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

[Wei Wang]

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 <mst@xxxxxxxxxx>
> > Signed-off-by: Wei Wang <wei.w.wang@xxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > ---
> >   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;
}

We need "bool page_poisoning" because "poison_value=0" doesn't tell 
whether page poising is in use by the guest. 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)
The host can discard free pages only when "page_poisoning=false".

Best,
Wei