Re: [RFC PATCH v4 05/17] vsock/virtio: MSG_ZEROCOPY flag support

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Jun 27, 2023 at 11:22:52AM +0300, Arseniy Krasnov wrote:
>
>
>On 27.06.2023 10:50, Stefano Garzarella wrote:
>> On Tue, Jun 27, 2023 at 07:41:51AM +0300, Arseniy Krasnov wrote:
>>>
>>>
>>> On 26.06.2023 19:03, Stefano Garzarella wrote:
>>>> On Sat, Jun 03, 2023 at 11:49:27PM +0300, Arseniy Krasnov wrote:
>>>>> This adds handling of MSG_ZEROCOPY flag on transmission path: if this
>>>>> flag is set and zerocopy transmission is possible, 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()'.
>>>>>
>>>>> Signed-off-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
>>>>> ---
>>>>> net/vmw_vsock/virtio_transport_common.c | 270 ++++++++++++++++++------
>>>>> 1 file changed, 208 insertions(+), 62 deletions(-)
>>>>>
>>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>>> index 0de562c1dc4b..f1ec38c72db7 100644
>>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>>> @@ -37,27 +37,100 @@ 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;
>>>>> +static bool virtio_transport_can_zcopy(struct virtio_vsock_pkt_info *info,
>>>>> +                       size_t max_to_send)
>>>>> +{
>>>>> +    struct iov_iter *iov_iter;
>>>>> +
>>>>> +    if (!info->msg)
>>>>> +        return false;
>>>>> +
>>>>> +    iov_iter = &info->msg->msg_iter;
>>>>> +
>>>>> +    /* 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;
>>>>> +    }
>>>>> +
>>>>> +    skb_zcopy_init(skb, uarg);
>>>>> +
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>>> +static int virtio_transport_fill_linear_skb(struct sk_buff *skb,
>>>>> +                        struct vsock_sock *vsk,
>>>>
>>>> `vsk` seems unused
>>>>
>>>>> +                        struct virtio_vsock_pkt_info *info,
>>>>> +                        size_t len)
>>>>> +{
>>>>>     void *payload;
>>>>>     int err;
>>>>>
>>>>> -    skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
>>>>> -    if (!skb)
>>>>> -        return NULL;
>>>>> +    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;
>>>>> +}
>>>>> +
>>>>> +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;
>>>>>
>>>>>     hdr = virtio_vsock_hdr(skb);
>>>>>     hdr->type    = cpu_to_le16(info->type);
>>>>> @@ -68,42 +141,6 @@ virtio_transport_alloc_skb(struct virtio_vsock_pkt_info *info,
>>>>>     hdr->dst_port    = cpu_to_le32(dst_port);
>>>>>     hdr->flags    = cpu_to_le32(info->flags);
>>>>>     hdr->len    = cpu_to_le32(len);
>>>>> -
>>>>> -    if (info->msg && len > 0) {
>>>>> -        payload = skb_put(skb, len);
>>>>> -        err = memcpy_from_msg(payload, info->msg, len);
>>>>> -        if (err)
>>>>> -            goto out;
>>>>> -
>>>>> -        if (msg_data_left(info->msg) == 0 &&
>>>>> -            info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
>>>>> -            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,
>>>>> -                     len,
>>>>> -                     info->type,
>>>>> -                     info->op,
>>>>> -                     info->flags);
>>>>> -
>>>>> -    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;
>>>>> -    }
>>>>> -
>>>>> -    return skb;
>>>>> -
>>>>> -out:
>>>>> -    kfree_skb(skb);
>>>>> -    return NULL;
>>>>> }
>>>>>
>>>>> static void virtio_transport_copy_nonlinear_skb(struct sk_buff *skb,
>>>>> @@ -214,6 +251,85 @@ static u16 virtio_transport_get_type(struct sock *sk)
>>>>>         return VIRTIO_VSOCK_TYPE_SEQPACKET;
>>>>> }
>>>>>
>>>>> +/* Returns a new packet on success, otherwise returns NULL.
>>>>> + *
>>>>> + * If NULL is returned, errp is set to a negative errno.
>>>>
>>>> I had noticed this in Bobby's patches, I think it's an old comment we
>>>> left around.
>>>>
>>>>> + */
>>>>> +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 dst_cid,
>>>>> +                          u32 dst_port,
>>>>> +                          u32 src_cid,
>>>>> +                          u32 src_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));
>>>>
>>>> why we are moving from skb_set_owner_sk_safe() to skb_set_owner_w()?
>>>>
>>>> We should mention this in the commit description.
>>>>
>>>>> +
>>>>> +    if (info->msg && payload_len > 0) {
>>>>> +        int err;
>>>>> +
>>>>> +        if (zcopy) {
>>>>> +            err = __zerocopy_sg_from_iter(info->msg, NULL, skb,
