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

From: Michael S. Tsirkin
Date: Tue Jul 25 2023 - 09:24:34 EST

On Tue, Jul 25, 2023 at 04:10:40PM +0300, Arseniy Krasnov wrote:
>
>
> On 25.07.2023 14:59, Michael S. Tsirkin wrote:
> > On Tue, Jul 25, 2023 at 11:39:22AM +0300, Arseniy Krasnov wrote:
> >>
> >>
> >> On 25.07.2023 11:25, Michael S. Tsirkin wrote:
> >>> On Fri, Jul 21, 2023 at 12:42:45AM +0300, 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)
> >>>> + res = true;
> >>>
> >>>
> >>> Yes but can't there already be buffers in the queue?
> >>> Then you can't stick num_max there.
> >>
> >> I think, that it is not critical, because vhost part always tries to process all
> >> incoming buffers (yes, 'vhost_exceeds_weight()' breaks at some moment, but it will
> >> reschedule tx kick ('vhost_vsock_handle_tx_kick()') work again), so current "too
> >> big" skb will wait until there will be enough space in queue and as it is requeued
> >> to the head of tx list it will be inserted to tx queue first.
> >>
> >> But anyway, I agree that comparing to 'num_free' may be more effective to the whole
> >> system performance...
> >>
> >> Thanks, Arseniy
> >
> > Oh I see. It makes sense then - instead of copying just so we can
> > stick it in the queue, wait a bit and send later.
> > Also - for stream transports can't the message be split
> > and sent chunk by chunk? Better than copying ...
>
> Technically yes, also we can split message for non-stream sockets (as vhost
> does when it copies data to rx buffers of the guest),

Won't breaking up messages break applications though?


> but it requires to rework
> current implementation by adding buffers one by one to the tx queue. I think
> it was not implemented here because until MSG_ZEROCOPY all skbs requires one
> (if it is control message) or two (with payload) buffers, so there is no big
> sense in processing max two buffers in "one-by-one" mode - we can just wait
> for space.
>
> May be, I can add this logic for non-linear skb's here:
>
> if (skb->len > vq->max_num)
> add buffers "one-by-one", incrementing internal offset in skb,
> if (new skb insertion fails)
> requeue skb, wait for space.
>
> In TX done callback I'll call consume skb only when above mentioned internal
> offset == skb->len. I think this approach allows to get rid of new 'check_iov'
> callback from this patch.
>
>
> Stefano, what do You think?
>
> Thanks, Arseniy
>
> >
> >
> >>>
> >>>
> >>>> + }
> >>>> +
> >>>> + rcu_read_unlock();
> >>>> +
> >>>> + 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),
> >>>> --
> >>>> 2.25.1
> >>>
> >