Re: [PATCH] virtio pmem: fix async flush ordering

Pankaj Gupta
2019-11-21T08:00:35Z

> > >
> > > > Remove logic to create child bio in the async flush function which
> > > > causes child bio to get executed after parent bio 'pmem_make_request'
> > > > completes. This resulted in wrong ordering of REQ_PREFLUSH with the
> > > > data write request.
> > > >
> > > > Instead we are performing flush from the parent bio to maintain the
> > > > correct order. Also, returning from function 'pmem_make_request' if
> > > > REQ_PREFLUSH returns an error.
> > > >
> > > > Reported-by: Jeff Moyer <jmoyer@redhat.com>
> > > > Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> > >
> > > There's a slight change in behavior for the error path in the
> > > virtio_pmem driver. Previously, all errors from virtio_pmem_flush were
> > > converted to -EIO. Now, they are reported as-is. I think this is
> > > actually an improvement.
> > >
> > > I'll also note that the current behavior can result in data corruption,
> > > so this should be tagged for stable.
> >
> > I added that and was about to push this out, but what about the fact
> > that now the guest will synchronously wait for flushing to occur. The
> > goal of the child bio was to allow that to be an I/O wait with
> > overlapping I/O, or at least not blocking the submission thread. Does
> > the block layer synchronously wait for PREFLUSH requests? If not I
> > think a synchronous wait is going to be a significant performance
> > regression. Are there any numbers to accompany this change?
>
> Why not just swap the parent child relationship in the PREFLUSH case?

I we are already inside parent bio "make_request" function and we create child
bio. How we exactly will swap the parent/child relationship for PREFLUSH case?

Child bio is queued after parent bio completes.

Thanks,
Pankaj

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