Re: [PATCH v2 3/3] virtiofs: emit uevents on filesystem events

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Feb 08, 2024 at 02:32:11PM -0500, Stefan Hajnoczi wrote:
> Alyssa Ross <hi@alyssa.is> requested that virtiofs notifies userspace
> when filesytems become available. This can be used to detect when a
> filesystem with a given tag is hotplugged, for example. uevents allow
> userspace to detect changes without resorting to polling.
> 
> The tag is included as a uevent property so it's easy for userspace to
> identify the filesystem in question even when the sysfs directory goes
> away during removal.
> 
> Here are example uevents:
> 
>   # udevadm monitor -k -p
> 
>   KERNEL[111.113221] add      /fs/virtiofs/2 (virtiofs)
>   ACTION=add
>   DEVPATH=/fs/virtiofs/2
>   SUBSYSTEM=virtiofs
>   TAG=test
> 
>   KERNEL[165.527167] remove   /fs/virtiofs/2 (virtiofs)
>   ACTION=remove
>   DEVPATH=/fs/virtiofs/2
>   SUBSYSTEM=virtiofs
>   TAG=test
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  fs/fuse/virtio_fs.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 28e96b7cde00..18a8f531e5d4 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -270,6 +270,17 @@ static void virtio_fs_start_all_queues(struct virtio_fs *fs)
>  	}
>  }
>  
> +static void virtio_fs_uevent(struct virtio_fs *fs, enum kobject_action action)
> +{
> +	char tag_str[sizeof("TAG=") +
> +		     sizeof_field(struct virtio_fs_config, tag) + 1];
> +	char *envp[] = {tag_str, NULL};
> +
> +	snprintf(tag_str, sizeof(tag_str), "TAG=%s", fs->tag);
> +
> +	kobject_uevent_env(&fs->kobj, action, envp);
> +}
> +
>  /* Add a new instance to the list or return -EEXIST if tag name exists*/
>  static int virtio_fs_add_instance(struct virtio_device *vdev,
>  				  struct virtio_fs *fs)
> @@ -309,6 +320,8 @@ static int virtio_fs_add_instance(struct virtio_device *vdev,
>  
>  	mutex_unlock(&virtio_fs_mutex);
>  
> +	virtio_fs_uevent(fs, KOBJ_ADD);

Why do you have to explicitly ask for the event?  Doesn't sysfs trigger
this for you automatically?  Set the kset uevent callback for this,
right?

thanks,

greg k-h