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Date:   Thu, 21 Nov 2019 23:38:41 -0500 (EST)
From:   Pankaj Gupta <pagupta@redhat.com>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     Jeff Moyer <jmoyer@redhat.com>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        Vishal L Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Len Brown <lenb@kernel.org>, Vivek Goyal <vgoyal@redhat.com>,
        Keith Busch <keith.busch@intel.com>
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Subject: Re: [PATCH] virtio pmem: fix async flush ordering
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> > > > >
> > > > > >  Remove logic to create child bio in the async flush function w=
hich
> > > > > >  causes child bio to get executed after parent bio
> > > > > >  'pmem_make_request'
> > > > > >  completes. This resulted in wrong ordering of REQ_PREFLUSH wit=
h
> > > > > >  the
> > > > > >  data write request.
> > > > > >
> > > > > >  Instead we are performing flush from the parent bio to maintai=
n
> > > > > >  the
> > > > > >  correct order. Also, returning from function 'pmem_make_reques=
t'
> > > > > >  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_flus=
h
> > > > > were
> > > > > converted to -EIO.  Now, they are reported as-is.  I think this i=
s
> > > > > 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 fac=
t
> > > > that now the guest will synchronously wait for flushing to occur. T=
he
> > > > 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. Do=
es
> > > > 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 creat=
e
> > child
> > bio. How we exactly will swap the parent/child relationship for PREFLUS=
H
> > case?
> >
> > Child bio is queued after parent bio completes.
>=20
> 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.

o.k. Thank you. Will try to post patch today to be considered for 5.4.

Best regards,
Pankaj
>=20
>=20