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Date:   Fri, 22 Nov 2019 00:37:00 -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>,
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        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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Thread-Topic: virtio pmem: fix async flush ordering
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> > > > > >
> > > > > > 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 occu=
r.
> > > > > > 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 no=
t I
> > > > > > think a synchronous wait is going to be a significant performan=
ce
> > > > > > regression. Are there any numbers to accompany this change?
> > > > >
> > > > > Why not just swap the parent child relationship in the PREFLUSH c=
ase?
> > > >
> > > > 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 reques=
t
> > > 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.
> >
>=20
> I think it is too late for v5.4-final, but we can get it in the
> -stable queue. Let's take the time to do it right and get some testing
> on it.

Sure.

Just sharing probable patch for early feedback, if I am doing it correctly?
I will test it thoroughly.

Thanks,
Pankaj

=3D=3D=3D=3D=3D=3D=3D=3D

diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
index 10351d5b49fa..c683e0e2515c 100644
--- a/drivers/nvdimm/nd_virtio.c
+++ b/drivers/nvdimm/nd_virtio.c
@@ -112,6 +112,12 @@ int async_pmem_flush(struct nd_region *nd_region, stru=
ct bio *bio)
                bio_copy_dev(child, bio);
                child->bi_opf =3D REQ_PREFLUSH;
                child->bi_iter.bi_sector =3D -1;
+
+               if (unlikely(bio->bi_opf & REQ_PREFLUSH)) {
+                       struct request_queue *q =3D bio->bi_disk->queue;
+                       q->make_request_fn(q, child);
+                       return 0;
+               }
                bio_chain(child, bio);
                submit_bio(child);
                return 0;

=20