>>>>> +                              &info->msg->msg_iter,
>>>>> +                              payload_len);
>>>>> +        } else {
>>>>> +            err = virtio_transport_fill_linear_skb(skb, vsk, info, payload_len);
>>>>> +        }
>>>>> +
>>>>> +        if (err)
>>>>> +            goto out;
>>>>> +
>>>>> +        VIRTIO_VSOCK_SKB_CB(skb)->frag_off = 0;
>>>>> +
>>>>> +        if (info->type == VIRTIO_VSOCK_TYPE_SEQPACKET) {
>>>>> +            struct virtio_vsock_hdr *hdr;
>>>>> +
>>>>> +            hdr = virtio_vsock_hdr(skb);
>>>>
>>>> Just `struct virtio_vsock_hdr *hdr = virtio_vsock_hdr(skb);` should be
>>>> fine.
>>>>
>>>>> +
>>>>> +            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.
>>>>>  *
>>>>> @@ -226,6 +342,8 @@ static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
>>>>>     const struct virtio_transport *t_ops;
>>>>>     struct virtio_vsock_sock *vvs;
>>>>>     u32 pkt_len = info->pkt_len;
>>>>> +    bool can_zcopy = false;
>>>>> +    u32 max_skb_cap;
>>>>>     u32 rest_len;
>>>>>     int ret;
>>>>>
>>>>> @@ -254,22 +372,49 @@ 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 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->flags &= ~MSG_ZEROCOPY;
>>>>> +
>>>>> +    if (info->flags & MSG_ZEROCOPY)
>>>>> +        can_zcopy = virtio_transport_can_zcopy(info, pkt_len);
>>>>> +
>>>>> +    if (can_zcopy)
>>>>> +        max_skb_cap = min_t(u32, VIRTIO_VSOCK_MAX_PKT_BUF_SIZE,
>>>>> +                    (MAX_SKB_FRAGS * PAGE_SIZE));
>>>>> +    else
>>>>> +        max_skb_cap = VIRTIO_VSOCK_MAX_PKT_BUF_SIZE;
>>>>> +
>>>>
>>>> We use `len` very often, what about `max_skb_len`?
>>>>
>>>>>     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_cap, rest_len);
>>>>>
>>>>> -        skb = virtio_transport_alloc_skb(info, skb_len,
>>>>> -                         src_cid, src_port,
>>>>> -                         dst_cid, dst_port);
>>>>> +        skb = virtio_transport_alloc_skb(vsk, info, skb_len, can_zcopy,
>>>>> +                         dst_cid, dst_port,
>>>>> +                         src_cid, src_port);
>>>>>         if (!skb) {
>>>>>             ret = -ENOMEM;
>>>>>             break;
>>>>>         }
>>>>>
>>>>> +        /* This is last skb to send this portion of data. */
>>>>> +        if (skb_len == rest_len &&
>>>>> +            info->flags & MSG_ZEROCOPY &&
>>>>> +            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);
>>>>> @@ -884,6 +1029,7 @@ virtio_transport_stream_enqueue(struct vsock_sock *vsk,
>>>>>         .msg = msg,
>>>>>         .pkt_len = len,
>>>>>         .vsk = vsk,
>>>>> +        .flags = msg->msg_flags,
>>>>
>>>> These flags then get copied into the virtio_vsock_hdr, which I don't
>>>> think is a good idea.
>>>>
>>>> Why not using directly info->msg->msg_flags?
>>>
>>> Ops, yes, it's a bug, You're right, this is really wrong as there are two different
>>> sets of flags - MSG_XXX passed to syscall and flags in the header of packet.
>>
>> Yep.
>>
>> What about the moving from skb_set_owner_sk_safe() to skb_set_owner_w()?
>> Was it voluntary? If so, can you explain why?
>
>Sure! here is what happens: difference between these functions is that 'skb_set_owner_w()'
>sets another destructor - 'sock_wfree()':
>
>https://elixir.bootlin.com/linux/v6.4/source/net/core/sock.c#L2408
>
>This destructor also updates 'sk->sk_wmem_alloc' during skb freeing. Why this is needed?
>Because '__zerocopy_sg_from_iter()' updates 'sk_wmem_alloc' if socket is NOT SOCK_STREAM
>or socket was NOT passed as input argument (it is not obvious why, but it is so):
>
>https://elixir.bootlin.com/linux/latest/source/net/core/datagram.c#L646
>
>So to support both types of sockets (seqpacket and stream) I decided to try pass NULL always
>as socket argument to '__zerocopy_sg_from_iter()', with this 'sk_wmem_alloc' will be always
>updated in '__zerocopy_sg_from_iter()' and then we decrease 'sk_wmem_alloc' in 'sock_wfree()'.
>
>May be this looks strange - I tried to understand logic of this code in '__zerocopy_sg_from_iter()',
>but it requires more deep analysis. Anyway it works:)

Yep, it should require also more analysis on my side :-)

Please document it in the commit description, so if we will find any
issue in the future, we know why we did this change.

Thanks,
Stefano