[Linux-cachefs] [PATCH] cachefiles: use private mounts in cache->mnt

[David Howells]

Christian Brauner <brauner at kernel.org> wrote:

> Besides that - and probably irrelevant from the perspective of a
> cachefiles developer - it also makes things simpler for a variety of
> other vfs features. One concrete example is fanotify.

What about cachefilesd?  That walks over the tree regularly, stats things and
maybe deletes things.  Should that be in a private mount/namespace too?

> This seems a rather desirable property as the underlying path can't e.g.
> suddenly go from read-write to read-only and in general it means that
> cachefiles is always in full control of the underlying mount after the
> user has allowed it to be used as a cache.

That's not entirely true, but I guess that emergency R/O conversion isn't a
case that's worrisome - and, in any case, only affects the superblock.

>  	ret = -EINVAL;
> -	if (mnt_user_ns(path.mnt) != &init_user_ns) {
> +	if (mnt_user_ns(cache->mnt) != &init_user_ns) {
>  		pr_warn("File cache on idmapped mounts not supported");
>  		goto error_unsupported;
>  	}

Is it worth doing this check before calling clone_private_mount()?

> +	cache_path = path;
> +	cache_path.mnt = cache->mnt;

Seems pointless to copy all of path into cache_path rather than just
path.dentry.

Apart from that, looks okay.

David