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

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 <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() &&
+		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.

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=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.


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