Re: [PATCH v4 2/3] QIOChannelSocket: Implement io_writev_zerocopy & io_flush_zerocopy for CONFIG_LINUX

[Leonardo Bras Soares Passos]

Hello Eric,

On Mon, Oct 11, 2021 at 4:28 PM Eric Blake <eblake@redhat.com> wrote:
>
> On Sat, Oct 09, 2021 at 04:56:12AM -0300, Leonardo Bras wrote:
> > For CONFIG_LINUX, implement the new optional callbacks io_write_zerocopy and
> > io_flush_zerocopy on QIOChannelSocket, but enables it only when MSG_ZEROCOPY
> > feature is available in the host kernel, which is checked on
> > qio_channel_socket_connect_sync()
> >
> > qio_channel_socket_writev() contents were moved to a helper function
> > qio_channel_socket_writev_flags() which accepts an extra argument for flags.
> > (This argument is passed directly to sendmsg().
> >
> > The above helper function is used to implement qio_channel_socket_writev(),
> > with flags = 0, keeping it's behavior unchanged, and
>
> its (remember, "it's" is shorthand for "it is", which does not fit here)
>
> > qio_channel_socket_writev_zerocopy() with flags = MSG_ZEROCOPY.
> >
> > qio_channel_socket_flush_zerocopy() was implemented by counting how many 
> > times
> > sendmsg(...,MSG_ZEROCOPY) was sucessfully called, and then reading the
> > socket's error queue, in order to find how many of them finished sending.
> > Flush will loop until those counters are the same, or until some error 
> > ocurs.
>
> occurs

Thanks!
(I will check if my codespell is enabled in this setup)

>
> >
> > A new function qio_channel_socket_poll() was also created in order to avoid
> > busy-looping recvmsg() in qio_channel_socket_flush_zerocopy() while waiting 
> > for
> > updates in socket's error queue.
> >
> > Notes on using writev_zerocopy():
> > 1: Buffer
> > - As MSG_ZEROCOPY tells the kernel to use the same user buffer to avoid 
> > copying,
> > some caution is necessary to avoid overwriting any buffer before it's sent.
> > If something like this happen, a newer version of the buffer may be sent 
> > instead.
> > - If this is a problem, it's recommended to call flush_zerocopy() before 
> > freeing
> > or re-using the buffer.
> >
> > 2: Locked memory
> > - When using MSG_ZERCOCOPY, the buffer memory will be locked after queued, 
> > and
> > unlocked after it's sent.
> > - Depending on the size of each buffer, and how often it's sent, it may 
> > require
> > a larger amount of locked memory than usually available to non-root user.
> > - If the required amount of locked memory is not available, writev_zerocopy
> > will return an error, which can abort an operation like migration,
> > - Because of this, when an user code wants to add zerocopy as a feature, it
> > requires a mechanism to disable it, so it can still be acessible to less
> > privileged users.
> >
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> >  include/io/channel-socket.h |   2 +
> >  include/io/channel.h        |   1 +
> >  io/channel-socket.c         | 180 ++++++++++++++++++++++++++++++++++--
> >  3 files changed, 173 insertions(+), 10 deletions(-)
> >
> > +static int qio_channel_socket_flush_zerocopy(QIOChannel *ioc,
> > +                                             Error **errp)
> > +{
>
> > +
> > +        /* No errors, count sucessfully finished sendmsg()*/
>
> Space before */

Thanks!
Also, typo on 'successfully'.

>
> > +        sioc->zerocopy_sent += serr->ee_data - serr->ee_info + 1;
> > +    }
> > +    return 0;
> > +}
> > +
> > +#endif /* CONFIG_LINUX */
> > +
> >  static int
> >  qio_channel_socket_set_blocking(QIOChannel *ioc,
> >                                  bool enabled,
> > @@ -787,6 +943,10 @@ static void qio_channel_socket_class_init(ObjectClass 
> > *klass,
> >      ioc_klass->io_set_delay = qio_channel_socket_set_delay;
> >      ioc_klass->io_create_watch = qio_channel_socket_create_watch;
> >      ioc_klass->io_set_aio_fd_handler = 
> > qio_channel_socket_set_aio_fd_handler;
> > +#ifdef CONFIG_LINUX
> > +    ioc_klass->io_writev_zerocopy = qio_channel_socket_writev_zerocopy;
> > +    ioc_klass->io_flush_zerocopy = qio_channel_socket_flush_zerocopy;
> > +#endif
> >  }
>
> I did a high-level look at the code, rather than an in-depth review of
> whether zero-copy was being used correctly.

It's so far been very helpful. Thanks!

Best regards,
Leo