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

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

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 <mst@redhat.com>
> > > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > > Cc: Michal Hocko <mhocko@suse.com>
> > > ---
> > >   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.

> The host can discard free pages only when "page_poisoning=false".
> 
> Best,
> Wei