[Virtio-fs] [PATCH] virtiofs: Fail dax mount if device does not support it

[Stefan Hajnoczi]

On Tue, Feb 09, 2021 at 05:47:54PM -0500, Vivek Goyal wrote:
> Right now "mount -t virtiofs -o dax myfs /mnt/virtiofs" succeeds even
> if filesystem deivce does not have a cache window and hence DAX can't
> be supported.
> 
> This gives a false sense to user that they are using DAX with virtiofs
> but fact of the matter is that they are not.
> 
> Fix this by returning error if dax can't be supported and user has asked
> for it.
> 
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> ---
>  fs/fuse/virtio_fs.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> Index: redhat-linux/fs/fuse/virtio_fs.c
> ===================================================================
> --- redhat-linux.orig/fs/fuse/virtio_fs.c	2021-02-04 10:40:21.704370721 -0500
> +++ redhat-linux/fs/fuse/virtio_fs.c	2021-02-09 15:56:45.693653979 -0500
> @@ -1324,8 +1324,15 @@ static int virtio_fs_fill_super(struct s
>  
>  	/* virtiofs allocates and installs its own fuse devices */
>  	ctx->fudptr = NULL;
> -	if (ctx->dax)
> +	if (ctx->dax) {
> +		if (!fs->dax_dev) {
> +			err = -EINVAL;
> +			pr_err("virtio-fs: dax can't be enabled as filesystem"
> +			       " device does not support it.\n");
> +			goto err_free_fuse_devs;
> +		}
>  		ctx->dax_dev = fs->dax_dev;
> +	}
>  	err = fuse_fill_super_common(sb, ctx);
>  	if (err < 0)
>  		goto err_free_fuse_devs;

Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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