Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Aug 31, 2021 at 08:02:39AM -0300, Leonardo Bras wrote:
> Call qio_channel_set_zerocopy(true) in the start of every multifd thread.
> 
> Change the send_write() interface of multifd, allowing it to pass down
> flags for qio_channel_write*().
> 
> Pass down MSG_ZEROCOPY flag for sending memory pages, while keeping the
> other data being sent at the default copying approach.
> 
> Signed-off-by: Leonardo Bras <leobras@redhat.com>
> ---
>  migration/multifd-zlib.c | 7 ++++---
>  migration/multifd-zstd.c | 7 ++++---
>  migration/multifd.c      | 9 ++++++---
>  migration/multifd.h      | 3 ++-
>  4 files changed, 16 insertions(+), 10 deletions(-)

> @@ -675,7 +676,8 @@ static void *multifd_send_thread(void *opaque)
>              }
>  
>              if (used) {
> -                ret = multifd_send_state->ops->send_write(p, used, &local_err);
> +                ret = multifd_send_state->ops->send_write(p, used, MSG_ZEROCOPY,
> +                                                          &local_err);

I don't think it is valid to unconditionally enable this feature due to the
resource usage implications

https://www.kernel.org/doc/html/v5.4/networking/msg_zerocopy.html

  "A zerocopy failure will return -1 with errno ENOBUFS. This happens 
   if the socket option was not set, the socket exceeds its optmem 
   limit or the user exceeds its ulimit on locked pages."

The limit on locked pages is something that looks very likely to be
exceeded unless you happen to be running a QEMU config that already
implies locked memory (eg PCI assignment)


Regards,
Daniel
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