Re: [PATCH net-next v3 4/4] vsock/virtio: MSG_ZEROCOPY flag support

[Michael S. Tsirkin]

On Fri, Jul 21, 2023 at 08:09:03AM +0300, Arseniy Krasnov wrote:
> 
> 
> On 21.07.2023 00:42, Arseniy Krasnov wrote:
> > This adds handling of MSG_ZEROCOPY flag on transmission path: if this
> > flag is set and zerocopy transmission is possible (enabled in socket
> > options and transport allows zerocopy), then non-linear skb will be
> > created and filled with the pages of user's buffer. Pages of user's
> > buffer are locked in memory by 'get_user_pages()'. Second thing that
> > this patch does is replace type of skb owning: instead of calling
> > 'skb_set_owner_sk_safe()' it calls 'skb_set_owner_w()'. Reason of this
> > change is that '__zerocopy_sg_from_iter()' increments 'sk_wmem_alloc'
> > of socket, so to decrease this field correctly proper skb destructor is
> > needed: 'sock_wfree()'. This destructor is set by 'skb_set_owner_w()'.
> > 
> > Signed-off-by: Arseniy Krasnov <AVKrasnov@xxxxxxxxxxxxxx>
> > ---
> >  Changelog:
> >  v5(big patchset) -> v1:
> >   * Refactorings of 'if' conditions.
> >   * Remove extra blank line.
> >   * Remove 'frag_off' field unneeded init.
> >   * Add function 'virtio_transport_fill_skb()' which fills both linear
> >     and non-linear skb with provided data.
> >  v1 -> v2:
> >   * Use original order of last four arguments in 'virtio_transport_alloc_skb()'.
> >  v2 -> v3:
> >   * Add new transport callback: 'msgzerocopy_check_iov'. It checks that
> >     provided 'iov_iter' with data could be sent in a zerocopy mode.
> >     If this callback is not set in transport - transport allows to send
> >     any 'iov_iter' in zerocopy mode. Otherwise - if callback returns 'true'
> >     then zerocopy is allowed. Reason of this callback is that in case of
> >     G2H transmission we insert whole skb to the tx virtio queue and such
> >     skb must fit to the size of the virtio queue to be sent in a single
> >     iteration (may be tx logic in 'virtio_transport.c' could be reworked
> >     as in vhost to support partial send of current skb). This callback
> >     will be enabled only for G2H path. For details pls see comment 
> >     'Check that tx queue...' below.
> > 
> >  include/net/af_vsock.h                  |   3 +
> >  net/vmw_vsock/virtio_transport.c        |  39 ++++
> >  net/vmw_vsock/virtio_transport_common.c | 257 ++++++++++++++++++------
> >  3 files changed, 241 insertions(+), 58 deletions(-)
> > 
> > diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> > index 0e7504a42925..a6b346eeeb8e 100644
> > --- a/include/net/af_vsock.h
> > +++ b/include/net/af_vsock.h
> > @@ -177,6 +177,9 @@ struct vsock_transport {
> >  
> >  	/* Read a single skb */
> >  	int (*read_skb)(struct vsock_sock *, skb_read_actor_t);
> > +
> > +	/* Zero-copy. */
> > +	bool (*msgzerocopy_check_iov)(const struct iov_iter *);
> >  };
> >  
> >  /**** CORE ****/
> > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> > index 7bbcc8093e51..23cb8ed638c4 100644
> > --- a/net/vmw_vsock/virtio_transport.c
> > +++ b/net/vmw_vsock/virtio_transport.c
> > @@ -442,6 +442,43 @@ static void virtio_vsock_rx_done(struct virtqueue *vq)
> >  	queue_work(virtio_vsock_workqueue, &vsock->rx_work);
> >  }
> >  
> > +static bool virtio_transport_msgzerocopy_check_iov(const struct iov_iter *iov)
> > +{
> > +	struct virtio_vsock *vsock;
> > +	bool res = false;
> > +
> > +	rcu_read_lock();
> > +
> > +	vsock = rcu_dereference(the_virtio_vsock);
> > +	if (vsock) {
> > +		struct virtqueue *vq;
> > +		int iov_pages;
> > +
> > +		vq = vsock->vqs[VSOCK_VQ_TX];
> > +
> > +		iov_pages = round_up(iov->count, PAGE_SIZE) / PAGE_SIZE;
> > +
> > +		/* Check that tx queue is large enough to keep whole
> > +		 * data to send. This is needed, because when there is
> > +		 * not enough free space in the queue, current skb to
> > +		 * send will be reinserted to the head of tx list of
> > +		 * the socket to retry transmission later, so if skb
> > +		 * is bigger than whole queue, it will be reinserted
> > +		 * again and again, thus blocking other skbs to be sent.
