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[79.176.6.42]) by smtp.gmail.com with ESMTPSA id 187sm10260541qkk.103.2019.11.19.05.03.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Nov 2019 05:03:24 -0800 (PST) Date: Tue, 19 Nov 2019 08:03:19 -0500 From: "Michael S. Tsirkin" To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, Khazhismel Kumykov , Wei Wang , Jason Wang , virtualization@lists.linux-foundation.org Subject: Re: [PATCH] virtio_balloon: fix shrinker scan number of pages Message-ID: <20191119080254-mutt-send-email-mst@kernel.org> References: <20191119101718.38976-1-mst@redhat.com> <34c84d6a-d200-c296-39bb-4770bf4517e9@redhat.com> MIME-Version: 1.0 In-Reply-To: <34c84d6a-d200-c296-39bb-4770bf4517e9@redhat.com> X-MC-Unique: CBkKTUcKM86WBjA878TyOA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org 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 get= s > > 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. >=20 > If it doesn't matter, why cc: stable? Oh right. Sorry. > >=20 > > Further, if we ever tried to iteratively leak pages as > > virtio_balloon_shrinker_scan tries to do, we'd run into issues - this i= s > > 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 shrinke= r") > > Reported-by: Khazhismel Kumykov > > Reviewed-by: Wei Wang > > Signed-off-by: Michael S. Tsirkin > > --- > > 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_ba= lloon.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 virt= io_balloon *vb, > > =09return blocks_freed << VIRTIO_BALLOON_FREE_PAGE_ORDER; > > } > > +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_PA= GE) / > > +=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 v= irtio_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); > > =09return pages_freed; > > @@ -799,7 +804,7 @@ static unsigned long virtio_balloon_shrinker_scan(s= truct shrinker *shrinker, > > =09struct virtio_balloon *vb =3D container_of(shrinker, > > =09=09=09=09=09struct virtio_balloon, shrinker); > > -=09pages_to_free =3D sc->nr_to_scan * VIRTIO_BALLOON_PAGES_PER_PAGE; > > +=09pages_to_free =3D sc->nr_to_scan; > > =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 >=20 > --=20 >=20 > Thanks, >=20 > David / dhildenb