Re: [Lsf-pc] [LSF/MM/BPF TOPIC] fuse passthrough solutions and status

[Amir Goldstein <amir73il@gmail.com>]

On Sat, Feb 25, 2023 at 2:59 AM Daniel Rosenberg via Lsf-pc
<lsf-pc@lists.linux-foundation.org> wrote:
>
> On Fri, Feb 10, 2023 at 7:52 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > Several fuse based filesystems pass file data from an underlying
> > filesystem without modification.  The added value can come from
> > changed directory structure, changed metadata or the ability to
> > intercept I/O only in special cases.  This pattern is very common, so
> > optimizing it could be very worthwhile.
> >
> > I'd like to discuss proposed solutions to enabling data passthrough.
> > There are several prototypes:
> >
> >  - fuse2[1] (myself, very old)
> >  - fuse-passthrough[2] (Alessio Balsini, more recent)
> >  - fuse-bpf[3] (Daniel Rosenberg, new)
> >
> > The scope of fuse-bpf is much wider, but it does offer conditional
> > passthrough behavior as well.
> >
> > One of the questions is how to reference underlying files.  Passing
> > open file descriptors directly in the fuse messages could be
> > dangerous[4].  Setting up the mapping from an open file descriptor to
> > the kernel using an ioctl() instead should be safe.
> >
> > Other open issues:
> >
> >  - what shall be the lifetime of the mapping?
> >
> >  - does the mapped open file need to be visible to userspace?
> > Remember, this is a kernel module, so there's no process involved
> > where you could look at /proc/PID/fd.  Adding a kernel thread for each
> > fuse instance that installs these mapped fds as actual file descriptor
> > might be the solution.
> >
> > Thanks,
> > Miklos
> >
> >
> > [1] https://lore.kernel.org/all/CAJfpegtjEoE7H8tayLaQHG9fRSBiVuaspnmPr2oQiOZXVB1+7g@mail.gmail.com/
> >
> > [2] https://lore.kernel.org/all/20210125153057.3623715-1-balsini@android.com/
> >
> > [3] https://lore.kernel.org/all/20221122021536.1629178-1-drosen@google.com/
> >
> > [4] https://lore.kernel.org/all/CAG48ez17uXtjCTa7xpa=JWz3iBbNDQTKO2hvn6PAZtfW3kXgcA@mail.gmail.com/
>
> I'd be very interested to discuss fuse-bpf, and how it aligns with the
> other efforts. I recall there being some io uring effort as well,
> though I haven't looked into that too much yet.
>

For the record, I scheduled a (30 min) session for this with you and Miklos
co-leads.

Bernd's session on FUSE uring will follow right after.

Thanks,
Amir.