> > +		 * Each page of the user provided buffer will be added
> > +		 * as a single buffer to the tx virtqueue, so compare
> > +		 * number of pages against maximum capacity of the queue.
> > +		 * +1 means buffer for the packet header.
> > +		 */
> > +		if (iov_pages + 1 <= vq->num_max)
> 
> I think this check is actual only for case one we don't have indirect buffer feature.
> With indirect mode whole data to send will be packed into one indirect buffer.
> 
> Thanks, Arseniy

Actually the reverse. With indirect you are limited to num_max.
Without you are limited to whatever space is left in the
queue (which you did not check here, so you should).


> > +			res = true;
> > +	}
> > +
> > +	rcu_read_unlock();

Just curious:
is the point of all this RCU dance to allow vsock
to change from under us? then why is it ok to
have it change? the virtio_transport_msgzerocopy_check_iov
will then refer to the old vsock ...


> > +
> > +	return res;
> > +}
> > +
> >  static bool virtio_transport_seqpacket_allow(u32 remote_cid);
> >  
> >  static struct virtio_transport virtio_transport = {
> > @@ -475,6 +512,8 @@ static struct virtio_transport virtio_transport = {
> >  		.seqpacket_allow          = virtio_transport_seqpacket_allow,
> >  		.seqpacket_has_data       = virtio_transport_seqpacket_has_data,
> >  
> > +		.msgzerocopy_check_iov	  = virtio_transport_msgzerocopy_check_iov,
> > +
> >  		.notify_poll_in           = virtio_transport_notify_poll_in,
> >  		.notify_poll_out          = virtio_transport_notify_poll_out,
> >  		.notify_recv_init         = virtio_transport_notify_recv_init,
> > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > index 26a4d10da205..e4e3d541aff4 100644
> > --- a/net/vmw_vsock/virtio_transport_common.c
> > +++ b/net/vmw_vsock/virtio_transport_common.c
> > @@ -37,73 +37,122 @@ virtio_transport_get_ops(struct vsock_sock *vsk)
> >  	return container_of(t, struct virtio_transport, transport);
> >  }
> >  
> > -/* Returns a new packet on success, otherwise returns NULL.
> > - *
> > - * If NULL is returned, errp is set to a negative errno.
> > - */
> > -static struct sk_buff *
> > -virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
> > -			   size_t len,
> > -			   u32 src_cid,
> > -			   u32 src_port,
> > -			   u32 dst_cid,
> > -			   u32 dst_port)
> > -{
> > -	const size_t skb_len = VIRTIO_VSOCK_SKB_HEADROOM + len;
> > -	struct virtio_vsock_hdr *hdr;
> > -	struct sk_buff *skb;
> > -	void *payload;
> > -	int err;
> > +static bool virtio_transport_can_zcopy(struct virtio_vsock_pkt_info *info,
> > +				       size_t max_to_send)
> > +{
> > +	const struct vsock_transport *t;
> > +	struct iov_iter *iov_iter;
> >  
> > -	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
> > -	if (!skb)
> > -		return NULL;
> > +	if (!info->msg)
> > +		return false;
> >  
> > -	hdr = virtio_vsock_hdr(skb);
> > -	hdr->type	= cpu_to_le16(info->type);
> > -	hdr->op		= cpu_to_le16(info->op);
> > -	hdr->src_cid	= cpu_to_le64(src_cid);
> > -	hdr->dst_cid	= cpu_to_le64(dst_cid);
> > -	hdr->src_port	= cpu_to_le32(src_port);
> > -	hdr->dst_port	= cpu_to_le32(dst_port);
> > -	hdr->flags	= cpu_to_le32(info->flags);
> > -	hdr->len	= cpu_to_le32(len);
> > +	iov_iter = &info->msg->msg_iter;
> >  
> > -	if (info->msg && len > 0) {
> > -		payload = skb_put(skb, len);
> > -		err = memcpy_from_msg(payload, info->msg, len);
> > -		if (err)
> > -			goto out;
> > +	t = vsock_core_get_transport(info->vsk);
> >  
> > -		if (msg_data_left(info->msg) == 0 &&
> > -		    info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
> > -			hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOM);
> > +	if (t->msgzerocopy_check_iov &&
> > +	    !t->msgzerocopy_check_iov(iov_iter))
> > +		return false;
> >  
> > -			if (info->msg->msg_flags & MSG_EOR)
> > -				hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOR);
> > -		}
> > +	/* Data is simple buffer. */
> > +	if (iter_is_ubuf(iov_iter))
> > +		return true;
> > +
> > +	if (!iter_is_iovec(iov_iter))
> > +		return false;
> > +
> > +	if (iov_iter->iov_offset)
> > +		return false;
> > +
> > +	/* We can't send whole iov. */
> > +	if (iov_iter->count > max_to_send)
> > +		return false;
> > +
> > +	return true;
> > +}
> > +
> > +static int virtio_transport_init_zcopy_skb(struct vsock_sock *vsk,
> > +					   struct sk_buff *skb,
> > +					   struct msghdr *msg,
> > +					   bool zerocopy)
> > +{
> > +	struct ubuf_info *uarg;
> > +
> > +	if (msg->msg_ubuf) {
> > +		uarg = msg->msg_ubuf;
> > +		net_zcopy_get(uarg);
> > +	} else {
> > +		struct iov_iter *iter = &msg->msg_iter;
> > +		struct ubuf_info_msgzc *uarg_zc;
> > +		int len;
> > +
> > +		/* Only ITER_IOVEC or ITER_UBUF are allowed and
> > +		 * checked before.
