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

[Arseniy Krasnov <avkrasnov@sberdevices.ru>]

On 25.07.2023 16:22, Michael S. Tsirkin wrote:
> On Tue, Jul 25, 2023 at 04:04:13PM +0300, Arseniy Krasnov wrote:
>>
>>
>> On 25.07.2023 14:50, Michael S. Tsirkin wrote:
>>> 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@sberdevices.ru>
>>>>> ---
>>>>>  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).
>>
>> I mean that with indirect, we only need one buffer, and we can just wait
>> for enough space - for this single buffer ( as we discussed a little bit before).
>> But if indirect buffer is not supported - we need that whole packet must fit
>> to the size of tx queue - otherwise it never be transmitted.
>>
>> Thanks, Arseniy
> 
> 
> yes but according to virtio spec it's illegal to add s/g that is bigger
> than queue size.

Aah, so even in case of indirect buffers feature, buffer descriptors stored in memory
pointed by indirect buffer must be accounted against queue size ?

Thanks, Arseniy

> 
>>>
>>>
>>>>> +			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),
>>>
>