Re: [PATCH v9 02/14] migration: Postcopy preemption preparation on channel creation

[Peter Xu]

On Tue, Jul 19, 2022 at 04:15:49PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > Create a new socket for postcopy to be prepared to send postcopy requested
> > pages via this specific channel, so as to not get blocked by precopy pages.
> > 
> > A new thread is also created on dest qemu to receive data from this new 
> > channel
> > based on the ram_load_postcopy() routine.
> > 
> > The ram_load_postcopy(POSTCOPY) branch and the thread has not started to
> > function, and that'll be done in follow up patches.
> > 
> > Cleanup the new sockets on both src/dst QEMUs, meanwhile look after the new
> > thread too to make sure it'll be recycled properly.
> 
> I'm hitting a CI failure here:
> https://gitlab.com/dagrh/qemu/-/jobs/2741659845
> 
> c.o -c ../migration/migration.c
> ../migration/migration.c: In function ‘migration_ioc_process_incoming’:
> ../migration/migration.c:766:8: error: ‘start_migration’ may be used 
> uninitialized in this function [-Werror=maybe-uninitialized]
>   766 |     if (start_migration) {
>       |        ^
> 
> > Reviewed-by: Daniel P. Berrang?? <berrange@redhat.com>
> > Reviewed-by: Juan Quintela <quintela@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/migration.c    | 62 +++++++++++++++++++++++----
> >  migration/migration.h    |  8 ++++
> >  migration/postcopy-ram.c | 92 ++++++++++++++++++++++++++++++++++++++--
> >  migration/postcopy-ram.h | 10 +++++
> >  migration/ram.c          | 25 ++++++++---
> >  migration/ram.h          |  4 +-
> >  migration/savevm.c       | 20 ++++-----
> >  migration/socket.c       | 22 +++++++++-
> >  migration/socket.h       |  1 +
> >  migration/trace-events   |  5 ++-
> >  10 files changed, 218 insertions(+), 31 deletions(-)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index ce7bb68cdc..9484fec0b2 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -321,6 +321,12 @@ void migration_incoming_state_destroy(void)
> >          mis->page_requested = NULL;
> >      }
> >  
> > +    if (mis->postcopy_qemufile_dst) {
> > +        
> > migration_ioc_unregister_yank_from_file(mis->postcopy_qemufile_dst);
> > +        qemu_fclose(mis->postcopy_qemufile_dst);
> > +        mis->postcopy_qemufile_dst = NULL;
> > +    }
> > +
> >      yank_unregister_instance(MIGRATION_YANK_INSTANCE);
> >  }
> >  
> > @@ -714,15 +720,21 @@ void migration_fd_process_incoming(QEMUFile *f, Error 
> > **errp)
> >      migration_incoming_process();
> >  }
> >  
> > +static bool migration_needs_multiple_sockets(void)
> > +{
> > +    return migrate_use_multifd() || migrate_postcopy_preempt();
> > +}
> > +
> >  void migration_ioc_process_incoming(QIOChannel *ioc, Error **errp)
> >  {
> >      MigrationIncomingState *mis = migration_incoming_get_current();
> >      Error *local_err = NULL;
> >      bool start_migration;
> > +    QEMUFile *f;
> >  
> >      if (!mis->from_src_file) {
> >          /* The first connection (multifd may have multiple) */
> > -        QEMUFile *f = qemu_file_new_input(ioc);
> > +        f = qemu_file_new_input(ioc);
> >  
> >          if (!migration_incoming_setup(f, errp)) {
> >              return;
> > @@ -730,13 +742,18 @@ void migration_ioc_process_incoming(QIOChannel *ioc, 
> > Error **errp)
> >  
> >          /*
> >           * Common migration only needs one channel, so we can start
> > -         * right now.  Multifd needs more than one channel, we wait.
> > +         * right now.  Some features need more than one channel, we wait.
> >           */
> > -        start_migration = !migrate_use_multifd();
> > +        start_migration = !migration_needs_multiple_sockets();
> >      } else {
> >          /* Multiple connections */
> > -        assert(migrate_use_multifd());
> > -        start_migration = multifd_recv_new_channel(ioc, &local_err);
> > +        assert(migration_needs_multiple_sockets());
> > +        if (migrate_use_multifd()) {
> > +            start_migration = multifd_recv_new_channel(ioc, &local_err);
> > +        } else if (migrate_postcopy_preempt()) {
> > +            f = qemu_file_new_input(ioc);
> > +            start_migration = postcopy_preempt_new_channel(mis, f);
> > +        }
> 
> So that doesn't always set start_migration?

Logically it should, because we asserted on
migration_needs_multiple_sockets(), while multifd and preempt mode are the
only two that may need multiple sockets.  So it must go into either of
them.

A fix could be like this:

diff --git a/migration/migration.c b/migration/migration.c
index 76cf2a72c0..7c7e529ca7 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -753,7 +753,8 @@ void migration_ioc_process_incoming(QIOChannel *ioc, Error 
**errp)
         assert(migration_needs_multiple_sockets());
         if (migrate_use_multifd()) {
             start_migration = multifd_recv_new_channel(ioc, &local_err);
-        } else if (migrate_postcopy_preempt()) {
+        } else {
+            assert(migrate_postcopy_preempt());
             f = qemu_file_new_input(ioc);
             start_migration = postcopy_preempt_new_channel(mis, f);
         }

Or we simply set start_migration=false as initial value.

Should I repost the series?

Thanks,

-- 
Peter Xu