Re: [PATCH v10 6/7] multifd: Send header packet without flags if zero-copy-send is enabled

[Leonardo Bras Soares Passos]

On Wed, Apr 27, 2022 at 5:44 AM Daniel P. Berrangé <berrange@redhat.com> wrote:
>
> On Tue, Apr 26, 2022 at 08:06:55PM -0300, Leonardo Bras wrote:
> > Since d48c3a0445 ("multifd: Use a single writev on the send side"),
> > sending the header packet and the memory pages happens in the same
> > writev, which can potentially make the migration faster.
> >
> > Using channel-socket as example, this works well with the default copying
> > mechanism of sendmsg(), but with zero-copy-send=true, it will cause
> > the migration to often break.
> >
> > This happens because the header packet buffer gets reused quite often,
> > and there is a high chance that by the time the MSG_ZEROCOPY mechanism get
> > to send the buffer, it has already changed, sending the wrong data and
> > causing the migration to abort.
> >
> > It means that, as it is, the buffer for the header packet is not suitable
> > for sending with MSG_ZEROCOPY.
> >
> > In order to enable zero copy for multifd, send the header packet on an
> > individual write(), without any flags, and the remanining pages with a
> > writev(), as it was happening before. This only changes how a migration
> > with zero-copy-send=true works, not changing any current behavior for
> > migrations with zero-copy-send=false.
> >
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> >  migration/multifd.c | 23 ++++++++++++++++++++---
> >  1 file changed, 20 insertions(+), 3 deletions(-)
>
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
>
>
> With regards,
> Daniel

Thanks for reviewing Daniel!

Best regards,
Leo

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