> > +		 */
> > +		if (iter_is_iovec(iter))
> > +			len = iov_length(iter->__iov, iter->nr_segs);
> > +		else
> > +			len = iter->count;
> > +
> > +		uarg = msg_zerocopy_realloc(sk_vsock(vsk),
> > +					    len,
> > +					    NULL);
> > +		if (!uarg)
> > +			return -1;
> > +
> > +		uarg_zc = uarg_to_msgzc(uarg);
> > +		uarg_zc->zerocopy = zerocopy ? 1 : 0;
> >  	}
> >  
> > -	if (info->reply)
> > -		virtio_vsock_skb_set_reply(skb);
> > +	skb_zcopy_init(skb, uarg);
> >  
> > -	trace_virtio_transport_alloc_pkt(src_cid, src_port,
> > -					 dst_cid, dst_port,
> > -					 len,
> > -					 info->type,
> > -					 info->op,
> > -					 info->flags);
> > +	return 0;
> > +}
> >  
> > -	if (info->vsk && !skb_set_owner_sk_safe(skb, sk_vsock(info->vsk))) {
> > -		WARN_ONCE(1, "failed to allocate skb on vsock socket with sk_refcnt == 0\n");
> > -		goto out;
> > +static int virtio_transport_fill_skb(struct sk_buff *skb,
> > +				     struct virtio_vsock_pkt_info *info,
> > +				     size_t len,
> > +				     bool zcopy)
> > +{
> > +	if (zcopy) {
> > +		return __zerocopy_sg_from_iter(info->msg, NULL, skb,
> > +					      &info->msg->msg_iter,
> > +					      len);
> > +	} else {
> > +		void *payload;
> > +		int err;
> > +
> > +		payload = skb_put(skb, len);
> > +		err = memcpy_from_msg(payload, info->msg, len);
> > +		if (err)
> > +			return -1;
> > +
> > +		if (msg_data_left(info->msg))
> > +			return 0;
> > +
> > +		return 0;
> >  	}
> > +}
> >  
> > -	return skb;
> > +static void virtio_transport_init_hdr(struct sk_buff *skb,
> > +				      struct virtio_vsock_pkt_info *info,
> > +				      u32 src_cid,
> > +				      u32 src_port,
> > +				      u32 dst_cid,
> > +				      u32 dst_port,
> > +				      size_t len)
> > +{
> > +	struct virtio_vsock_hdr *hdr;
> >  
> > -out:
> > -	kfree_skb(skb);
> > -	return NULL;
> > +	hdr = virtio_vsock_hdr(skb);
> > +	hdr->type	= cpu_to_le16(info->type);
> > +	hdr->op		= cpu_to_le16(info->op);
> > +	hdr->src_cid	= cpu_to_le64(src_cid);
> > +	hdr->dst_cid	= cpu_to_le64(dst_cid);
> > +	hdr->src_port	= cpu_to_le32(src_port);
> > +	hdr->dst_port	= cpu_to_le32(dst_port);
> > +	hdr->flags	= cpu_to_le32(info->flags);
> > +	hdr->len	= cpu_to_le32(len);
> >  }
> >  
> >  static void virtio_transport_copy_nonlinear_skb(const struct sk_buff *skb,
> > @@ -214,6 +263,70 @@ static u16 virtio_transport_get_type(struct sock *sk)
> >  		return VIRTIO_VSOCK_TYPE_SEQPACKET;
> >  }
> >  
> > +static struct sk_buff *virtio_transport_alloc_skb(struct vsock_sock *vsk,
> > +						  struct virtio_vsock_pkt_info *info,
> > +						  size_t payload_len,
> > +						  bool zcopy,
> > +						  u32 src_cid,
> > +						  u32 src_port,
> > +						  u32 dst_cid,
> > +						  u32 dst_port)
> > +{
> > +	struct sk_buff *skb;
> > +	size_t skb_len;
> > +
> > +	skb_len = VIRTIO_VSOCK_SKB_HEADROOM;
> > +
> > +	if (!zcopy)
> > +		skb_len += payload_len;
> > +
> > +	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
> > +	if (!skb)
> > +		return NULL;
> > +
> > +	virtio_transport_init_hdr(skb, info, src_cid, src_port,
> > +				  dst_cid, dst_port,
> > +				  payload_len);
> > +
> > +	/* Set owner here, because '__zerocopy_sg_from_iter()' uses
> > +	 * owner of skb without check to update 'sk_wmem_alloc'.
