netdev - Re: [PATCH net-next v3 4/4] vsock/virtio: MSG

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Date: Tue, 25 Jul 2023 08:39:17 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Arseniy Krasnov <avkrasnov@...rdevices.ru>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Bobby Eshleman <bobby.eshleman@...edance.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel@...rdevices.ru,
	oxffffaa@...il.com
Subject: Re: [PATCH net-next v3 4/4] vsock/virtio: MSG_ZEROCOPY flag support

On Tue, Jul 25, 2023 at 02:28:02PM +0200, Stefano Garzarella wrote:
> On Tue, Jul 25, 2023 at 12:16:11PM +0300, Arseniy Krasnov wrote:
> > 
> > 
> > On 25.07.2023 11:46, Arseniy Krasnov wrote:
> > > 
> > > 
> > > On 25.07.2023 11:43, Stefano Garzarella wrote:
> > > > On Fri, Jul 21, 2023 at 08:09:03AM +0300, Arseniy Krasnov wrote:
> > > > > 
> > > > > 
> > > > > On 21.07.2023 00:42, Arseniy Krasnov wrote:
> > > > > > This adds handling of MSG_ZEROCOPY flag on transmission path: if this
> > > > > > flag is set and zerocopy transmission is possible (enabled in socket
> > > > > > options and transport allows zerocopy), 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()'. Second thing that
> > > > > > this patch does is replace type of skb owning: instead of calling
> > > > > > 'skb_set_owner_sk_safe()' it calls 'skb_set_owner_w()'. Reason of this
> > > > > > change is that '__zerocopy_sg_from_iter()' increments 'sk_wmem_alloc'
> > > > > > of socket, so to decrease this field correctly proper skb destructor is
> > > > > > needed: 'sock_wfree()'. This destructor is set by 'skb_set_owner_w()'.
> > > > > > 
> > > > > > Signed-off-by: Arseniy Krasnov <AVKrasnov@...rdevices.ru>
> > > > > > ---
> > > > > >  Changelog:
> > > > > >  v5(big patchset) -> v1:
> > > > > >   * Refactorings of 'if' conditions.
> > > > > >   * Remove extra blank line.
> > > > > >   * Remove 'frag_off' field unneeded init.
> > > > > >   * Add function 'virtio_transport_fill_skb()' which fills both linear
> > > > > >     and non-linear skb with provided data.
> > > > > >  v1 -> v2:
> > > > > >   * Use original order of last four arguments in 'virtio_transport_alloc_skb()'.
> > > > > >  v2 -> v3:
> > > > > >   * Add new transport callback: 'msgzerocopy_check_iov'. It checks that
> > > > > >     provided 'iov_iter' with data could be sent in a zerocopy mode.
> > > > > >     If this callback is not set in transport - transport allows to send
> > > > > >     any 'iov_iter' in zerocopy mode. Otherwise - if callback returns 'true'
> > > > > >     then zerocopy is allowed. Reason of this callback is that in case of
> > > > > >     G2H transmission we insert whole skb to the tx virtio queue and such
> > > > > >     skb must fit to the size of the virtio queue to be sent in a single
> > > > > >     iteration (may be tx logic in 'virtio_transport.c' could be reworked
> > > > > >     as in vhost to support partial send of current skb). This callback
> > > > > >     will be enabled only for G2H path. For details pls see comment
> > > > > >     'Check that tx queue...' below.
> > > > > > 
> > > > > >  include/net/af_vsock.h                  |   3 +
> > > > > >  net/vmw_vsock/virtio_transport.c        |  39 ++++
> > > > > >  net/vmw_vsock/virtio_transport_common.c | 257 ++++++++++++++++++------
> > > > > >  3 files changed, 241 insertions(+), 58 deletions(-)
> > > > > > 
> > > > > > diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
> > > > > > index 0e7504a42925..a6b346eeeb8e 100644
> > > > > > --- a/include/net/af_vsock.h
> > > > > > +++ b/include/net/af_vsock.h
> > > > > > @@ -177,6 +177,9 @@ struct vsock_transport {
> > > > > > 
> > > > > >      /* Read a single skb */
> > > > > >      int (*read_skb)(struct vsock_sock *, skb_read_actor_t);
> > > > > > +
> > > > > > +    /* Zero-copy. */
> > > > > > +    bool (*msgzerocopy_check_iov)(const struct iov_iter *);
> > > > > >  };
> > > > > > 
> > > > > >  /**** CORE ****/
> > > > > > diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> > > > > > index 7bbcc8093e51..23cb8ed638c4 100644
> > > > > > --- a/net/vmw_vsock/virtio_transport.c
> > > > > > +++ b/net/vmw_vsock/virtio_transport.c
> > > > > > @@ -442,6 +442,43 @@ static void virtio_vsock_rx_done(struct virtqueue *vq)
> > > > > >      queue_work(virtio_vsock_workqueue, &vsock->rx_work);
> > > > > >  }
> > > > > > 
> > > > > > +static bool
> > > > > > virtio_transport_msgzerocopy_check_iov(const struct
> > > > > > iov_iter *iov)
> > > > > > +{
> > > > > > +    struct virtio_vsock *vsock;
> > > > > > +    bool res = false;
> > > > > > +
> > > > > > +    rcu_read_lock();
> > > > > > +
> > > > > > +    vsock = rcu_dereference(the_virtio_vsock);
> > > > > > +    if (vsock) {
> 
> Just noted, what about the following to reduce the indentation?
> 
>         if (!vsock) {
>             goto out;
>         }

no {} pls

>             ...
>             ...
>     out:
>         rcu_read_unlock();
>         return res;

indentation is quite modest here. Not sure goto is worth it.