[Virtio-fs] [PATCH] virtiofs: Export filesystem tags through sysfs

[Alyssa Ross]

On Thu, Oct 05, 2023 at 04:30:30PM -0400, Vivek Goyal wrote:
> virtiofs filesystem is mounted using a "tag" which is exported by the
> virtiofs device. virtiofs driver knows about all the available tags but
> these are not exported to user space.
>
> People have asked these tags to be exported to user space. Most recently
> Lennart Poettering has asked for it as he wants to scan the tags and mount
> virtiofs automatically in certain cases.
>
> https://gitlab.com/virtio-fs/virtiofsd/-/issues/128

Hi, I was one of those people. :)

> This patch exports tags through sysfs. One tag is associated with each
> virtiofs device. A new "tag" file appears under virtiofs device dir.
> Actual filesystem tag can be obtained by reading this "tag" file.
>
> For example, if a virtiofs device exports tag "myfs", a new file "tag"
> will show up here.
>
> /sys/bus/virtio/devices/virtio<N>/tag
>
> # cat /sys/bus/virtio/devices/virtio<N>/tag
> myfs
>
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>

Are you still thinking about exposing this in the uevent as well?
That would be much more convenient for me, because with this approach
by the time the "remove" uevent arrives, it's no longer possible to
check what tag was associated with the device — you have to store it
somewhere when the device appears, so you can look it up again when the
device is removed.  (Not everybody uses udev.)

Regardless,

Tested-by: Alyssa Ross <hi at alyssa.is>

… and a review comment below.

> ---
>  fs/fuse/virtio_fs.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 5f1be1da92ce..a5b11e18f331 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -107,6 +107,21 @@ static const struct fs_parameter_spec virtio_fs_parameters[] = {
>  	{}
>  };
>
> +/* Forward Declarations */
> +static void virtio_fs_stop_all_queues(struct virtio_fs *fs);
> +
> +/* sysfs related */
> +static ssize_t tag_show(struct device *dev, struct device_attribute *attr,
> +			char *buf)
> +{
> +	struct virtio_device *vdev = container_of(dev, struct virtio_device,
> +						  dev);
> +	struct virtio_fs *fs = vdev->priv;
> +
> +	return sysfs_emit(buf, "%s", fs->tag);

All of the other files in the device directory end with trailing
newlines.  Should this one be an exception?

> +}
> +static DEVICE_ATTR_RO(tag);
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