[PATCH RFC v2 11/12] packet: enable sendmsg zerocopy

[Willem de Bruijn]

From: Willem de Bruijn <will...@google.com>

Support MSG_ZEROCOPY on PF_PACKET transmission.

Tested:
  pf_packet loopback test snd_zerocopy_lo -p -z produces:

  without zerocopy (-p):
    rx=0 (0 MB) tx=221696 txc=0
    rx=0 (0 MB) tx=443880 txc=0
    rx=0 (0 MB) tx=661056 txc=0
    rx=0 (0 MB) tx=877152 txc=0

  with zerocopy (-p -z):
    rx=0 (0 MB) tx=528548 txc=528544
    rx=0 (0 MB) tx=1052364 txc=1052360
    rx=0 (0 MB) tx=1571956 txc=1571952
    rx=0 (0 MB) tx=2094144 txc=2094140

  Packets do not arrive at the Rx socket due to a martian test:

    IPv4: martian destination 127.0.0.1 from 127.0.0.1, dev lo

  I'll need to revise snd_zerocopy_lo to bypass that.

Signed-off-by: Willem de Bruijn <will...@google.com>
---
 net/packet/af_packet.c | 52 ++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 42 insertions(+), 10 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 2bd0d1949312..af9ecc1edf72 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2754,28 +2754,55 @@ static int tpacket_snd(struct packet_sock *po, struct 
msghdr *msg)
 
 static struct sk_buff *packet_alloc_skb(struct sock *sk, size_t prepad,
                                        size_t reserve, size_t len,
-                                       size_t linear, int noblock,
+                                       size_t linear, int flags,
                                        int *err)
 {
        struct sk_buff *skb;
+       size_t data_len;
 
-       /* Under a page?  Don't bother with paged skb. */
-       if (prepad + len < PAGE_SIZE || !linear)
-               linear = len;
+       if (flags & MSG_ZEROCOPY) {
+               /* Minimize linear, but respect header lower bound */
+               linear = reserve + min(len, max_t(size_t, linear, MAX_HEADER));
+               data_len = 0;
+       } else {
+               /* Under a page? Don't bother with paged skb. */
+               if (prepad + len < PAGE_SIZE || !linear)
+                       linear = len;
+               data_len = len - linear;
+       }
 
-       skb = sock_alloc_send_pskb(sk, prepad + linear, len - linear, noblock,
-                                  err, 0);
+       skb = sock_alloc_send_pskb(sk, prepad + linear, data_len,
+                                  flags & MSG_DONTWAIT, err, 0);
        if (!skb)
                return NULL;
 
        skb_reserve(skb, reserve);
        skb_put(skb, linear);
-       skb->data_len = len - linear;
-       skb->len += len - linear;
+       skb->data_len = data_len;
+       skb->len += data_len;
 
        return skb;
 }
 
+static int packet_zerocopy_sg_from_iovec(struct sk_buff *skb,
+                                        struct msghdr *msg,
+                                        int offset, size_t size)
+{
+       int ret;
+
+       /* if SOCK_DGRAM, head room was alloc'ed and holds ll-headers */
+       __skb_pull(skb, offset);
+       ret = zerocopy_sg_from_iter(skb, &msg->msg_iter);
+       __skb_push(skb, offset);
+       if (unlikely(ret))
+               return ret == -EMSGSIZE ? ret : -EIO;
+
+       if (!skb_zerocopy_alloc(skb, size))
+               return -ENOMEM;
+
+       return 0;
+}
+
 static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
 {
        struct sock *sk = sock->sk;
@@ -2853,7 +2880,7 @@ static int packet_snd(struct socket *sock, struct msghdr 
*msg, size_t len)
        linear = __virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len);
        linear = max(linear, min_t(int, len, dev->hard_header_len));
        skb = packet_alloc_skb(sk, hlen + tlen, hlen, len, linear,
-                              msg->msg_flags & MSG_DONTWAIT, &err);
+                              msg->msg_flags, &err);
        if (skb == NULL)
                goto out_unlock;
 
@@ -2867,7 +2894,11 @@ static int packet_snd(struct socket *sock, struct msghdr 
*msg, size_t len)
        }
 
        /* Returns -EFAULT on error */
-       err = skb_copy_datagram_from_iter(skb, offset, &msg->msg_iter, len);
+       if (msg->msg_flags & MSG_ZEROCOPY)
+               err = packet_zerocopy_sg_from_iovec(skb, msg, offset, len);
+       else
+               err = skb_copy_datagram_from_iter(skb, offset, &msg->msg_iter,
+                                                 len);
        if (err)
                goto out_free;
 
@@ -2913,6 +2944,7 @@ static int packet_snd(struct socket *sock, struct msghdr 
*msg, size_t len)
        return len;
 
 out_free:
+       skb_zcopy_abort(skb);
        kfree_skb(skb);
 out_unlock:
        if (dev)
-- 
2.11.0.483.g087da7b7c-goog