Re: [PATCH v3 2/8] migration/savevm: Prepare vmdesc json writer in qemu_savevm_state_setup()

[Peter Xu <peterx@redhat.com>]

On Thu, Jan 12, 2023 at 06:40:00PM +0000, Dr. David Alan Gilbert wrote:
> * David Hildenbrand (david@redhat.com) wrote:
> > On 12.01.23 18:43, Dr. David Alan Gilbert wrote:
> > > * David Hildenbrand (david@redhat.com) wrote:
> > > > ... and store it in the migration state. This is a preparation for
> > > > storing selected vmds's already in qemu_savevm_state_setup().
> > > > 
> > > > Signed-off-by: David Hildenbrand <david@redhat.com>
> > > > ---
> > > >   migration/migration.c |  4 ++++
> > > >   migration/migration.h |  4 ++++
> > > >   migration/savevm.c    | 18 ++++++++++++------
> > > >   3 files changed, 20 insertions(+), 6 deletions(-)
> > > > 
> > 
> > [1]
> > 
> > > > diff --git a/migration/migration.c b/migration/migration.c
> > > > index 52b5d39244..1d33a7efa0 100644
> > > > --- a/migration/migration.c
> > > > +++ b/migration/migration.c
> > > > @@ -2170,6 +2170,9 @@ void migrate_init(MigrationState *s)
> > > >       s->vm_was_running = false;
> > > >       s->iteration_initial_bytes = 0;
> > > >       s->threshold_size = 0;
> > > > +
> > > > +    json_writer_free(s->vmdesc);
> > > > +    s->vmdesc = NULL;
> > > >   }
> > 
> > [...]
> > 
> > > >       trace_savevm_state_setup();
> > > >       QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
> > > >           if (!se->ops || !se->ops->save_setup) {
> > > > @@ -1390,15 +1395,12 @@ int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
> > > >                                                       bool in_postcopy,
> > > >                                                       bool inactivate_disks)
> > > >   {
> > > > -    g_autoptr(JSONWriter) vmdesc = NULL;
> > > > +    MigrationState *ms = migrate_get_current();
> > > > +    JSONWriter *vmdesc = ms->vmdesc;
> > > >       int vmdesc_len;
> > > >       SaveStateEntry *se;
> > > >       int ret;
> > > > -    vmdesc = json_writer_new(false);
> > > > -    json_writer_start_object(vmdesc, NULL);
> > > > -    json_writer_int64(vmdesc, "page_size", qemu_target_page_size());
> > > > -    json_writer_start_array(vmdesc, "devices");
> > > >       QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
> > > >           ret = vmstate_save(f, se, vmdesc);
> > > >           if (ret) {
> > > > @@ -1433,6 +1435,10 @@ int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
> > > >           qemu_put_buffer(f, (uint8_t *)json_writer_get(vmdesc), vmdesc_len);
> > > >       }
> > > > +    /* Free it now to detect any inconsistencies. */
> > > > +    json_writer_free(vmdesc);
> > > > +    ms->vmdesc = NULL;
> > > 
> > > and this only happens when this succesfully exits;  so if this errors
> > > out, and then you retry an outwards migration, I think you've leaked a
> > > writer.
> > 
> > Shouldn't the change [1] to migrate_init() cover that?
> 
> Hmm OK, yes it does - I guess it does mean you keep the allocation
> around for a bit longer, but that's OK in practice since normally you'll
> be quitting soon.

Instead of json_writer_free() here and there, how about free it in
migrate_fd_cleanup() once and for all?

-- 
Peter Xu