Re: [PATCH v8 5/5] multifd: Implement zero copy write in multifd migration (multifd-zero-copy)

[Leonardo Bras Soares Passos <leobras@redhat.com>]

Hello Peter, thanks for reviewing!

On Mon, Feb 7, 2022 at 11:22 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Feb 01, 2022 at 03:29:03AM -0300, Leonardo Bras wrote:
> > -void multifd_send_sync_main(QEMUFile *f)
> > +int multifd_send_sync_main(QEMUFile *f)
> >  {
> >      int i;
> > +    bool flush_zero_copy;
> >
> >      if (!migrate_use_multifd()) {
> > -        return;
> > +        return 0;
> >      }
> >      if (multifd_send_state->pages->num) {
> >          if (multifd_send_pages(f) < 0) {
> >              error_report("%s: multifd_send_pages fail", __func__);
> > -            return;
> > +            return 0;
>
> I've not checked how it used to do if multifd_send_pages() failed, but.. should
> it returns -1 rather than 0 when there will be a return code?

Yeah, that makes sense.
The point here is that I was trying not to modify much of the current behavior.

I mean, multifd_send_sync_main() would previously return void, so any
other errors would not matter to the caller of this function, which
will continue to run as if nothing happened.

Now, if it fails with flush_zero_copy, the operation needs to be aborted.

Maybe, I should make it different:
- In any error, return -1.
- Create/use a specific error code in the case of a failing
flush_zero_copy, so I can test the return value for it on the caller
function and return early.

Or alternatively, the other errors could also return early, but since
this will change how the code currently works, I would probably need
another patch for that change. (so it can be easily reverted if
needed)

What do you think is better?


> >          }
> >      }
> > +
> > +    /*
> > +     * When using zero-copy, it's necessary to flush after each iteration to
> > +     * make sure pages from earlier iterations don't end up replacing newer
> > +     * pages.
> > +     */
> > +    flush_zero_copy = migrate_use_zero_copy_send();
> > +
> >      for (i = 0; i < migrate_multifd_channels(); i++) {
> >          MultiFDSendParams *p = &multifd_send_state->params[i];
> >
> > @@ -591,7 +600,7 @@ void multifd_send_sync_main(QEMUFile *f)
> >          if (p->quit) {
> >              error_report("%s: channel %d has already quit", __func__, i);
> >              qemu_mutex_unlock(&p->mutex);
> > -            return;
> > +            return 0;
>
> Same question here.

Please see above,

>
> >          }
>
> The rest looks good.  Thanks,

Thank you!

Best regards,
Leo