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Date:   Thu, 29 Jun 2023 14:32:40 +0200
From:   Stefano Garzarella <sgarzare@redhat.com>
To:     Arseniy Krasnov <avkrasnov@sberdevices.ru>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Bobby Eshleman <bobby.eshleman@bytedance.com>,
        kvm@vger.kernel.org, virtualization@lists.linux-foundation.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel@sberdevices.ru, oxffffaa@gmail.com
Subject: Re: [RFC PATCH v4 05/17] vsock/virtio: MSG_ZEROCOPY flag support
Message-ID: <kkz3hl5ebhblyyerduybbrdcntdilsecjlhq4y5cymayw55q22@w4mph6yu2x7d>
References: <20230603204939.1598818-1-AVKrasnov@sberdevices.ru>
 <20230603204939.1598818-6-AVKrasnov@sberdevices.ru>
 <rbyt6uomvimurmgchxpuyoqjehdleqzzohzzdnajgadrwkbwsf@qwlcvzqhqxqw>
 <b25f0c4d-828c-ffe8-2780-2c954ed8770a@sberdevices.ru>
 <m5q3fqqvur4pcvkcxx36ivoqu77tsrjd4xna6zszmzq34dbqq5@6wfrhllk6tsq>
 <7bce4bbc-6549-1284-e027-05d5a9d100e1@sberdevices.ru>
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In-Reply-To: <7bce4bbc-6549-1284-e027-05d5a9d100e1@sberdevices.ru>
Precedence: bulk

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