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

Ira Weiny
2026-01-30T20:49:03Z

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.

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]