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From:   Daniel Rosenberg <drosen@google.com>
Date:   Tue, 22 Nov 2022 12:56:38 -0800
Message-ID: <CA+PiJmRLTXfjJmgJm9VRBQeLVkWgaqSq0RMrRY1Vj7q6pV+omw@mail.gmail.com>
Subject: Re: [RFC PATCH v2 00/21] FUSE BPF: A Stacked Filesystem Extension for FUSE
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-unionfs@vger.kernel.org,
        bpf@vger.kernel.org, kernel-team@android.com
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I've been running the generic xfstests against it, with some
modifications to do things like mount/unmount the lower and upper fs
at once. Most of the failures I see there are related to missing
opcodes, like FUSE_SETLK, FUSE_GETLK, and FUSE_IOCTL. The main failure
I have been seeing is generic/126, which is happening due to some
additional checks we're doing in fuse_open_backing. I figured at some
point we'd add some tests into libfuse, and that sounds like a good
place to start.

On Tue, Nov 22, 2022 at 3:13 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> 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.