Re: [PATCH 2/2] virtio_balloon: submit reported pages as individual buffers

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 26, 2026 at 11:09:59AM -0400, Michael S. Tsirkin wrote:
> The virtio_balloon driver reports a scatter-gather list of free pages
> to the VMM so it may discard unused pages.
> 
> For an arm64 kernel with 64 KiB pages, the default reporting and
> allocation orders allow entries ranging from 2 MiB to 512 MiB. If the
> report contains a sufficient number of large entries, we will end up
> with a descriptor chain that represents over 4 GiB of memory. This
> violates virtio spec, which says:
> 	Drivers MUST NOT add a descriptor chain longer than $2^{32}$ bytes in total;
> 
> Specifically, submitting $2^{32}$ or more breaks Rust-VMM.
> 
> Submit each reported page as a separate virtqueue buffer instead of one
> buffer with an sg list of all pages.  This also avoids indirect
> descriptor allocation (kmalloc in the reporting path).
> 
> To avoid multiple buffers causing interrupt storms, keep callbacks
> disabled while we wait for an event.
> 
> Note: if the virtqueue is broken, wait_event on
> virtqueue_get_buf hangs.  This is a pre-existing issue:
> the old single-buffer code had the same hang.
> 
> Reported-by: Ruben Hakobyan <hruben@meta.com>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

brown paperbag post,forgot to commit.

> ---
> 
> Lightly tested. Ruben, could you pls help test?
> 
>  drivers/virtio/virtio_balloon.c | 48 ++++++++++++++++++++++++---------
>  1 file changed, 35 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index f6c2dff33f8a..363519fa890d 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -202,25 +202,47 @@ static int virtballoon_free_page_report(struct page_reporting_dev_info *pr_dev_i
>  	struct virtio_balloon *vb =
>  		container_of(pr_dev_info, struct virtio_balloon, pr_dev_info);
>  	struct virtqueue *vq = vb->reporting_vq;
> -	unsigned int unused, err;
> +	unsigned int i, err = 0;
>  
>  	/* We should always be able to add these buffers to an empty queue. */
> -	err = virtqueue_add_inbuf(vq, sg, nents, vb, GFP_NOWAIT);
> +	for (i = 0; i < nents; i++) {
> +		struct scatterlist one;
>  
> -	/*
> -	 * In the extremely unlikely case that something has occurred and we
> -	 * are able to trigger an error we will simply display a warning
> -	 * and exit without actually processing the pages.
> -	 */
> -	if (WARN_ON_ONCE(err))
> -		return err;
> +		sg_init_table(&one, 1);
> +		sg_set_page(&one, sg_page(&sg[i]), sg[i].length,
> +			    sg[i].offset);
> +		err = virtqueue_add_inbuf(vq, &one, 1, &sg[i], GFP_NOWAIT);
>  
> -	virtqueue_kick(vq);
> +		/*
> +		 * In the extremely unlikely case that something has occurred
> +		 * and we are able to trigger an error we will simply display
> +		 * a warning and exit without actually processing the pages.
> +		 */
> +		if (WARN_ON_ONCE(err)) {
> +			nents = i;
> +			break;
> +		}
> +	}
>  
> -	/* When host has read buffer, this completes via balloon_ack */
> -	wait_event(vb->acked, virtqueue_get_buf(vq, &unused));
> +	if (nents) {
> +		virtqueue_kick(vq);
>  
> -	return 0;
> +		/* When host has read buffer, this completes via balloon_ack.
> +		 * We report pages as individual buffers, so arm the callback
> +		 * to only interrupt once all the queued buffers have been
> +		 * used: otherwise we would get one interrupt per page.
> +		 */
> +		for (i = 0; i < nents; i++) {
> +			unsigned int unused;
> +
> +			virtqueue_enable_cb_delayed_max(vq);
> +			wait_event(vb->acked,
> +				   virtqueue_get_buf(vq, &unused));
> +			virtqueue_disable_cb(vq);
> +		}
> +	}
> +
> +	return err;
>  }
>  
>  static void set_page_pfns(struct virtio_balloon *vb,
> -- 
> MST
>