[PATCH 6.1 165/232] nvdimm: virtio_pmem: serialize flush requests

Sasha Levin
2026-02-28T18:13:48Z

From: Li Chen <me@linux.beauty>

[ Upstream commit a9ba6733c7f1096c4506bf4e34a546e07242df74 ]

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@linux.beauty>
Acked-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20260203021353.121091-1-me@linux.beauty
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 a78e17a43a9d2..90b0fdb72024a 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 20da455d2ef63..ed5376128bbd8 100644
--- a/drivers/nvdimm/virtio_pmem.c
+++ b/drivers/nvdimm/virtio_pmem.c
@@ -50,6 +50,7 @@ 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 0dddefe594c46..f72cf17f9518f 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;
--
2.51.0