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

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Date:   Thu, 21 Nov 2019 03:00:27 -0500 (EST)
From:   Pankaj Gupta <pagupta@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Jeff Moyer <jmoyer@...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>,
        Ira Weiny <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


> > >
> > > >  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?

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  

> 
>