linux-kernel - Re: [RFC v2 1/2] virtio-pmem: Async virtio-pmem flush

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Date:   Wed, 25 Aug 2021 15:08:30 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Pankaj Gupta <pankaj.gupta@...os.com>
Cc:     Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Linux NVDIMM <nvdimm@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jmoyer <jmoyer@...hat.com>, David Hildenbrand <david@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>
Subject: Re: [RFC v2 1/2] virtio-pmem: Async virtio-pmem flush

On Wed, Aug 25, 2021 at 3:01 PM Pankaj Gupta <pankaj.gupta@...os.com> wrote:
>
> > > Hi Dan,
> > >
> > > Thank you for the review. Please see my reply inline.
> > >
> > > > > Implement asynchronous flush for virtio pmem using work queue
> > > > > to solve the preflush ordering issue. Also, coalesce the flush
> > > > > requests when a flush is already in process.
> > > > >
> > > > > Signed-off-by: Pankaj Gupta <pankaj.gupta@...os.com>
> > > > > ---
> > > > >  drivers/nvdimm/nd_virtio.c   | 72 ++++++++++++++++++++++++++++--------
> > > > >  drivers/nvdimm/virtio_pmem.c | 10 ++++-
> > > > >  drivers/nvdimm/virtio_pmem.h | 14 +++++++
> > > > >  3 files changed, 79 insertions(+), 17 deletions(-)
> > > > >
> > > > > diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> > > > > index 10351d5b49fa..61b655b583be 100644
> > > > > --- a/drivers/nvdimm/nd_virtio.c
> > > > > +++ b/drivers/nvdimm/nd_virtio.c
> > > > > @@ -97,29 +97,69 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
> > > > >         return err;
> > > > >  };
> > > > >
> > > > > +static void submit_async_flush(struct work_struct *ws);
> > > > > +
> > > > >  /* The asynchronous flush callback function */
> > > > >  int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
> > > > >  {
> > > > > -       /*
> > > > > -        * Create child bio for asynchronous flush and chain with
> > > > > -        * parent bio. Otherwise directly call nd_region flush.
> > > > > +       /* queue asynchronous flush and coalesce the flush requests */
> > > > > +       struct virtio_device *vdev = nd_region->provider_data;
> > > > > +       struct virtio_pmem *vpmem  = vdev->priv;
> > > > > +       ktime_t req_start = ktime_get_boottime();
> > > > > +
> > > > > +       spin_lock_irq(&vpmem->lock);
> > > > > +       /* flush requests wait until ongoing flush completes,
> > > > > +        * hence coalescing all the pending requests.
> > > > >          */
> > > > > -       if (bio && bio->bi_iter.bi_sector != -1) {
> > > > > -               struct bio *child = bio_alloc(GFP_ATOMIC, 0);
> > > > > -
> > > > > -               if (!child)
> > > > > -                       return -ENOMEM;
> > > > > -               bio_copy_dev(child, bio);
> > > > > -               child->bi_opf = REQ_PREFLUSH;
> > > > > -               child->bi_iter.bi_sector = -1;
> > > > > -               bio_chain(child, bio);
> > > > > -               submit_bio(child);
> > > > > -               return 0;
> > > > > +       wait_event_lock_irq(vpmem->sb_wait,
> > > > > +                           !vpmem->flush_bio ||
> > > > > +                           ktime_before(req_start, vpmem->prev_flush_start),
> > > > > +                           vpmem->lock);
> > > > > +       /* new request after previous flush is completed */
> > > > > +       if (ktime_after(req_start, vpmem->prev_flush_start)) {
> > > > > +               WARN_ON(vpmem->flush_bio);
> > > > > +               vpmem->flush_bio = bio;
> > > > > +               bio = NULL;
> > > > > +       }
> > > >
> > > > Why the dance with ->prev_flush_start vs just calling queue_work()
> > > > again. queue_work() is naturally coalescing in that if the last work
> > > > request has not started execution another queue attempt will be
> > > > dropped.
> > >
> > > How parent flush request will know when corresponding flush is completed?
> >
> > The eventual bio_endio() is what signals upper layers that the flush
> > completed...
> >
> >
> > Hold on... it's been so long that I forgot that you are copying
> > md_flush_request() here. It would help immensely if that was mentioned
> > in the changelog and at a minimum have a comment in the code that this
> > was copied from md. In fact it would be extra helpful if you
>
> My bad. I only mentioned this in the cover letter.

Yeah, sorry about that. Having come back to this after so long I just
decided to jump straight into the patches, but even if I had read that
cover I still would have given the feedback that md_flush_request()
heritage should also be noted with a comment in the code.