> > +	 */
> > +	if (vsk)
> > +		skb_set_owner_w(skb, sk_vsock(vsk));
> > +
> > +	if (info->msg && payload_len > 0) {
> > +		int err;
> > +
> > +		err = virtio_transport_fill_skb(skb, info, payload_len, zcopy);
> > +		if (err)
> > +			goto out;
> > +
> > +		if (info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
> > +			struct virtio_vsock_hdr *hdr = virtio_vsock_hdr(skb);
> > +
> > +			hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOM);
> > +
> > +			if (info->msg->msg_flags & MSG_EOR)
> > +				hdr->flags |= cpu_to_le32(VIRTIO_VSOCK_SEQ_EOR);
> > +		}
> > +	}
> > +
> > +	if (info->reply)
> > +		virtio_vsock_skb_set_reply(skb);
> > +
> > +	trace_virtio_transport_alloc_pkt(src_cid, src_port,
> > +					 dst_cid, dst_port,
> > +					 payload_len,
> > +					 info->type,
> > +					 info->op,
> > +					 info->flags);
> > +
> > +	return skb;
> > +out:
> > +	kfree_skb(skb);
> > +	return NULL;
> > +}
> > +
> >  /* This function can only be used on connecting/connected sockets,
> >   * since a socket assigned to a transport is required.
> >   *
> > @@ -222,10 +335,12 @@ static u16 virtio_transport_get_type(struct sock *sk)
> >  static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
> >  					  struct virtio_vsock_pkt_info *info)
> >  {
> > +	u32 max_skb_len = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
> >  	u32 src_cid, src_port, dst_cid, dst_port;
> >  	const struct virtio_transport *t_ops;
> >  	struct virtio_vsock_sock *vvs;
> >  	u32 pkt_len = info->pkt_len;
> > +	bool can_zcopy = false;
> >  	u32 rest_len;
> >  	int ret;
> >  
> > @@ -254,15 +369,30 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
> >  	if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW)
> >  		return pkt_len;
> >  
> > +	if (info->msg) {
> > +		/* If zerocopy is not enabled by 'setsockopt()', we behave as
> > +		 * there is no MSG_ZEROCOPY flag set.
> > +		 */
> > +		if (!sock_flag(sk_vsock(vsk), SOCK_ZEROCOPY))
> > +			info->msg->msg_flags &= ~MSG_ZEROCOPY;
> > +
> > +		if (info->msg->msg_flags & MSG_ZEROCOPY)
> > +			can_zcopy = virtio_transport_can_zcopy(info, pkt_len);
> > +
> > +		if (can_zcopy)
> > +			max_skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE,
> > +					    (MAX_SKB_FRAGS * PAGE_SIZE));
> > +	}
> > +
> >  	rest_len = pkt_len;
> >  
> >  	do {
> >  		struct sk_buff *skb;
> >  		size_t skb_len;
> >  
> > -		skb_len = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE, rest_len);
> > +		skb_len = min(max_skb_len, rest_len);
> >  
> > -		skb = virtio_transport_alloc_skb(info, skb_len,
> > +		skb = virtio_transport_alloc_skb(vsk, info, skb_len, can_zcopy,
> >  						 src_cid, src_port,
> >  						 dst_cid, dst_port);
> >  		if (!skb) {
> > @@ -270,6 +400,17 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
> >  			break;
> >  		}
> >  
> > +		/* This is last skb to send this portion of data. */
> > +		if (info->msg && info->msg->msg_flags & MSG_ZEROCOPY &&
> > +		    skb_len == rest_len && info->op == VIRTIO_VSOCK_OP_RW) {
> > +			if (virtio_transport_init_zcopy_skb(vsk, skb,
> > +							    info->msg,
> > +							    can_zcopy)) {
> > +				ret = -ENOMEM;
> > +				break;
> > +			}
> > +		}
> > +
> >  		virtio_transport_inc_tx_pkt(vvs, skb);
> >  
> >  		ret = t_ops->send_pkt(skb);
> > @@ -934,7 +1075,7 @@ static int virtio_transport_reset_no_sock(const struct virtio_transport *t,
> >  	if (!t)
> >  		return -ENOTCONN;
> >  
> > -	reply = virtio_transport_alloc_skb(&info, 0,
> > +	reply = virtio_transport_alloc_skb(NULL, &info, 0, false,
> >  					   le64_to_cpu(hdr->dst_cid),
> >  					   le32_to_cpu(hdr->dst_port),
> >  					   le64_to_cpu(hdr->src_cid),