Re: [PATCH v4 10/25] migration: Add Error** argument to qemu_savevm_state_setup()

[Cédric Le Goater]

On 3/11/24 20:03, Fabiano Rosas wrote:
> Cédric Le Goater <clg@redhat.com> writes:
>
> > On 3/8/24 15:36, Fabiano Rosas wrote:
> > > Cédric Le Goater <clg@redhat.com> writes:
> > >
> > > > This prepares ground for the changes coming next which add an Error**
> > > > argument to the .save_setup() handler. Callers of qemu_savevm_state_setup()
> > > > now handle the error and fail earlier setting the migration state from
> > > > MIGRATION_STATUS_SETUP to MIGRATION_STATUS_FAILED.
> > > >
> > > > In qemu_savevm_state(), move the cleanup to preserve the error
> > > > reported by .save_setup() handlers.
> > > >
> > > > Since the previous behavior was to ignore errors at this step of
> > > > migration, this change should be examined closely to check that
> > > > cleanups are still correctly done.
> > > >
> > > > Signed-off-by: Cédric Le Goater <clg@redhat.com>
> > > > ---
> > > >
> > > >    Changes in v4:
> > > >    
> > > >    - Merged cleanup change in qemu_savevm_state()
> > > >      
> > > >    Changes in v3:
> > > >    
> > > >    - Set migration state to MIGRATION_STATUS_FAILED
> > > >    - Fixed error handling to be done under lock in bg_migration_thread()
> > > >    - Made sure an error is always set in case of failure in
> > > >      qemu_savevm_state_setup()
> > > >      
> > > >    migration/savevm.h    |  2 +-
> > > >    migration/migration.c | 27 ++++++++++++++++++++++++---
> > > >    migration/savevm.c    | 26 +++++++++++++++-----------
> > > >    3 files changed, 40 insertions(+), 15 deletions(-)
> > > >
> > > > diff --git a/migration/savevm.h b/migration/savevm.h
> > > > index 
> > > > 74669733dd63a080b765866c703234a5c4939223..9ec96a995c93a42aad621595f0ed58596c532328
> > > >  100644
> > > > --- a/migration/savevm.h
> > > > +++ b/migration/savevm.h
> > > > @@ -32,7 +32,7 @@
> > > >    bool qemu_savevm_state_blocked(Error **errp);
> > > >    void qemu_savevm_non_migratable_list(strList **reasons);
> > > >    int qemu_savevm_state_prepare(Error **errp);
> > > > -void qemu_savevm_state_setup(QEMUFile *f);
> > > > +int qemu_savevm_state_setup(QEMUFile *f, Error **errp);
> > > >    bool qemu_savevm_state_guest_unplug_pending(void);
> > > >    int qemu_savevm_state_resume_prepare(MigrationState *s);
> > > >    void qemu_savevm_state_header(QEMUFile *f);
> > > > diff --git a/migration/migration.c b/migration/migration.c
> > > > index 
> > > > a49fcd53ee19df1ce0182bc99d7e064968f0317b..6d1544224e96f5edfe56939a9c8395d88ef29581
> > > >  100644
> > > > --- a/migration/migration.c
> > > > +++ b/migration/migration.c
> > > > @@ -3408,6 +3408,8 @@ static void *migration_thread(void *opaque)
> > > >        int64_t setup_start = qemu_clock_get_ms(QEMU_CLOCK_HOST);
> > > >        MigThrError thr_error;
> > > >        bool urgent = false;
> > > > +    Error *local_err = NULL;
> > > > +    int ret;
> > > >    
> > > >        thread = migration_threads_add("live_migration", qemu_get_thread_id());
> > > >    
> > > > @@ -3451,9 +3453,17 @@ static void *migration_thread(void *opaque)
> > > >        }
> > > >    
> > > >        bql_lock();
> > > > -    qemu_savevm_state_setup(s->to_dst_file);
> > > > +    ret = qemu_savevm_state_setup(s->to_dst_file, &local_err);
> > > >        bql_unlock();
> > > >    
> > > > +    if (ret) {
> > > > +        migrate_set_error(s, local_err);
> > > > +        error_free(local_err);
> > > > +        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> > > > +                          MIGRATION_STATUS_FAILED);
> > > > +        goto out;
> > > > +     }
> > > > +
> > > >        qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
> > > >                                   MIGRATION_STATUS_ACTIVE);
> > >
> > > This^ should be before the new block it seems:
> > >
> > > GOOD:
> > > migrate_set_state new state setup
> > > migrate_set_state new state wait-unplug
> > > migrate_fd_cancel
> > > migrate_set_state new state cancelling
> > > migrate_fd_cleanup
> > > migrate_set_state new state cancelled
> > > migrate_fd_cancel
> > > ok 1 /x86_64/failover-virtio-net/migrate/abort/wait-unplug
> > >
> > > BAD:
> > > migrate_set_state new state setup
> > > migrate_fd_cancel
> > > migrate_set_state new state cancelling
> > > migrate_fd_cleanup
> > > migrate_set_state new state cancelled
> > > qemu-system-x86_64: ram_save_setup failed: Input/output error
> > > **
> > > ERROR:../tests/qtest/virtio-net-failover.c:1203:test_migrate_abort_wait_unplug:
> > > assertion failed (status == "cancelling"): ("cancelled" == "cancelling")
> > >
> > > Otherwise migration_iteration_finish() will schedule the cleanup BH and
> > > that will run concurrently with migrate_fd_cancel() issued by the test
> > > and bad things happens.
> >
> > This hack makes things work :
> >
> > @@ -3452,6 +3452,9 @@ static void *migration_thread(void *opaq
> >            qemu_savevm_send_colo_enable(s->to_dst_file);
> >        }
> >    
> > +    qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
> > +                            MIGRATION_STATUS_SETUP);
> > +
>
> Why move it all the way up here? Has moving the wait_unplug before the
> 'if (ret)' block not worked for you?

We could be sleeping while holding the BQL. It looked wrong.


Thanks,

C.


> >        bql_lock();
> >        ret = qemu_savevm_state_setup(s->to_dst_file, &local_err);
> >        bql_unlock();
> >
> > We should fix the test instead :) Unless waiting for failover devices
> > to unplug before the save_setup handlers and not after is ok.
> >
> > commit c7e0acd5a3f8 ("migration: add new migration state wait-unplug")
> > is not clear about the justification.:
> >
> >       This patch adds a new migration state called wait-unplug.  It is entered
> >       after the SETUP state if failover devices are present. It will transition
> >       into ACTIVE once all devices were succesfully unplugged from the guest.
>
> This is not clear indeed, but to me it seems having the wait-unplug
> after setup was important.
>
> > > =====
> > > PS: I guess the next level in our Freestyle Concurrency video-game is to
> > > make migrate_fd_cancel() stop setting state and poking files and only
> > > set a flag that's tested in the other parts of the code.
> >
> > Is that a new item on the TODO list?
>
> Yep, I'll add it to the wiki.