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

[Arseniy Krasnov <avkrasnov@sberdevices.ru>]

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:)

Thanks, Arseniy

> 
> 
> Thanks,
> Stefano
>