virtiofs: Fail dax mount if device does not support it

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@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 8868ac31a3c0a1c111ece76fc92958f1d4631a6b..4ee6f734ba8380f05ad8ee6411c3f828a68d2b72 100644 (file)
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -1324,8 +1324,15 @@ static int virtio_fs_fill_super(struct super_block *sb, struct fs_context *fsc)
 
        /* 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;