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

[Pankaj Gupta]

> > >
> > > >  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@xxxxxxxxxx>
> > > > Signed-off-by: Pankaj Gupta <pagupta@xxxxxxxxxx>
> > >
> > > 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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