Re: [Qemu-devel] [virtio-dev] Re: [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

[Michael S. Tsirkin]

On Thu, Nov 16, 2017 at 09:27:24PM +0800, Wei Wang wrote:
> On 11/16/2017 04:32 AM, Michael S. Tsirkin wrote:
> > On Fri, Nov 03, 2017 at 04:13:06PM +0800, Wei Wang wrote:
> > > Negotiation of the VIRTIO_BALLOON_F_FREE_PAGE_VQ feature indicates the
> > > support of reporting hints of guest free pages to the host via
> > > virtio-balloon. The host requests the guest to report the free pages by
> > > sending commands via the virtio-balloon configuration registers.
> > > 
> > > When the guest starts to report, the first element added to the free page
> > > vq is a sequence id of the start reporting command. The id is given by
> > > the host, and it indicates whether the following free pages correspond
> > > to the command. For example, the host may stop the report and start again
> > > with a new command id. The obsolete pages for the previous start command
> > > can be detected by the id dismatching on the host. The id is added to the
> > > vq using an output buffer, and the free pages are added to the vq using
> > > input buffer.
> > > 
> > > Here are some explainations about the added configuration registers:
> > > - host2guest_cmd: a register used by the host to send commands to the
> > > guest.
> > > - guest2host_cmd: written by the guest to ACK to the host about the
> > > commands that have been received. The host will clear the corresponding
> > > bits on the host2guest_cmd register. The guest also uses this register
> > > to send commands to the host (e.g. when finish free page reporting).
> > > - free_page_cmd_id: the sequence id of the free page report command
> > > given by the host.
> > > 
> > > Signed-off-by: Wei Wang <address@hidden>
> > > Signed-off-by: Liang Li <address@hidden>
> > > Cc: Michael S. Tsirkin <address@hidden>
> > > Cc: Michal Hocko <address@hidden>
> > > ---
> > > 
> > > +
> > > +static void report_free_page(struct work_struct *work)
> > > +{
> > > + struct virtio_balloon *vb;
> > > +
> > > + vb = container_of(work, struct virtio_balloon, report_free_page_work);
> > > + report_free_page_cmd_id(vb);
> > > + walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
> > > + /*
> > > +  * The last few free page blocks that were added may not reach the
> > > +  * batch size, but need a kick to notify the device to handle them.
> > > +  */
> > > + virtqueue_kick(vb->free_page_vq);
> > > + report_free_page_end(vb);
> > > +}
> > > +
> > I think there's an issue here: if pages are poisoned and hypervisor
> > subsequently drops them, testing them after allocation will
> > trigger a false positive.
> > 
> > The specific configuration:
> > 
> > PAGE_POISONING on
> > PAGE_POISONING_NO_SANITY off
> > PAGE_POISONING_ZERO off
> > 
> > 
> > Solutions:
> > 1. disable the feature in that configuration
> >     suggested as an initial step
> 
> Thanks for the finding.
> Similar to this option: I'm thinking could we make walk_free_mem_block()
> simply return if that option is on?
> That is, at the beginning of the function:
>     if (!page_poisoning_enabled())
>                 return;
> 
> I think in most usages, people would not choose to use the poisoning option
> due to the added overhead.
> 
> 
> Probably we could make it a separate fix patch of this report following
> patch 5 to explain the above reasons in the commit.
> 
> > 2. pass poison value to host so it can validate page content
> >     before it drops it
> > 3. pass poison value to host so it can init allocated pages with that value
> > 
> > In fact one nice side effect would be that unmap
> > becomes safe even though free list is not locked anymore.
> 
> I haven't got this point yet,  how would it bring performance benefit?

Upon getting a free page, host could check that its content
matches the poison value. If it doesn't page has been used.

But let's ignore this for now.

> > It would be interesting to see whether this last has
> > any value performance-wise.
> > 
> 
> Best,
> Wei