[PATCH] nvdimm: virtio_pmem: serialize flush requests

[Li Chen <me@linux.beauty>]

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>
---
 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);
+
 	/*
 	 * 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;
 };
 
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