> > > > > > 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.gu...@ionos.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.
Sure.
Thanks,
Pankaj
