On Thu, Sep 06, 2018 at 12:05:19PM +0800, Jason Wang wrote:
> There's no need to duplicate page get logic in each action. So this
> patch tries to get page and calculate the offset before processing XDP
> actions, and undo them when meet errors (we don't care the performance
> on errors). This will be used for factoring out XDP logic.
>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
I see some issues with this one.
> ---
> drivers/net/tun.c | 37 ++++++++++++++++---------------------
> 1 file changed, 16 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 372caf7d67d9..f8cdcfa392c3 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1642,7 +1642,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
> int len, int *skb_xdp)
> {
> struct page_frag *alloc_frag = ¤t->task_frag;
> - struct sk_buff *skb;
> + struct sk_buff *skb = NULL;
> struct bpf_prog *xdp_prog;
> int buflen = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> unsigned int delta = 0;
> @@ -1668,6 +1668,9 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
> if (copied != len)
> return ERR_PTR(-EFAULT);
>
> + get_page(alloc_frag->page);
> + alloc_frag->offset += buflen;
> +
This adds an atomic op on XDP_DROP which is a data path
operation for some workloads.
> /* There's a small window that XDP may be set after the check
> * of xdp_prog above, this should be rare and for simplicity
> * we do XDP on skb in case the headroom is not enough.
> @@ -1695,23 +1698,15 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
>
> switch (act) {
> case XDP_REDIRECT:
> - get_page(alloc_frag->page);
> - alloc_frag->offset += buflen;
> err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
> xdp_do_flush_map();
> if (err)
> - goto err_redirect;
> - rcu_read_unlock();
> - local_bh_enable();
> - return NULL;
> + goto err_xdp;
> + goto out;
> case XDP_TX:
> - get_page(alloc_frag->page);
> - alloc_frag->offset += buflen;
> if (tun_xdp_tx(tun->dev, &xdp) < 0)
> - goto err_redirect;
> - rcu_read_unlock();
> - local_bh_enable();
> - return NULL;
> + goto err_xdp;
> + goto out;
> case XDP_PASS:
> delta = orig_data - xdp.data;
> len = xdp.data_end - xdp.data;
> @@ -1730,23 +1725,23 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
> local_bh_enable();
>
> skb = build_skb(buf, buflen);
> - if (!skb)
> - return ERR_PTR(-ENOMEM);
> + if (!skb) {
> + skb = ERR_PTR(-ENOMEM);
> + goto out;
So goto out will skip put_page, and we did
do get_page above. Seems wrong. You should
goto err_skb or something like this.
> + }
>
> skb_reserve(skb, pad - delta);
> skb_put(skb, len);
> - get_page(alloc_frag->page);
> - alloc_frag->offset += buflen;
>
> return skb;
>
> -err_redirect:
> - put_page(alloc_frag->page);
> err_xdp:
> + alloc_frag->offset -= buflen;
> + put_page(alloc_frag->page);
> +out:
Out here isn't an error at all, is it? You should not mix return and
error handling IMHO.
> rcu_read_unlock();
> local_bh_enable();
> - this_cpu_inc(tun->pcpu_stats->rx_dropped);
Doesn't this break rx_dropped accounting?
> - return NULL;
> + return skb;
> }
>
> /* Get packet from user space buffer */
> --
> 2.17.1
