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

[Dan Williams]

On Thu, Nov 21, 2019 at 12:00 AM Pankaj Gupta <pagupta@xxxxxxxxxx> wrote:
>
>
> > > >
> > > > >  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.

Sorry, I didn't quite mean with bio_split, but issuing another request
in front of the real bio. See md_flush_request() for inspiration.