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

From: Michael S. Tsirkin
Date: Tue Jul 25 2023 - 09:23:24 EST

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@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).
>
> 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.

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