Re: [Qemu-devel] [PATCH v18 10/10] virtio-balloon: don't report free pages when page poisoning is enabled

[Wei Wang]

On 12/11/2017 09:24 PM, Michael S. Tsirkin wrote:
> On Mon, Dec 11, 2017 at 02:38:45PM +0800, Wei Wang wrote:
> > On 12/01/2017 11:49 PM, Michael S. Tsirkin wrote:
> > > On Wed, Nov 29, 2017 at 09:55:26PM +0800, Wei Wang wrote:
> > > > The guest free pages should not be discarded by the live migration thread
> > > > when page poisoning is enabled with PAGE_POISONING_NO_SANITY=n, because
> > > > skipping the transfer of such poisoned free pages will trigger false
> > > > positive when new pages are allocated and checked on the destination.
> > > > This patch skips the reporting of free pages in the above case.
> > > >
> > > > Reported-by: Michael S. Tsirkin <address@hidden>
> > > > Signed-off-by: Wei Wang <address@hidden>
> > > > Cc: Michal Hocko <address@hidden>
> > > > ---
> > > >    drivers/virtio/virtio_balloon.c | 4 +++-
> > > >    1 file changed, 3 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > > > index 035bd3a..6ac4cff 100644
> > > > --- a/drivers/virtio/virtio_balloon.c
> > > > +++ b/drivers/virtio/virtio_balloon.c
> > > > @@ -652,7 +652,9 @@ static void report_free_page(struct work_struct *work)
> > > >         /* Start by sending the obtained cmd id to the host with an outbuf */
> > > >         send_one_desc(vb, vb->free_page_vq, virt_to_phys(&vb->start_cmd_id),
> > > >                       sizeof(uint32_t), false, true, false);
> > > > -       walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
> > > > +       if (!(page_poisoning_enabled() &&
> > > > +           !IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY)))
> > > > +               walk_free_mem_block(vb, 0, &virtio_balloon_send_free_pages);
> > > >         /*
> > > >          * End by sending the stop id to the host with an outbuf. Use the
> > > >          * non-batching mode here to trigger a kick after adding the stop id.
> > > PAGE_POISONING_ZERO is actually OK.
> > >
> > > But I really would prefer it that we still send pages to host,
> > > otherwise debugging becomes much harder.
> > >
> > > And it does not have to be completely useless, even though
> > > you can not discard them as they would be zero-filled then.
> > >
> > > How about a config field telling host what should be there in the free
> > > pages? This way even though host can not discard them, host can send
> > > them out without reading them, still a win.
> > Since this poison value comes with the free page reporting feature, how
> > about sending the poison value via the free_page_vq, along with the cmd id
> > in the outbuf? That is, use the following interface:
> >
> > struct virtio_balloon_free_page_vq_hdr {
> >      bool page_poisoning;
> >      __virtio32 poison_value;
> >      __virtio32 cmd_id;
> > }
> Can we put the value in config space instead?
>
> > We need "bool page_poisoning" because "poison_value=0" doesn't tell whether
> > page poising is in use by the guest.
> Can we use a feature bit for this?
>
> > PAGE_POISONING_ZERO sets
> > "page_poisoning=true, poisoning_value=0", and the host will send the
> > 0-filled pages to the destination (if not sending 0-filled pages, the
> > destination host would offer non-zero pages to the guest)
> Why would it? Linux zeroes all pages on alloc.

Thanks, that is the case. I think we don't need a feature bit then. 
Please have a check the v19 patches.

Best,
Wei