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

[Wei Wang]

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 <wei.w.wang@xxxxxxxxx>
> > Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > ---
> >
> > +
> > +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?

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

Best,
Wei