Re: [PATCH v10 7/7] multifd: Implement zero copy write in multifd migration (multifd-zero-copy)

[Leonardo Bras Soares Passos]

On Tue, Apr 26, 2022 at 8:26 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Apr 26, 2022 at 08:06:56PM -0300, Leonardo Bras wrote:
> > Implement zero copy send on nocomp_send_write(), by making use of QIOChannel
> > writev + flags & flush interface.
> >
> > Change multifd_send_sync_main() so flush_zero_copy() can be called
> > after each iteration in order to make sure all dirty pages are sent before
> > a new iteration is started. It will also flush at the beginning and at the
> > end of migration.
> >
> > Also make it return -1 if flush_zero_copy() fails, in order to cancel
> > the migration process, and avoid resuming the guest in the target host
> > without receiving all current RAM.
> >
> > This will work fine on RAM migration because the RAM pages are not usually 
> > freed,
> > and there is no problem on changing the pages content between 
> > writev_zero_copy() and
> > the actual sending of the buffer, because this change will dirty the page 
> > and
> > cause it to be re-sent on a next iteration anyway.
> >
> > A lot of locked memory may be needed in order to use multifd migration
> > with zero-copy enabled, so disabling the feature should be necessary for
> > low-privileged users trying to perform multifd migrations.
> >
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
>
> --
> Peter Xu
>

Thanks for reviewing Peter!

Best regards,
Leo