Re: [PATCH v2 3/3] virtio_balloon: Specify page reporting order if needed

[Gavin Shan]

On 6/23/21 3:44 AM, Alexander Duyck wrote:
> On Mon, Jun 21, 2021 at 10:49 PM Gavin Shan <gshan@xxxxxxxxxx> wrote:
> > The page reporting won't be triggered if the freeing page can't come
> > up with a free area, whose size is equal or bigger than the threshold
> > (page reporting order). The default page reporting order, equal to
> > @pageblock_order, is too huge on some architectures to trigger page
> > reporting. One example is ARM64 when 64KB base page size is used.
> >
> >        PAGE_SIZE:          64KB
> >        pageblock_order:    13       (512MB)
> >        MAX_ORDER:          14
> >
> > This specifies the page reporting order to 5 (2MB) for this specific
> > case so that page reporting can be triggered.
> >
> > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > ---
> >   drivers/virtio/virtio_balloon.c | 17 +++++++++++++++++
> >   1 file changed, 17 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 510e9318854d..fd419780cc23 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -993,6 +993,23 @@ static int virtballoon_probe(struct virtio_device *vdev)
> >                          goto out_unregister_oom;
> >                  }
> >
> > +               /*
> > +                * The default page reporting order is @pageblock_order, which
> > +                * corresponds to 512MB in size on ARM64 when 64KB base page
> > +                * size is used. The page reporting won't be triggered if the
> > +                * freeing page can't come up with a free area like that huge.
> > +                * So we specify the page reporting order to 5, corresponding
> > +                * to 2MB. It helps to avoid THP splitting if 4KB base page
> > +                * size is used by host.
> > +                *
> > +                * Ideallh, the page reporting order is selected based on the
>
> "Ideally"

Yeah, I noticed it right after the patch was posted. Will be fixed
in v3.

> > +                * host's base page size. However, it needs more work to report
> > +                * that value. The hardcoded order would be fine currently.
> > +                */
> > +#if defined(CONFIG_ARM64) && defined(CONFIG_ARM64_64K_PAGES)
> > +               vb->pr_dev_info.order = 5;
> > +#endif
> > +
> >                  err = page_reporting_register(&vb->pr_dev_info);
> >                  if (err)
> >                          goto out_unregister_oom;
>
> This works for now. However my preference would be to look into seeing
> if we can add a value that the host can report that would override the
> value you selected here. Then in situations where the host has a
> smaller THP page size then the guest it can report the preferred
> reporting order via the virtio_balloon interface and have greater
> flexibility.
>
> Reviewed-by: Alexander Duyck <alexanderduyck@xxxxxx>

Yes, It's something for later. Lets fix the particular case
(ARM64 and 64KB page size) for now.

Thanks,
Gavin