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

[Daniel P . Berrangé]

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
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