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

[Alexander Duyck <alexander.duyck@gmail.com>]

On Mon, Jun 21, 2021 at 10:49 PM Gavin Shan <gshan@redhat.com> 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@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: virtualization@lists.linux-foundation.org
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>  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"

> +                * 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@fb.com>