Re: [PATCH] virtio_balloon: fix shrinker scan number of pages

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

On Tue, Nov 19, 2019 at 12:39:51PM +0100, David Hildenbrand wrote:
> On 19.11.19 11:17, Michael S. Tsirkin wrote:
> > virtio_balloon_shrinker_scan should return number of system pages freed,
> > but because it's calling functions that deal with balloon pages, it gets
> > confused and sometimes returns the number of balloon pages.
> > 
> > It does not matter practically as the exact number isn't
> > used, but it seems better to be consistent in case someone
> > starts using this API.
> 
> If it doesn't matter, why cc: stable?

Oh right. Sorry.

> > 
> > Further, if we ever tried to iteratively leak pages as
> > virtio_balloon_shrinker_scan tries to do, we'd run into issues - this is
> > because freed_pages was accumulating total freed pages, but was also
> > subtracted on each iteration from pages_to_free, which can result in
> > either leaking less memory than we were supposed to free, or or more if
> > pages_to_free underruns.
> > 
> > On a system with 4K pages we are lucky that we are never asked to leak
> > more than 128 pages while we can leak up to 256 at a time,
> > but it looks like a real issue for systems with page size != 4K.
> > 
> > Cc: stable@vger.kernel.org
> > Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinker")
> > Reported-by: Khazhismel Kumykov <khazhy@google.com>
> > Reviewed-by: Wei Wang <wei.w.wang@intel.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >   drivers/virtio/virtio_balloon.c | 17 +++++++++++------
> >   1 file changed, 11 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 226fbb995fb0..7cee05cdf3fb 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -772,6 +772,13 @@ static unsigned long shrink_free_pages(struct virtio_balloon *vb,
> >   	return blocks_freed << VIRTIO_BALLOON_FREE_PAGE_ORDER;
> >   }
> > +static unsigned long leak_balloon_pages(struct virtio_balloon *vb,
> > +                                          unsigned long pages_to_free)
> > +{
> > +	return leak_balloon(vb, pages_to_free * VIRTIO_BALLOON_PAGES_PER_PAGE) /
> > +		VIRTIO_BALLOON_PAGES_PER_PAGE;
> > +}
> > +
> >   static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
> >   					  unsigned long pages_to_free)
> >   {
> > @@ -782,11 +789,9 @@ static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
> >   	 * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
> >   	 * multiple times to deflate pages till reaching pages_to_free.
> >   	 */
> > -	while (vb->num_pages && pages_to_free) {
> > -		pages_freed += leak_balloon(vb, pages_to_free) /
> > -					VIRTIO_BALLOON_PAGES_PER_PAGE;
> > -		pages_to_free -= pages_freed;
> > -	}
> > +	while (vb->num_pages && pages_freed < pages_to_free)
> > +		pages_freed += leak_balloon_pages(vb, pages_to_free);
> > +
> >   	update_balloon_size(vb);
> >   	return pages_freed;
> > @@ -799,7 +804,7 @@ static unsigned long virtio_balloon_shrinker_scan(struct shrinker *shrinker,
> >   	struct virtio_balloon *vb = container_of(shrinker,
> >   					struct virtio_balloon, shrinker);
> > -	pages_to_free = sc->nr_to_scan * VIRTIO_BALLOON_PAGES_PER_PAGE;
> > +	pages_to_free = sc->nr_to_scan;
> >   	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> >   		pages_freed = shrink_free_pages(vb, pages_to_free);
> > 
> 
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb