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Subject: Re: [PATCH] virtio_balloon: fix shrinker scan number of pages
To:     "Michael S. Tsirkin" <mst@redhat.com>, linux-kernel@vger.kernel.org
Cc:     stable@vger.kernel.org, Khazhismel Kumykov <khazhy@google.com>,
        Wei Wang <wei.w.wang@intel.com>,
        Jason Wang <jasowang@redhat.com>,
        virtualization@lists.linux-foundation.org
References: <20191119101718.38976-1-mst@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <34c84d6a-d200-c296-39bb-4770bf4517e9@redhat.com>
Date:   Tue, 19 Nov 2019 12:39:51 +0100
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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.
>=20
> 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?

>=20
> 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.
>=20
> 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 !=3D 4K.
>=20
> 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(-)
>=20
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_ball=
oon.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,
>   =09return blocks_freed << VIRTIO_BALLOON_FREE_PAGE_ORDER;
>   }
>  =20
> +static unsigned long leak_balloon_pages(struct virtio_balloon *vb,
> +                                          unsigned long pages_to_free)
> +{
> +=09return leak_balloon(vb, pages_to_free * VIRTIO_BALLOON_PAGES_PER_PAGE=
) /
> +=09=09VIRTIO_BALLOON_PAGES_PER_PAGE;
> +}
> +
>   static unsigned long shrink_balloon_pages(struct virtio_balloon *vb,
>   =09=09=09=09=09  unsigned long pages_to_free)
>   {
> @@ -782,11 +789,9 @@ static unsigned long shrink_balloon_pages(struct vir=
tio_balloon *vb,
>   =09 * VIRTIO_BALLOON_ARRAY_PFNS_MAX balloon pages, so we call it
>   =09 * multiple times to deflate pages till reaching pages_to_free.
>   =09 */
> -=09while (vb->num_pages && pages_to_free) {
> -=09=09pages_freed +=3D leak_balloon(vb, pages_to_free) /
> -=09=09=09=09=09VIRTIO_BALLOON_PAGES_PER_PAGE;
> -=09=09pages_to_free -=3D pages_freed;
> -=09}
> +=09while (vb->num_pages && pages_freed < pages_to_free)
> +=09=09pages_freed +=3D leak_balloon_pages(vb, pages_to_free);
> +
>   =09update_balloon_size(vb);
>  =20
>   =09return pages_freed;
> @@ -799,7 +804,7 @@ static unsigned long virtio_balloon_shrinker_scan(str=
uct shrinker *shrinker,
>   =09struct virtio_balloon *vb =3D container_of(shrinker,
>   =09=09=09=09=09struct virtio_balloon, shrinker);
>  =20
> -=09pages_to_free =3D sc->nr_to_scan * VIRTIO_BALLOON_PAGES_PER_PAGE;
> +=09pages_to_free =3D sc->nr_to_scan;
>  =20
>   =09if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
>   =09=09pages_freed =3D shrink_free_pages(vb, pages_to_free);
>=20


--=20

Thanks,

David / dhildenb