RE: [PATCH bpf-next] xsk: build skb by page

John Fastabend
2020-12-31T16:30:10Z

Xuan Zhuo wrote:
> This patch is used to construct skb based on page to save memory copy
> overhead.
>
> Taking into account the problem of addr unaligned, and the
> possibility of frame size greater than page in the future.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
> net/xdp/xsk.c | 68 ++++++++++++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 51 insertions(+), 17 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index ac4a317..7cab40f 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -430,6 +430,55 @@ static void xsk_destruct_skb(struct sk_buff *skb)
> sock_wfree(skb);
> }
>
> +static struct sk_buff *xsk_build_skb_bypage(struct xdp_sock *xs, struct xdp_desc *desc)
> +{
> + char *buffer;
> + u64 addr;
> + u32 len, offset, copy, copied;
> + int err, i;
> + struct page *page;
> + struct sk_buff *skb;
> +
> + skb = sock_alloc_send_skb(&xs->sk, 0, 1, &err);

Because this is just grabbing an skb did you consider build_skb?

> + if (unlikely(!skb))
> + return NULL;

I think it would be best to push err back to caller here with ERR_PTR().

> +
> + addr = desc->addr;
> + len = desc->len;
> +
> + buffer = xsk_buff_raw_get_data(xs->pool, addr);
> + offset = offset_in_page(buffer);
> + addr = buffer - (char *)xs->pool->addrs;
> +
> + for (copied = 0, i = 0; copied < len; ++i) {
> + page = xs->pool->umem->pgs[addr >> PAGE_SHIFT];
> +
> + get_page(page);

Is it obvious why this get_page() is needed? Maybe a small comment would
be nice. Something like, "we need to inc refcnt on page to ensure skb
does not release page from pool".

> +
> + copy = min((u32)(PAGE_SIZE - offset), len - copied);
> +

nit: take it or leave it, seems like a lot of new lines imo. I would
just put all these together. Not really important though.

> + skb_fill_page_desc(skb, i, page, offset, copy);
> +
> + copied += copy;
> + addr += copy;
> + offset = 0;
> + }
> +
> + skb->len += len;
> + skb->data_len += len;
> + skb->truesize += len;
> +
> + refcount_add(len, &xs->sk.sk_wmem_alloc);
> +
> + skb->dev = xs->dev;
> + skb->priority = xs->sk.sk_priority;
> + skb->mark = xs->sk.sk_mark;
> + skb_shinfo(skb)->destructor_arg = (void *)(long)addr;
> + skb->destructor = xsk_destruct_skb;
> +
> + return skb;
> +}
> +
> static int xsk_generic_xmit(struct sock *sk)
> {
> struct xdp_sock *xs = xdp_sk(sk);
> @@ -445,40 +494,25 @@ static int xsk_generic_xmit(struct sock *sk)
> goto out;
>
> while (xskq_cons_peek_desc(xs->tx, &desc, xs->pool)) {
> - char *buffer;
> - u64 addr;
> - u32 len;
> -
> if (max_batch-- == 0) {
> err = -EAGAIN;
> goto out;
> }
>
> - len = desc.len;
> - skb = sock_alloc_send_skb(sk, len, 1, &err);
> + skb = xsk_build_skb_bypage(xs, &desc);
> if (unlikely(!skb))

Is err set here? Either way if skb is an ERR_PTR we can use that
here for better error handling.

> goto out;
>
> - skb_put(skb, len);
> - addr = desc.addr;
> - buffer = xsk_buff_raw_get_data(xs->pool, addr);
> - err = skb_store_bits(skb, 0, buffer, len);
> /* This is the backpressure mechanism for the Tx path.
> * Reserve space in the completion queue and only proceed
> * if there is space in it. This avoids having to implement
> * any buffering in the Tx path.
> */
> - if (unlikely(err) || xskq_prod_reserve(xs->pool->cq)) {
> + if (xskq_prod_reserve(xs->pool->cq)) {
> kfree_skb(skb);

Same here, do we need to set err now that its not explicit above in
err = skb_store_bits...

> goto out;
> }
>
> - skb->dev = xs->dev;
> - skb->priority = sk->sk_priority;
> - skb->mark = sk->sk_mark;
> - skb_shinfo(skb)->destructor_arg = (void *)(long)desc.addr;
> - skb->destructor = xsk_destruct_skb;
> -
> err = __dev_direct_xmit(skb, xs->queue_id);
> if (err == NETDEV_TX_BUSY) {
> /* Tell user-space to retry the send */
> --
> 1.8.3.1
>