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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 26, 2012 at 04:21:58PM -0700, Frank Swiderski wrote:
> On Tue, Jun 26, 2012 at 2:47 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Tue, Jun 26, 2012 at 02:31:26PM -0700, Frank Swiderski wrote:
> >> On Tue, Jun 26, 2012 at 1:40 PM, Rik van Riel <riel@redhat.com> wrote:
> >> > On 06/26/2012 04:32 PM, Frank Swiderski wrote:
> >> >>
> >> >> 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<fes@google.com>
> >> >
> >> >
> >> > It is a great idea, but how can this memory balancing
> >> > possibly work if someone uses memory cgroups inside a
> >> > guest?
> >>
> >> Thanks and good point--this isn't something that I considered in the
> >> implementation.
> >>
> >> > Having said that, we currently do not have proper
> >> > memory reclaim balancing between cgroups at all, so
> >> > requiring that of this balloon driver would be
> >> > unreasonable.
> >> >
> >> > The code looks good to me, my only worry is the
> >> > code duplication. We now have 5 balloon drivers,
> >> > for 4 hypervisors, all implementing everything
> >> > from scratch...
> >>
> >> Do you have any recommendations on this?  I could (I think reasonably
> >> so) modify the existing virtio_balloon.c and have it change behavior
> >> based on a feature bit or other configuration.  I'm not sure that
> >> really addresses the root of what you're pointing out--it's still
> >> adding a different implementation, but doing so as an extension of an
> >> existing one.
> >>
> >> fes
> >
> > Let's assume it's a feature bit: how would you
> > formulate what the feature does *from host point of view*?
> >
> > --
> > MST
> 
> In this implementation, the host doesn't keep track of pages in the
> balloon, as there is no explicit deflate path.  The host device for
> this implementation should merely, for example, MADV_DONTNEED on the
> pages sent in an inflate.  Thus, the inflate becomes a notification
> that the guest doesn't need those pages mapped in, but that they
> should be available if the guest touches them.

So guest access removes the page from the balloon,
since it cancels MADV_DONTNEED, right?
Okay. But what is the meaning of num_pages then?
For example, let's assume I set num_pages to 1,
then guest gives me a page and later accesses this
page. Is guest also required to give me another
page now? Later I send a config interrupt without
changing num_pages. Is guest required to give me another
page now?

> In that sense, it's
> not a rigid shrink of guest memory.  I'm not sure what I'd call the
> feature bit though.
> 
> Was that the question you were asking, or did I misread?
> 
> fes

Yes. It would be a good idea for you to try and write a spec IMO.
Send a patch to virtio.lyx

-- 
MST