Subject: Re: [PATCH rfc] packet: zerocopy packet_snd

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 26 Nov 2014 23:20:38 +0200

On Wed, Nov 26, 2014 at 02:59:34PM -0500, Willem de Bruijn wrote:
> > The main problem with zero copy ATM is with queueing disciplines
> > which might keep the socket around essentially forever.
> > The case was described here:
> > https://lkml.org/lkml/2014/1/17/105
> > and of course this will make it more serious now that
> > more applications will be able to do this, so
> > chances that an administrator enables this
> > are higher.
> 
> The denial of service issue raised there, that a single queue can
> block an entire virtio-net device, is less problematic in the case of
> packet sockets. A socket can run out of sk_wmem_alloc, but a prudent
> application can increase the limit or use separate sockets for
> separate flows.

Sounds like this interface is very hard to use correctly.

> > One possible solution is some kind of timer orphaning frags
> > for skbs that have been around for too long.
> 
> Perhaps this can be approximated without an explicit timer by calling
> skb_copy_ubufs on enqueue whenever qlen exceeds a threshold value?

Not sure.  I'll have to see that patch to judge.

> >
> >> ---
> >>  include/linux/skbuff.h        |   1 +
> >>  include/linux/socket.h        |   1 +
> >>  include/uapi/linux/errqueue.h |   1 +
> >>  net/packet/af_packet.c        | 110 ++++++++++++++++++++++++++++++++++++++----
> >>  4 files changed, 103 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> >> index 78c299f..8e661d2 100644
> >> --- a/include/linux/skbuff.h
> >> +++ b/include/linux/skbuff.h
> >> @@ -295,6 +295,7 @@ struct ubuf_info {
> >>       void (*callback)(struct ubuf_info *, bool zerocopy_success);
> >>       void *ctx;
> >>       unsigned long desc;
> >> +     void *callback_priv;
> >>  };
> >>
> >>  /* This data is invariant across clones and lives at
> >> diff --git a/include/linux/socket.h b/include/linux/socket.h
> >> index de52228..0a2e0ea 100644
> >> --- a/include/linux/socket.h
> >> +++ b/include/linux/socket.h
> >> @@ -265,6 +265,7 @@ struct ucred {
> >>  #define MSG_SENDPAGE_NOTLAST 0x20000 /* sendpage() internal : not the last page */
> >>  #define MSG_EOF         MSG_FIN
> >>
> >> +#define MSG_ZEROCOPY 0x4000000       /* Pin user pages */
> >>  #define MSG_FASTOPEN 0x20000000      /* Send data in TCP SYN */
> >>  #define MSG_CMSG_CLOEXEC 0x40000000  /* Set close_on_exec for file
> >>                                          descriptor received through
> >> diff --git a/include/uapi/linux/errqueue.h b/include/uapi/linux/errqueue.h
> >> index 07bdce1..61bf318 100644
> >> --- a/include/uapi/linux/errqueue.h
> >> +++ b/include/uapi/linux/errqueue.h
> >> @@ -19,6 +19,7 @@ struct sock_extended_err {
> >>  #define SO_EE_ORIGIN_ICMP6   3
> >>  #define SO_EE_ORIGIN_TXSTATUS        4
> >>  #define SO_EE_ORIGIN_TIMESTAMPING SO_EE_ORIGIN_TXSTATUS
> >> +#define SO_EE_ORIGIN_UPAGE   5
> >>
> >>  #define SO_EE_OFFENDER(ee)   ((struct sockaddr*)((ee)+1))
> >>
> >> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> >> index 58af5802..367c23a 100644
> >> --- a/net/packet/af_packet.c
> >> +++ b/net/packet/af_packet.c
> >> @@ -2370,28 +2370,112 @@ out:
> >>
> >>  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 = 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;
> >>  }
> >>
> >> +/* release zerocopy resources and avoid destructor callback on kfree */
> >> +static void packet_snd_zerocopy_free(struct sk_buff *skb)
> >> +{
> >> +     struct ubuf_info *uarg = skb_shinfo(skb)->destructor_arg;
> >> +
> >> +     if (uarg) {
> >> +             kfree_skb(uarg->callback_priv);
> >> +             sock_put((struct sock *) uarg->ctx);
> >> +             kfree(uarg);
> >> +
> >> +             skb_shinfo(skb)->destructor_arg = NULL;
