Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread

[Peter Xu <peterx@redhat.com>]

On Tue, Mar 23, 2021 at 10:51:57AM +0300, Andrey Gruzdev wrote:
> On 22.03.2021 23:17, Peter Xu wrote:
> > On Fri, Mar 19, 2021 at 05:52:47PM +0300, Andrey Gruzdev wrote:
> > > Added missing qemu_fflush() on buffer file holding precopy device state.
> > > Increased initial QIOChannelBuffer allocation to 512KB to avoid reallocs.
> > > Typical configurations often require >200KB for device state and VMDESC.
> > > 
> > > Signed-off-by: Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>
> > > ---
> > >   migration/migration.c | 4 +++-
> > >   1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/migration/migration.c b/migration/migration.c
> > > index ca8b97baa5..32b48fe9f5 100644
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -3812,7 +3812,7 @@ static void *bg_migration_thread(void *opaque)
> > >        * with vCPUs running and, finally, write stashed non-RAM part of
> > >        * the vmstate from the buffer to the migration stream.
> > >        */
> > > -    s->bioc = qio_channel_buffer_new(128 * 1024);
> > > +    s->bioc = qio_channel_buffer_new(512 * 1024);
> > >       qio_channel_set_name(QIO_CHANNEL(s->bioc), "vmstate-buffer");
> > >       fb = qemu_fopen_channel_output(QIO_CHANNEL(s->bioc));
> > >       object_unref(OBJECT(s->bioc));
> > > @@ -3866,6 +3866,8 @@ static void *bg_migration_thread(void *opaque)
> > >       if (qemu_savevm_state_complete_precopy_non_iterable(fb, false, false)) {
> > >           goto fail;
> > >       }
> > > +    qemu_fflush(fb);
> > What will happen if the vmstates are bigger than 512KB?  Would the extra data
> > be dropped?
> 
> No, the buffer shall be reallocated and the original content shall be kept.

Oh right..

Would you comment above qemu_fflush() about it (maybe also move it right before
the call to qemu_put_buffer)?  Otherwise it indeed looks more like an
optimization rather than a bugfix.

For the long term I think we'd better have a helper:

        qemu_put_qio_channel_buffer(QEMUFile *file, QIOChannelBuffer *bioc)

So as to hide this flush operation, which is tricky. We'll have two users so
far:

        bg_migration_completion
        colo_do_checkpoint_transaction

IMHO it'll be nicer if you'd do it in this patch altogether!

Thanks,

-- 
Peter Xu