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

[John Fastabend <john.fastabend@gmail.com>]

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
>