Re: [PATCH] nvdimm: virtio_pmem: serialize flush requests

Michael S. Tsirkin
2026-01-31T17:47:22Z

On Fri, Jan 30, 2026 at 02:52:12PM -0600, Ira Weiny wrote:
> Li Chen wrote:
> > Under heavy concurrent flush traffic, virtio-pmem can overflow its request
> > virtqueue (req_vq): virtqueue_add_sgs() starts returning -ENOSPC and the
> > driver logs "no free slots in the virtqueue". Shortly after that the
> > device enters VIRTIO_CONFIG_S_NEEDS_RESET and flush requests fail with
> > "virtio pmem device needs a reset".
> >
> > Serialize virtio_pmem_flush() with a per-device mutex so only one flush
> > request is in-flight at a time. This prevents req_vq descriptor overflow
> > under high concurrency.
> >
> > Reproducer (guest with virtio-pmem):
> > - mkfs.ext4 -F /dev/pmem0
> > - mount -t ext4 -o dax,noatime /dev/pmem0 /mnt/bench
> > - fio: ioengine=io_uring rw=randwrite bs=4k iodepth=64 numjobs=64
> > direct=1 fsync=1 runtime=30s time_based=1
>
> I don't see this error.
>
> <file>
> 13:28:50 > cat foo.fio
> # test http://lore.kernel.org/20260113034552.62805-1-me@linux.beauty
>
> [global]
> filename=/mnt/bench/foo
> ioengine=io_uring
> size=1G
> bs=4K
> iodepth=64
> numjobs=64
> direct=1
> fsync=1
> runtime=30s
> time_based=1
>
> [rand-write]
> rw=randwrite
> </file>
>
> It's possible I'm doing something wrong. Can you share your qemu cmdline
> or more details on the bug yall see.
>
> > - dmesg: "no free slots in the virtqueue"
> > "virtio pmem device needs a reset"
> >
> > Fixes: 6e84200c0a29 ("virtio-pmem: Add virtio pmem driver")
> > Signed-off-by: Li Chen <me@linux.beauty>
> > ---
> > drivers/nvdimm/nd_virtio.c | 15 +++++++++++----
> > drivers/nvdimm/virtio_pmem.c | 1 +
> > drivers/nvdimm/virtio_pmem.h | 4 ++++
> > 3 files changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> > index c3f07be4aa22..827a17fe7c71 100644
> > --- a/drivers/nvdimm/nd_virtio.c
> > +++ b/drivers/nvdimm/nd_virtio.c
> > @@ -44,19 +44,24 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
> > unsigned long flags;
> > int err, err1;
> >
> > + might_sleep();
> > + mutex_lock(&vpmem->flush_lock);
>
> Assuming this does fix a bug I'd rather use guard here.

Do you, from code review, agree with the logic that
it's racy right now?
Whether the bug is reproducible isn't really the question.

> guard(mutex)(&vpmem->flush_lock);
>
> Then skip all the gotos and out_unlock stuff.
>
> Also, does this affect performance at all?
>
> Ira
>
> > +
> > /*
> > * Don't bother to submit the request to the device if the device is
> > * not activated.
> > */
> > if (vdev->config->get_status(vdev) & VIRTIO_CONFIG_S_NEEDS_RESET) {
> > dev_info(&vdev->dev, "virtio pmem device needs a reset\n");
> > - return -EIO;
> > + err = -EIO;
> > + goto out_unlock;
> > }
> >
> > - might_sleep();
> > req_data = kmalloc(sizeof(*req_data), GFP_KERNEL);
> > - if (!req_data)
> > - return -ENOMEM;
> > + if (!req_data) {
> > + err = -ENOMEM;
> > + goto out_unlock;
> > + }
> >
> > req_data->done = false;
> > init_waitqueue_head(&req_data->host_acked);
> > @@ -103,6 +108,8 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
> > }
> >
> > kfree(req_data);
> > +out_unlock:
> > + mutex_unlock(&vpmem->flush_lock);
> > return err;
> > };
>
> [snip]