Re: [Qemu-devel] [PATCH] blockdev-backup: don't check aio_context too early

[John Snow <jsnow@redhat.com>]

On 5/7/19 4:50 AM, Kevin Wolf wrote:
> Am 06.05.2019 um 22:33 hat John Snow geschrieben:
>> in blockdev_backup_prepare, we check to make sure that the target is
>> associated with a compatible aio context. However, do_blockdev_backup is
>> called later and has some logic to move the target to a compatible
>> aio_context. The transaction version will fail certain commands
>> needlessly early as a result.
>>
>> Allow blockdev_backup_prepare to simply call do_blockdev_backup, which
>> will ultimately decide if the contexts are compatible or not.
>>
>> Note: the transaction version has always disallowed this operation since
>> its initial commit bd8baecd (2014), whereas the version of
>> qmp_blockdev_backup at the time, from commit c29c1dd312f, tried to
>> enforce the aio_context switch instead. It's not clear, and I can't see
>> from the mailing list archives at the time, why the two functions take a
>> different approach. It wasn't until later in efd7556708b (2016) that the
>> standalone version tried to determine if it could set the context or
>> not.
>>
>> Reported-by: aihua liang <aliang@redhat.com>
>> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1683498
> 
> Signed-off-by is missing, and a testcase, too. :-)
> 
>> diff --git a/blockdev.c b/blockdev.c
>> index 79fbac8450..a81d88980c 100644
>> --- a/blockdev.c
>> +++ b/blockdev.c
>> @@ -1872,10 +1872,6 @@ static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
>>      }
>>  
>>      aio_context = bdrv_get_aio_context(bs);
>> -    if (aio_context != bdrv_get_aio_context(target)) {
>> -        error_setg(errp, "Backup between two IO threads is not implemented");
>> -        return;
>> -    }
>>      aio_context_acquire(aio_context);
>>      state->bs = bs;
> 
> The actual change looks good to me.
> 
> Kevin
> 

(See the Red Hat BZ for details on the test I am more or less trying to
replicate in iotests -- but the jist of it is using an iothread on one
device and not specifying one for the other, then backing up both to two
blockdev-add created nodes not attached to any device.)

Is there some trick to getting this to fail with accel=qtest? I'm
noticing that if the VM is paused or if I use accel=qtest, the iothread
contexts for the drives appear to go ... unresolved? and the test passes.

I've only had luck (so far) reproducing this with accel=kvm on a running
VM (the drives can be empty) and I don't actually know why that is just
yet -- I guess these aio_context objects get resolved at runtime?

Anyway, this seems to be a little tricky so far, maybe you have some
advice for me?

--js

(Also note: doing a full backup and an incremental backup for two
perfectly empty 64GB qcow2 drives takes over 6 seconds in total. It
probably shouldn't, right? There's something about the initial backup
that appears to take a pretty long time.)