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

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

On Fri, Nov 17, 2017 at 07:35:03PM +0800, Wei Wang wrote:
> On 11/16/2017 09:27 PM, 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 <wei.w.wang@intel.com>
> > > > Signed-off-by: Liang Li <liang.z.li@intel.com>
> > > > Cc: Michael S. Tsirkin <mst@redhat.com>
> > > > Cc: Michal Hocko <mhocko@kernel.org>
> > > > ---
> > > > 
> > > > +
> > > > +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;
> > 
> 
> 
> Thought about it more, I think it would be better to put this logic to
> virtio_balloon:
> 
>         send_free_page_cmd_id(vb, &vb->start_cmd_id);
>         if (page_poisoning_enabled() &&
>             !IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY))
>                 walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
>         send_free_page_cmd_id(vb, &vb->stop_cmd_id);
> 
> 
> walk_free_mem_block() should be a more generic API, and this potential page
> poisoning issue is specific to live migration which is only one use case of
> this function, so I think it is better to handle it in the special use case
> itself.
> 
> Best,
> Wei
> 

It's a quick work-around but it doesn't make me very happy.

AFAIK e.g. RHEL has a debug kernel with poisoning enabled.
If this never uses free page hinting at all, it will
be much less useful for debugging guests.

-- 
MST