> >> +             skb_shinfo(skb)->tx_flags &= ~SKBTX_DEV_ZEROCOPY;
> >> +     }
> >> +}
> >> +
> >> +static void packet_snd_zerocopy_callback(struct ubuf_info *uarg,
> >> +                                      bool zerocopy_success)
> >> +{
> >> +     struct sk_buff *err_skb;
> >> +     struct sock *err_sk;
> >> +     struct sock_exterr_skb *serr;
> >> +
> >> +     err_sk = uarg->ctx;
> >> +     err_skb = uarg->callback_priv;
> >> +
> >> +     serr = SKB_EXT_ERR(err_skb);
> >> +     memset(serr, 0, sizeof(*serr));
> >> +     serr->ee.ee_errno = ENOMSG;
> >> +     serr->ee.ee_origin = SO_EE_ORIGIN_UPAGE;
> >> +     serr->ee.ee_data = uarg->desc & 0xFFFFFFFF;
> >> +     serr->ee.ee_info = ((u64) uarg->desc) >> 32;
> >> +     if (sock_queue_err_skb(err_sk, err_skb))
> >> +             kfree_skb(err_skb);
> >> +
> >> +     kfree(uarg);
> >> +     sock_put(err_sk);
> >> +}
> >> +
> >> +static int packet_zerocopy_sg_from_iovec(struct sk_buff *skb,
> >> +                                      struct msghdr *msg)
> >> +{
> >> +     struct ubuf_info *uarg = NULL;
> >> +     int ret;
> >> +
> >> +     if (iov_pages(msg->msg_iov, 0, msg->msg_iovlen) > MAX_SKB_FRAGS)
> >> +             return -EMSGSIZE;
> >> +
> >> +     uarg = kzalloc(sizeof(*uarg), GFP_KERNEL);
> >> +     if (!uarg)
> >> +             return -ENOMEM;
> >> +
> >> +     uarg->callback_priv = alloc_skb(0, GFP_KERNEL);
> >> +     if (!uarg->callback_priv) {
> >> +             kfree(uarg);
> >> +             return -ENOMEM;
> >> +     }
> >> +
> >> +     ret = zerocopy_sg_from_iovec(skb, msg->msg_iov, 0, msg->msg_iovlen);
> >> +     if (ret) {
> >> +             kfree_skb(uarg->callback_priv);
> >> +             kfree(uarg);
> >> +             return -EIO;
> >> +     }
> >> +
> >> +     sock_hold(skb->sk);
> >> +     uarg->ctx = skb->sk;
> >> +     uarg->callback = packet_snd_zerocopy_callback;
> >> +     uarg->desc = (unsigned long) msg->msg_iov[0].iov_base;
> >> +
> >> +     skb_shinfo(skb)->destructor_arg = uarg;
> >> +     skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
> >> +     skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
> >> +
> >> +     return 0;
> >> +}
> >> +
> >>  static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> >>  {
> >>       struct sock *sk = sock->sk;
> >> @@ -2408,6 +2492,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> >>       unsigned short gso_type = 0;
> >>       int hlen, tlen;
> >>       int extra_len = 0;
> >> +     bool zerocopy = msg->msg_flags & MSG_ZEROCOPY;
> >>
> >>       /*
> >>        *      Get and verify the address.
> >> @@ -2501,7 +2586,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> >>       hlen = LL_RESERVED_SPACE(dev);
> >>       tlen = dev->needed_tailroom;
> >>       skb = packet_alloc_skb(sk, hlen + tlen, hlen, len, vnet_hdr.hdr_len,
> >> -                            msg->msg_flags & MSG_DONTWAIT, &err);
> >> +                            msg->msg_flags, &err);
> >>       if (skb == NULL)
> >>               goto out_unlock;
> >>
> >> @@ -2518,7 +2603,11 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> >>       }
> >>
> >>       /* Returns -EFAULT on error */
> >> -     err = skb_copy_datagram_from_iovec(skb, offset, msg->msg_iov, 0, len);
> >> +     if (zerocopy)
> >> +             err = packet_zerocopy_sg_from_iovec(skb, msg);
> >> +     else
> >> +             err = skb_copy_datagram_from_iovec(skb, offset, msg->msg_iov,
> >> +                                                0, len);
> >>       if (err)
> >>               goto out_free;
> >>
> >> @@ -2578,6 +2667,7 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
> >>       return len;
> >>
> >>  out_free:
> >> +     packet_snd_zerocopy_free(skb);
> >>       kfree_skb(skb);
> >>  out_unlock:
> >>       if (dev)
> >> --
> >> 2.1.0.rc2.206.gedb03e5