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

[Li Chen <me@linux.beauty>]

Hi Ira,

On Sat, 31 Jan 2026 04:52:12 +0800,
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.

Thanks for taking a look.

I can reproduce the issue here, but it is timing dependent. A single fio run
does not always hit it, so I suspect that's why you're not seeing the dmesg
messages.

Environment:
QEMU: 10.1.2
virtio-pmem backend: memory-backend-ram (shared)

The virtio-pmem relevant QEMU bits:
  -object memory-backend-ram,id=pmem0,size=10G,share=on
  -device virtio-pmem-pci,id=virtio-pmem0,memdev=pmem0

For completeness, this is the full QEMU command line I used (paths replaced
with placeholders):
  qemu-system-x86_64 -enable-kvm -cpu host -smp 16 -m 10G,maxmem=20G \\
    -netdev user,id=net0,hostfwd=tcp::<ssh_port>-:22 \\
    -device virtio-net,netdev=net0 \\
    -drive file=<guest.qcow2>,if=none,id=boot0,format=qcow2 \\
    -device virtio-blk-pci,drive=boot0,num-queues=4 \\
    -object memory-backend-ram,id=pmem0,size=10G,share=on \\
    -device virtio-pmem-pci,id=virtio-pmem0,memdev=pmem0 \\
    -nographic -kernel <bzImage> -append "<cmdline>"

Kernel under test (baseline, no patch):
  v6.18-764-g7aa104c7e8e9

I used the same fio parameters from the cover letter. The only difference is
that I run it in a loop so it has multiple chances to trigger. Each iteration
does a fresh mkfs + mount and clears dmesg before running fio:
This should be equivalent to the foo.fio you posted.

  for i in $(seq 1 10); do
    umount -l /mnt/bench 2>/dev/null || true
    mkfs.ext4 -F /dev/pmem0
    mkdir -p /mnt/bench
    dmesg -C
    mount -t ext4 -o dax,noatime /dev/pmem0 /mnt/bench
    fio --name=randwrite_fsync --filename=/mnt/bench/foo --size=1G \\
      --ioengine=io_uring --rw=randwrite --bs=4k --iodepth=64 --numjobs=64 \\
      --direct=1 --fsync=1 --runtime=30 --time_based=1
    dmesg | egrep -i \\
      -e "no free slots in the virtqueue" \\
      -e "virtio pmem device needs a reset" && break
  done

If it does not trigger in 10 iterations, reboot the guest and repeat.

On the baseline kernel, I see:
"failed to send command to virtio pmem device, no free slots in the virtqueue"
and "virtio pmem device needs a reset"
Typically within a few iterations (often on the first one).

With the fix applied, I ran 10 iterations back-to-back and did not see the
above messages.
 
> >   - 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.
> 
> 	guard(mutex)(&vpmem->flush_lock);
> 
> Then skip all the gotos and out_unlock stuff.

Agreed. I'll use guard in v2.
 
> Also, does this affect performance at all?

I did a quick sanity check. With a smaller numjobs value (numjobs=16,
iodepth=64, fsync=1, bs=4k, runtime=30s), I did not see a regression on this
setup. At numjobs=64 the baseline frequently hits NEEDS_RESET, so correctness
is the primary motivation here.

Regards,
Li​