linux-kernel - Re: [RFC v3 1/2] virtio-pmem: Async virtio-pmem flush

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Message-ID: <CAM9Jb+hbds3b+nY9APU40Fpd9pt4CyFuZ3SU4ZB05subnaJQvQ@mail.gmail.com>
Date:   Wed, 16 Feb 2022 18:04:19 +0100
From:   Pankaj Gupta <pankaj.gupta.linux@...il.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux NVDIMM <nvdimm@...ts.linux.dev>,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jmoyer <jmoyer@...hat.com>,
        Stefan Hajnoczi <stefanha@...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>,
        Pankaj Gupta <pankaj.gupta@...os.com>
Subject: Re: [RFC v3 1/2] virtio-pmem: Async virtio-pmem flush

> >
> > > >
> > > > Enable asynchronous flush for virtio pmem using work queue. Also,
> > > > coalesce the flush requests when a flush is already in process.
> > > > This functionality is copied from md/RAID code.
> > > >
> > > > When a flush is already in process, new flush requests wait till
> > > > previous flush completes in another context (work queue). For all
> > > > the requests come between ongoing flush and new flush start time, only
> > > > single flush executes, thus adhers to flush coalscing logic. This is
> > >
> > > s/adhers/adheres/
> > >
> > > s/coalscing/coalescing/
> > >
> > > > important for maintaining the flush request order with request coalscing.
> > >
> > > s/coalscing/coalescing/
> >
> > o.k. Sorry for the spelling mistakes.
> >
> > >
> > > >
> > > > Signed-off-by: Pankaj Gupta <pankaj.gupta.linux@...il.com>
> > > > ---
> > > >  drivers/nvdimm/nd_virtio.c   | 74 +++++++++++++++++++++++++++---------
> > > >  drivers/nvdimm/virtio_pmem.c | 10 +++++
> > > >  drivers/nvdimm/virtio_pmem.h | 16 ++++++++
> > > >  3 files changed, 83 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> > > > index 10351d5b49fa..179ea7a73338 100644
> > > > --- a/drivers/nvdimm/nd_virtio.c
> > > > +++ b/drivers/nvdimm/nd_virtio.c
> > > > @@ -100,26 +100,66 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
> > > >  /* 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();
> > > > +       int ret = -EINPROGRESS;
> > > > +
> > > > +       spin_lock_irq(&vpmem->lock);
> > >
> > > Why a new lock and not continue to use ->pmem_lock?
> >
> > This spinlock is to protect entry in 'wait_event_lock_irq'
> > and the Other spinlock is to protect virtio queue data.
>
> Understood, but md shares the mddev->lock for both purposes, so I
> would ask that you either document what motivates the locking split,
> or just reuse the lock until a strong reason to split them arises.

O.k. Will check again if we could use same lock Or document it.

Thanks,
Pankaj