netdev - Re: [RFC PATCH v3 1/9] vsock: SO_RCVLOWAT transport set callback

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Date:   Mon, 8 Aug 2022 12:30:11 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <AVKrasnov@...rdevices.ru>
Cc:     "David S. Miller" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
        "sthemmin@...rosoft.com" <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        Dexuan Cui <decui@...rosoft.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Bryan Tan <bryantan@...are.com>,
        Vishnu Dasa <vdasa@...are.com>,
        VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
        Krasnov Arseniy <oxffffaa@...il.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        kernel <kernel@...rdevices.ru>
Subject: Re: [RFC PATCH v3 1/9] vsock: SO_RCVLOWAT transport set callback

On Mon, Aug 8, 2022 at 12:23 PM Stefano Garzarella <sgarzare@...hat.com> wrote:
>
> On Wed, Aug 03, 2022 at 01:51:05PM +0000, Arseniy Krasnov wrote:
> >This adds transport specific callback for SO_RCVLOWAT, because in some
> >transports it may be difficult to know current available number of bytes
> >ready to read. Thus, when SO_RCVLOWAT is set, transport may reject it.
> >
> >Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
> >---
> > include/net/af_vsock.h   |  1 +
> > net/vmw_vsock/af_vsock.c | 25 +++++++++++++++++++++++++
> > 2 files changed, 26 insertions(+)
> >
> >diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> >index f742e50207fb..eae5874bae35 100644
> >--- a/include/net/af_vsock.h
> >+++ b/include/net/af_vsock.h
> >@@ -134,6 +134,7 @@ struct vsock_transport {
> >       u64 (*stream_rcvhiwat)(struct vsock_sock *);
> >       bool (*stream_is_active)(struct vsock_sock *);
> >       bool (*stream_allow)(u32 cid, u32 port);
> >+      int (*set_rcvlowat)(struct vsock_sock *, int);
>
> checkpatch suggests to add identifier names. For some we put them in,
> for others we didn't, but I suggest putting them in for the new ones
> because I think it's clearer too.
>
> WARNING: function definition argument 'struct vsock_sock *' should also
> have an identifier name
> #25: FILE: include/net/af_vsock.h:137:
> +       int (*set_rcvlowat)(struct vsock_sock *, int);
>
> WARNING: function definition argument 'int' should also have an identifier name
> #25: FILE: include/net/af_vsock.h:137:
> +       int (*set_rcvlowat)(struct vsock_sock *, int);
>
> total: 0 errors, 2 warnings, 0 checks, 44 lines checked
>
> >
> >       /* SEQ_PACKET. */
> >       ssize_t (*seqpacket_dequeue)(struct vsock_sock *vsk, struct msghdr *msg,
> >diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
> >index f04abf662ec6..016ad5ff78b7 100644
> >--- a/net/vmw_vsock/af_vsock.c
> >+++ b/net/vmw_vsock/af_vsock.c
> >@@ -2129,6 +2129,30 @@ vsock_connectible_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
> >       return err;
> > }
> >
> >+static int vsock_set_rcvlowat(struct sock *sk, int val)
> >+{
> >+      const struct vsock_transport *transport;
> >+      struct vsock_sock *vsk;
> >+      int err = 0;
> >+
> >+      vsk = vsock_sk(sk);
> >+
> >+      if (val > vsk->buffer_size)
> >+              return -EINVAL;
> >+
> >+      transport = vsk->transport;
> >+
> >+      if (!transport)
> >+              return -EOPNOTSUPP;
>
> I don't know whether it is better in this case to write it in
> sk->sk_rcvlowat, maybe we can return EOPNOTSUPP only when the trasport
> is assigned and set_rcvlowat is not defined. This is because usually the
> options are set just after creation, when the transport is practically
> unassigned.
>
> I mean something like this:
>
>          if (transport) {
>                  if (transport->set_rcvlowat)
>                          return transport->set_rcvlowat(vsk, val);
>                  else
>                          return -EOPNOTSUPP;
>          }
>
>          WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
>
>          return 0;

Since hv_sock implements `set_rcvlowat` to return EOPNOTSUPP. maybe we 
can just do the following:

        if (transport && transport->set_rcvlowat)
                return transport->set_rcvlowat(vsk, val);

        WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
        return 0;

That is, the default behavior is to set sk->sk_rcvlowat, but for 
transports that want a different behavior, they need to define 
set_rcvlowat() (like hv_sock).

Thanks,
Stefano