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Date:   Mon, 18 Nov 2019 00:30:21 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Wei Wang <wei.w.wang@intel.com>
Cc:     Khazhismel Kumykov <khazhy@google.com>, jasowang@redhat.com,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] virtio_balloon: fix shrinker pages_to_free calculation
Message-ID: <20191118002652-mutt-send-email-mst@kernel.org>
References: <20191115225557.61847-1-khazhy@google.com>
 <5DD21784.8020506@intel.com>
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On Mon, Nov 18, 2019 at 12:01:08PM +0800, Wei Wang wrote:
> On 11/16/2019 06:55 AM, Khazhismel Kumykov wrote:
> > To my reading, we're accumulating total freed pages in pages_freed, but
> > subtracting it every iteration from pages_to_free, meaning we'll count
> > earlier iterations multiple times, freeing fewer pages than expected.
> > Just accumulate in pages_freed, and compare to pages_to_free.
>=20
> Not sure about the above. But the following unit mismatch is a good captu=
re,
> thanks!
>=20
> >=20
> > There's also a unit mismatch, where pages_to_free seems to be virtio
> > balloon pages, and pages_freed is system pages (We divide by
> > VIRTIO_BALLOON_PAGES_PER_PAGE), so sutracting pages_freed from
> > pages_to_free may result in freeing too much.
> >=20
> > There also seems to be a mismatch between shrink_free_pages() and
> > shrink_balloon_pages(), where in both pages_to_free is given as # of
> > virtio pages to free, but free_pages() returns virtio pages, and
> > balloon_pages returns system pages.
> >=20
> > (For 4K PAGE_SIZE, this mismatch wouldn't be noticed since
> > VIRTIO_BALLOON_PAGES_PER_PAGE would be 1)
> >=20
> > Have both return virtio pages, and divide into system pages when
> > returning from shrinker_scan()
>=20
> Sounds good.
>=20
> >=20
> > Fixes: 71994620bb25 ("virtio_balloon: replace oom notifier with shrinke=
r")
> > Cc: Wei Wang <wei.w.wang@intel.com>
> > Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
> > ---
> >=20
> > Tested this under memory pressure conditions and the shrinker seemed to
> > shrink.
> >=20
> >   drivers/virtio/virtio_balloon.c | 11 ++++-------
> >   1 file changed, 4 insertions(+), 7 deletions(-)
> >=20
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_ba=
lloon.c
> > index 226fbb995fb0..7951ece3fe24 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -782,11 +782,8 @@ 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_to_free > pages_freed)
> > +=09=09pages_freed +=3D leak_balloon(vb, pages_to_free - pages_freed);
> >   =09update_balloon_size(vb);
> >   =09return pages_freed;
> > @@ -805,11 +802,11 @@ static unsigned long virtio_balloon_shrinker_scan=
(struct shrinker *shrinker,
> >   =09=09pages_freed =3D shrink_free_pages(vb, pages_to_free);
>=20
> We also need a fix here then:
>=20
> pages_freed =3D shrink_free_pages(vb, sc->nr_to_scan) *
> VIRTIO_BALLOON_PAGES_PER_PAGE;

No let's do accounting in pages please. virtio page is a legacy
thing we just did not fix it in time to get rid of it by now.

>=20
> Btw, there is another mistake, in virtio_balloon_shrinker_count:
>=20
> -       count +=3D vb->num_free_page_blocks >> VIRTIO_BALLOON_FREE_PAGE_O=
RDER;
> +      count +=3D vb->num_free_page_blocks << VIRTIO_BALLOON_FREE_PAGE_OR=
DER;
>=20
> You may want to include it in this fix patch as well.

OMG. should be a separate patch.
But really this just shows why shifts are such a bad idea.

Let's define
VIRTIO_BALLOON_PAGES_PER_FREE_PAGE

and use it with * and / consistently instead of shifts.


> Best,
> Wei
>=20