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

[Wang, Wei W]

On Friday, November 17, 2017 8:45 PM, Michael S. Tsirkin wrote:
> 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@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;
> > >
> >
> >
> > 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.
> 

I understand your concern. I think people who use debugging guests don't regard performance as the first priority, and most vendors usually wouldn't use debugging guests for their products.

How about taking it as the initial solution? We can exploit more solutions after this series is done.

Best,
Wei