Re: [RFC PATCH v2 00/21] FUSE BPF: A Stacked Filesystem Extension for FUSE

Amir Goldstein
2022-11-22T11:15:47Z

On Tue, Nov 22, 2022 at 4:15 AM Daniel Rosenberg <drosen@google.com> wrote:
>
> These patches extend FUSE to be able to act as a stacked filesystem. This
> allows pure passthrough, where the fuse file system simply reflects the lower
> filesystem, and also allows optional pre and post filtering in BPF and/or the
> userspace daemon as needed. This can dramatically reduce or even eliminate
> transitions to and from userspace.
>
> For this patch set, I have removed the code related to the bpf side of things
> since that is undergoing some large reworks to get it in line with the more
> recent BPF developements. This set of patches implements direct passthrough to
> the lower filesystem with no alteration. Looking at the v1 code should give a
> pretty good idea of what the general shape of the bpf calls will look like.
> Without the bpf side, it's like a less efficient bind mount. Not very useful
> on its own, but still useful to get eyes on it since the backing calls will be
> larglely the same when bpf is in the mix.
>
> This changes the format of adding a backing file/bpf slightly from v1. It's now
> a bit more modular. You add a block of data at the end of a lookup response to
> give the bpf fd and backing id, but there is now a type header to both blocks,
> and a reserved value for future additions. In the future, we may allow for
> multiple bpfs or backing files, and this will allow us to extend it without any
> UAPI breaking changes. Multiple BPFs would be useful for combining fuse-bpf
> implementations without needing to manually combine bpf fragments. Multiple
> backing files would allow implementing things like a limited overlayfs.
> In this patch set, this is only a single block, with only backing supported,
> although I've left the definitions reflecting the BPF case as well.
> For bpf, the plan is to have two blocks, with the bpf one coming first.
> Any further extensions are currently just speculative.
>
> You can run this without needing to set up a userspace daemon by adding these
> mount options: root_dir=[fd],no_daemon where fd is an open file descriptor
> pointing to the folder you'd like to use as the root directory. The fd can be
> immediately closed after mounting. This is useful for running various fs tests.
>

Which tests did you run?

My recommendation (if you haven't done that already):
Add a variant to libfuse test_passthrough (test_examples.py):
@pytest.mark.parametrize("name", ('passthrough', 'passthrough_plus',
'passthrough_fh', 'passthrough_ll',
'passthrough_bpf'))

and compose the no_daemon cmdline for the 'passthrough_bpf' mount.

This gives pretty good basic test coverage for FUSE passthrough operations.

I've extended test_passthrough_hp() for my libfuse_passthrough patches [1],
but it's the same principle.

Thanks,
Amir.

[1] https://github.com/amir73il/libfuse/commits/fuse_passthrough
* 'passthrough_module' uses 'libfuse_passthrough' which enables
Allesio's FUSE_DEV_IOC_PASSTHROUGH_OPEN by default.