linux-kernel - Re: [PATCH v2] nvdimm: virtio_pmem: serialize flush requests

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Message-ID: <CAM9Jb+iBD1=BgPjN6Jku7=G8qSoavCfB9ZB+rmb_aqgmhQdctA@mail.gmail.com>
Date: Tue, 3 Feb 2026 11:43:44 +0100
From: Pankaj Gupta <pankaj.gupta.linux@...il.com>
To: Li Chen <me@...ux.beauty>
Cc: Dan Williams <dan.j.williams@...el.com>, Vishal Verma <vishal.l.verma@...el.com>, 
	Dave Jiang <dave.jiang@...el.com>, Ira Weiny <ira.weiny@...el.com>, 
	Cornelia Huck <cohuck@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, Yuval Shaia <yuval.shaia@...cle.com>, 
	virtualization@...ts.linux.dev, nvdimm@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] nvdimm: virtio_pmem: serialize flush requests

> 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
>   - 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@...ux.beauty>

Acked-by: Pankaj Gupta <pankaj.gupta.linux@...il.com

> ---
> v2:
> - Use guard(mutex)() for flush_lock (as suggested by Ira Weiny).
> - Drop redundant might_sleep() next to guard(mutex)() (as suggested by Michael S. Tsirkin).
>
>  drivers/nvdimm/nd_virtio.c   | 3 ++-
>  drivers/nvdimm/virtio_pmem.c | 1 +
>  drivers/nvdimm/virtio_pmem.h | 4 ++++
>  3 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/nvdimm/nd_virtio.c b/drivers/nvdimm/nd_virtio.c
> index c3f07be4aa22..af82385be7c6 100644
> --- a/drivers/nvdimm/nd_virtio.c
> +++ b/drivers/nvdimm/nd_virtio.c
> @@ -44,6 +44,8 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
>         unsigned long flags;
>         int err, err1;
>
> +       guard(mutex)(&vpmem->flush_lock);
> +
>         /*
>          * Don't bother to submit the request to the device if the device is
>          * not activated.
> @@ -53,7 +55,6 @@ static int virtio_pmem_flush(struct nd_region *nd_region)
>                 return -EIO;
>         }
>
> -       might_sleep();
>         req_data = kmalloc(sizeof(*req_data), GFP_KERNEL);
>         if (!req_data)
>                 return -ENOMEM;
> diff --git a/drivers/nvdimm/virtio_pmem.c b/drivers/nvdimm/virtio_pmem.c
> index 2396d19ce549..77b196661905 100644
> --- a/drivers/nvdimm/virtio_pmem.c
> +++ b/drivers/nvdimm/virtio_pmem.c
> @@ -64,6 +64,7 @@ static int virtio_pmem_probe(struct virtio_device *vdev)
>                 goto out_err;
>         }
>
> +       mutex_init(&vpmem->flush_lock);
>         vpmem->vdev = vdev;
>         vdev->priv = vpmem;
>         err = init_vq(vpmem);
> diff --git a/drivers/nvdimm/virtio_pmem.h b/drivers/nvdimm/virtio_pmem.h
> index 0dddefe594c4..f72cf17f9518 100644
> --- a/drivers/nvdimm/virtio_pmem.h
> +++ b/drivers/nvdimm/virtio_pmem.h
> @@ -13,6 +13,7 @@
>  #include <linux/module.h>
>  #include <uapi/linux/virtio_pmem.h>
>  #include <linux/libnvdimm.h>
> +#include <linux/mutex.h>
>  #include <linux/spinlock.h>
>
>  struct virtio_pmem_request {
> @@ -35,6 +36,9 @@ struct virtio_pmem {
>         /* Virtio pmem request queue */
>         struct virtqueue *req_vq;
>
> +       /* Serialize flush requests to the device. */
> +       struct mutex flush_lock;
> +
>         /* nvdimm bus registers virtio pmem device */
>         struct nvdimm_bus *nvdimm_bus;
>         struct nvdimm_bus_descriptor nd_desc;
> --
> 2.52.0