Re: [RFC PATCH v4 05/17] vsock/virtio: MSG_ZEROCOPY flag support
From: Arseniy Krasnov
Date: Tue Jun 27 2023 - 04:29:36 EST
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@xxxxxxxxxxxxxx>
>>>> ---
>>>> 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
>