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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 25 Jul 2023 09:06:02 -0400

On Tue, Jul 25, 2023 at 02:53:39PM +0200, Stefano Garzarella wrote:
> On Tue, Jul 25, 2023 at 07:50:53AM -0400, Michael S. Tsirkin 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 <[email-masked]>
> > > > ---
> > > >  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) {
> > > > +		struct virtqueue *vq;
> > > > +		int iov_pages;
> > > > +
> > > > +		vq = vsock->vqs[VSOCK_VQ_TX];
> > > > +
> > > > +		iov_pages = round_up(iov->count, PAGE_SIZE) / PAGE_SIZE;
> > > > +
> > > > +		/* Check that tx queue is large enough to keep whole
> > > > +		 * data to send. This is needed, because when there is
> > > > +		 * not enough free space in the queue, current skb to
> > > > +		 * send will be reinserted to the head of tx list of
> > > > +		 * the socket to retry transmission later, so if skb
> > > > +		 * is bigger than whole queue, it will be reinserted
> > > > +		 * again and again, thus blocking other skbs to be sent.
> > > > +		 * Each page of the user provided buffer will be added
> > > > +		 * as a single buffer to the tx virtqueue, so compare
> > > > +		 * number of pages against maximum capacity of the queue.
> > > > +		 * +1 means buffer for the packet header.
> > > > +		 */
> > > > +		if (iov_pages + 1 <= vq->num_max)
> > > 
> > > I think this check is actual only for case one we don't have indirect buffer feature.
> > > With indirect mode whole data to send will be packed into one indirect buffer.
> > > 
> > > Thanks, Arseniy
> > 
> > Actually the reverse. With indirect you are limited to num_max.
> > Without you are limited to whatever space is left in the
> > queue (which you did not check here, so you should).
> > 
> > 
> > > > +			res = true;
> > > > +	}
> > > > +
> > > > +	rcu_read_unlock();
> > 
> > Just curious:
> > is the point of all this RCU dance to allow vsock
> > to change from under us? then why is it ok to
> > have it change? the virtio_transport_msgzerocopy_check_iov
> > will then refer to the old vsock ...
> 
> IIRC we introduced the RCU to handle hot-unplug issues:
> commit 0deab087b16a ("vsock/virtio: use RCU to avoid use-after-free on
> the_virtio_vsock")
> 
> When we remove the device, we flush all the works, etc. so we should
> not be in this case (referring the old vsock), except for an irrelevant
> transient as the device is disappearing.
> 
> Stefano

what if old device goes away then new one appears?

-- 
MST