this patch is merge
it with the existing virtio balloon driver with a feature bit for
determining which behavior to use.

I also think the idea of a generic balloon that the different balloon
drivers use for the inflate/deflate operations is interesting and
useful, though I think the suggestion of pending that until later is
correct.

Sounds reasonable?

Regards,
fes

I think a spec patch would be a good spec at this point.
You can get the spec from Rusty, or a mirror
from my git:

git://git.kernel.org/pub/scm/virt/kvm/mst/virtio-spec.git