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: 6e84200 ("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>

Author: FirstLoveLife

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;

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);

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;