Re: [PATCH net-next v3 4/4] vsock/virtio: MSG_ZEROCOPY flag support

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Jul 25, 2023 at 08:39:17AM -0400, Michael S. Tsirkin wrote:
>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@sberdevices.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

ooops, true, too much QEMU code today, but luckily checkpatch would have
spotted it ;-)

>
>>             ...
>>             ...
>>     out:
>>         rcu_read_unlock();
>>         return res;
>
>indentation is quite modest here. Not sure goto is worth it.

It's a pattern we follow a lot in this file, and I find the early
return/goto more readable.
Anyway, I don't have a strong opinion, it's fine the way it is now,
actually we don't have too many indentations for now in this function.

Thanks,
Stefano