linux-kernel - Re: [PATCH] virtio pmem: fix async flush ordering

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Date:   Wed, 20 Nov 2019 23:32:16 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     Pankaj Gupta <pagupta@...hat.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>, Vivek Goyal <vgoyal@...hat.com>,
        Keith Busch <keith.busch@...el.com>
Subject: Re: [PATCH] virtio pmem: fix async flush ordering

On Wed, Nov 20, 2019 at 11:23 PM Dan Williams <dan.j.williams@...el.com> wrote:
>
> On Wed, Nov 20, 2019 at 9:26 AM Jeff Moyer <jmoyer@...hat.com> wrote:
> >
> > Pankaj Gupta <pagupta@...hat.com> writes:
> >
> > >  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@...hat.com>
> > > Signed-off-by: Pankaj Gupta <pagupta@...hat.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?

Why not just swap the parent child relationship in the PREFLUSH case?