Re: [PATCH v2 1/4] Defer skb allocation -- add destroy buffers function for virtio

Rusty Russell
2009-12-14T03:25:59Z

On Fri, 11 Dec 2009 11:03:25 pm Shirley Ma wrote:
> Signed-off-by: <xma@us.ibm.com>

Hi Shirley,

These patches look quite close. More review to follow :)

This title needs revision. It should start with virtio: (all the virtio
patches do, for easy identification after merge), eg:

Subject: virtio: Add destroy callback for unused buffers

It also needs a commit message which explains the problem and the solution.
Something like this:

There's currently no way for a virtio driver to ask for unused
buffers, so it has to keep a list itself to reclaim them at shutdown.
This is redundant, since virtio_ring stores that information. So
add a new hook to do this: virtio_net will be the first user.

> -------------
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c708ecc..bb5eb7b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -107,6 +107,16 @@ static struct page *get_a_page(struct virtnet_info *vi, gfp_t gfp_mask)
> return p;
> }
>
> +static void virtio_net_free_pages(void *buf)
> +{
> + struct page *page, *next;
> +
> + for (page = buf; page; page = next) {
> + next = (struct page *)page->private;
> + __free_pages(page, 0);
> + }
> +}
> +
> static void skb_xmit_done(struct virtqueue *svq)
> {
> struct virtnet_info *vi = svq->vdev->priv;

This belongs in one of the future patches: it will cause an unused warning
and is logically not part of this patch anyway.

> +static int vring_destroy_bufs(struct virtqueue *_vq, void (*destroy)(void *))
> +{
> + struct vring_virtqueue *vq = to_vvq(_vq);
> + void *buf;
> + unsigned int i;
> + int freed = 0;

unsigned int for return and freed counter? Means changing prototype, but
negative return isn't useful here.

> +
> + START_USE(vq);
> +
> + for (i = 0; i < vq->vring.num; i++) {
> + if (vq->data[i]) {
> + /* detach_buf clears data, so grab it now. */
> + buf = vq->data[i];
> + detach_buf(vq, i);
> + destroy(buf);
> + freed++;

You could simplify this a bit, since the queue must be inactive:

destroy(vq->data[i]);
detach_buf(vq, i);
freed++;

> + }
> + }
> +
> + END_USE(vq);
> + return freed;

Perhaps add a:
/* That should have freed everything. */
BUG_ON(vq->num_free != vq->vring.num)

> void (*disable_cb)(struct virtqueue *vq);
> bool (*enable_cb)(struct virtqueue *vq);
> + int (*destroy_bufs)(struct virtqueue *vq, void (*destory)(void *));

destory typo :)

Cheers,
Rusty.