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@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> > >> > > >> > > >> > 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
