Re: [RFC PATCH 2/4] migration: Fix state transition in postcopy_start() error handling

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Thu, Aug 07, 2025 at 01:49:10PM +0200, Juraj Marcin wrote:
>> From: Juraj Marcin <jmarcin@redhat.com>
>> 
>> Depending on where an error during postcopy_start() happens, the state
>> can be either "active", "device" or "cancelling", but never
>> "postcopy-active". Migration state is transitioned to "postcopy-active"
>> only just before a successful return from the function.
>> 
>> Accept any state except "cancelling" when transitioning to "failed"
>> state.
>> 
>> Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
>> ---
>>  migration/migration.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>> 
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 10c216d25d..e5ce2940d5 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -2872,8 +2872,9 @@ static int postcopy_start(MigrationState *ms, Error **errp)
>>  fail_closefb:
>>      qemu_fclose(fb);
>>  fail:
>> -    migrate_set_state(&ms->state, MIGRATION_STATUS_POSTCOPY_ACTIVE,
>> -                          MIGRATION_STATUS_FAILED);
>> +    if ( ms->state != MIGRATION_STATUS_CANCELLING) {
>> +        migrate_set_state(&ms->state, ms->state, MIGRATION_STATUS_FAILED);
>> +    }
>
> Hmm, this might have been overlooked from my commit 48814111366b.  Maybe
> worth a Fixes and copy stable?
>
> For example, I would expect the old code (prior of 48814111366b) still be
> able to fail postcopy and resume src QEMU if qemu_savevm_send_packaged()
> failed.  Now, looks like it'll be stuck at "device" state..
>
> The other thing is it also looks like a common pattern to set FAILED
> meanwhile not messing with a CANCELLING stage.  It's not easy to always
> remember this, so maybe we should consider having a helper function?
>
>   migrate_set_failure(MigrationState *, Error *err);
>

We didn't do it back then because at there would be some logical
conflict with this series:

https://lore.kernel.org/r/20250110100707.4805-1-shivam.kumar1@nutanix.com

But I don't remember the details. If it works this time I'm all for it.

> Which could set err with migrate_set_error() (likely we could also
> error_report() the error), and update FAILED iff it's not CANCELLING.
>
> I saw three of such occurances that such helper may apply, but worth double
> check:
>
> postcopy_start[2725]           if (ms->state != MIGRATION_STATUS_CANCELLING) {
> migration_completion[3069]     if (s->state != MIGRATION_STATUS_CANCELLING) {
> igration_connect[4064]        if (s->state != MIGRATION_STATUS_CANCELLING) {
>
> If the cleanup looks worthwhile, and if the Fixes apply, we could have the
> cleanup patch on top of the fixes patch so patch 1 is easier to backport.
>
> Thanks,
>
>>      migration_block_activate(NULL);
>>      migration_call_notifiers(ms, MIG_EVENT_PRECOPY_FAILED, NULL);
>>      bql_unlock();
>> -- 
>> 2.50.1
>>