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

Pankaj Gupta
2019-11-21T08:02:05Z

> >
> > > 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?

My bad, I missed this point completely.

Thanks,
Pankaj

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