Re: [PATCH net-next v9 11/14] tcp: RX path for devmem TCP

David Wei
2024-05-23T06:02:57Z

On 2024-05-10 16:21, Mina Almasry wrote:
> +/* On error, returns the -errno. On success, returns number of bytes sent to the
> + * user. May not consume all of @remaining_len.
> + */
> +static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
> + unsigned int offset, struct msghdr *msg,
> + int remaining_len)
> +{
> + struct dmabuf_cmsg dmabuf_cmsg = { 0 };
> + struct tcp_xa_pool tcp_xa_pool;
> + unsigned int start;
> + int i, copy, n;
> + int sent = 0;
> + int err = 0;
> +
> + tcp_xa_pool.max = 0;
> + tcp_xa_pool.idx = 0;
> + do {
> + start = skb_headlen(skb);
> +
> + if (skb_frags_readable(skb)) {
> + err = -ENODEV;
> + goto out;
> + }
> +
> + /* Copy header. */
> + copy = start - offset;
> + if (copy > 0) {
> + copy = min(copy, remaining_len);
> +
> + n = copy_to_iter(skb->data + offset, copy,
> + &msg->msg_iter);
> + if (n != copy) {
> + err = -EFAULT;
> + goto out;
> + }
> +
> + offset += copy;
> + remaining_len -= copy;
> +
> + /* First a dmabuf_cmsg for # bytes copied to user
> + * buffer.
> + */
> + memset(&dmabuf_cmsg, 0, sizeof(dmabuf_cmsg));
> + dmabuf_cmsg.frag_size = copy;
> + err = put_cmsg(msg, SOL_SOCKET, SO_DEVMEM_LINEAR,
> + sizeof(dmabuf_cmsg), &dmabuf_cmsg);
> + if (err || msg->msg_flags & MSG_CTRUNC) {
> + msg->msg_flags &= ~MSG_CTRUNC;
> + if (!err)
> + err = -ETOOSMALL;
> + goto out;
> + }
> +
> + sent += copy;
> +
> + if (remaining_len == 0)
> + goto out;
> + }
> +
> + /* after that, send information of dmabuf pages through a
> + * sequence of cmsg
> + */
> + for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
> + skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
> + struct net_iov *niov;
> + u64 frag_offset;
> + int end;
> +
> + /* !skb_frags_readable() should indicate that ALL the
> + * frags in this skb are dmabuf net_iovs. We're checking
> + * for that flag above, but also check individual frags
> + * here. If the tcp stack is not setting
> + * skb_frags_readable() correctly, we still don't want
> + * to crash here.
> + */
> + if (!skb_frag_net_iov(frag)) {
> + net_err_ratelimited("Found non-dmabuf skb with net_iov");
> + err = -ENODEV;
> + goto out;
> + }
> +
> + niov = skb_frag_net_iov(frag);

Sorry if we've already discussed this.

We have this additional hunk:

+ if (niov->pp->mp_ops != &dmabuf_devmem_ops) {
+ err = -ENODEV;
+ goto out;
+ }

In case one of our skbs end up here, skb_frag_is_net_iov() and
!skb_frags_readable(). Does this even matter? And if so then is there a
better way to distinguish between our two types of net_iovs?