Re: [PATCH] virtio pmem: fix async flush ordering

[Pankaj Gupta <pagupta@redhat.com>]

> > > > > >
> > > > > > 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.
> > >
> > > Sorry, I didn't quite mean with bio_split, but issuing another request
> > > in front of the real bio. See md_flush_request() for inspiration.
> >
> > o.k. Thank you. Will try to post patch today to be considered for 5.4.
> >
> 
> I think it is too late for v5.4-final, but we can get it in the
> -stable queue. Let's take the time to do it right and get some testing
> on it.

Sure.

Just sharing probable patch for early feedback, if I am doing it correctly?
I will test it thoroughly.

Thanks,
Pankaj

========

diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
index 10351d5b49fa..c683e0e2515c 100644
--- a/drivers/nvdimm/nd_virtio.c
+++ b/drivers/nvdimm/nd_virtio.c
@@ -112,6 +112,12 @@ int async_pmem_flush(struct nd_region *nd_region, struct bio *bio)
                bio_copy_dev(child, bio);
                child->bi_opf = REQ_PREFLUSH;
                child->bi_iter.bi_sector = -1;
+
+               if (unlikely(bio->bi_opf & REQ_PREFLUSH)) {
+                       struct request_queue *q = bio->bi_disk->queue;
+                       q->make_request_fn(q, child);
+                       return 0;
+               }
                bio_chain(child, bio);
                submit_bio(child);
                return 0;