Re: [LSF/MM/BPF TOPIC] Where is fuse going? API cleanup, restructuring and more

[Bernd Schubert <bernd@bsbernd.com>]

On 2/4/26 20:06, Darrick J. Wong wrote:
> On Mon, Feb 02, 2026 at 02:51:04PM +0100, Miklos Szeredi wrote:
>> I propose a session where various topics of interest could be
>> discussed including but not limited to the below list
>>
>> New features being proposed at various stages of readiness:
>>
>>  - fuse4fs: exporting the iomap interface to userspace
> 
> FYI, I took a semi-break from fuse-iomap for 7.0 because I was too busy
> working on xfs_healer, but I was planning to repost the patchbomb with
> many many cleanups and reorganizations (thanks Joanne!) as soon as
> possible after Linus tags 7.0-rc1.
> 
> I don't think LSFMM is a good venue for discussing a gigantic pile of
> code, because (IMO) LSF is better spent either (a) retrying in person to
> reach consensus on things that we couldn't do online; or (b) discussing
> roadmaps and/or people problems.  In other words, I'd rather use
> in-person time to go through broader topics that affect multiple people,
> and the mailing lists for detailed examination of a large body of text.
> 
> However -- do you have questions about the design?  That could be a good
> topic for email /and/ for a face to face meeting.  Though I strongly
> suspect that there are so many other sub-topics that fuse-iomap could
> eat up an entire afternoon at LSFMM:
> 
>  0 How do we convince $managers to spend money on porting filesystems
>    to fuse?  Even if they use the regular slow mode?
> 
>  1 What's the process for merging all the code changes into libfuse?
>    The iomap parts are pretty straightforward because libfuse passes
>    the request/reply straight through to fuse server, but...

To be honest, I'm rather lost with your patch bomb - in which order do I
need to review what? And what can be merged without?
Regarding libfuse patches - certainly helpful if you also post them
here, but I don't want to create PRs out of your series, which then
might fail the PR tests and I would have to fix it on my own ;)
So the right order is to create libfuse PRs, let the test run, let
everyone review here or via PR and then it gets merged.

> 
>  2 ...the fuse service container part involves a bunch of architecture
>    shifts to libfuse.  First you need a new mount helper to connect to
>    a unix socket to start the service, pass some resources (fds and
>    mount options) through the unix socket to the service.  Obviously
>    that requires new library code for a fuse server to see the unix
>    socket and request those resources.  After that you also need to
>    define a systemd service file that stands up the appropriate
>    sandboxing.  I've not written examples, but that needs to be in the
>    final product.
> 
>  3 What tooling changes to we need to make to /sbin/mount so that it
>    can discover fuse-service-container support and the caller's
>    preferences in using the f-s-c vs. the kernel and whatnot?  Do we
>    add another weird x-foo-bar "mount option" so that preferences may
>    be specified explicitly?
> 
>  4 For defaults situations, where do we make policy about when to use
>    f-s-c and when do we allow use of the kernel driver?  I would guess
>    that anything in /etc/fstab could use the kernel driver, and
>    everything else should use a fuse container if possible.  For
>    unprivileged non-root-ns mounts I think we'd only allow the
>    container?
> 
> <shrug> If we made progress on merging the kernel code in the next three
> months, does that clear the way for discussions of 2-4 at LSF?
> 
> Also, I hear that FOSSY 2026 will have kernel and KDE tracks, and it's
> in Vancouver BC, which could be a good venu to talk to the DE people.
> 
>>  - famfs: export distributed memory
> 
> This has been, uh, hanging out for an extraordinarily long time.
> 
>>  - zero copy for fuse-io-uring
>>
>>  - large folios
>>
>>  - file handles on the userspace API
> 
> (also all that restart stuff, but I think that was already proposed)
> 
>>  - compound requests
>>
>>  - BPF scripts
> 
> Is this an extension of the fuse-bpf filtering discussion that happened
> in 2023?  (I wondered why you wouldn't just do bpf hooks in the vfs
> itself, but maybe hch already NAKed that?)
> 
> As for fuse-iomap -- this week Joanne and I have been working on making
> it so that fuse servers can upload ->iomap_{begin,end,ioend} functions
> into the kernel as BPF programs to avoid server upcalls.  This might be
> a better way to handle the repeating-pattern-iomapping pattern that
> seems to exist in famfs than hardcoding things in yet another "upload
> iomap mappings" fuse request.
> 
> (Yes I see you FUSE_SETUPMAPPING...)
> 
>> How do these fit into the existing codebase?
>>
>> Cleaner separation of layers:
>>
>>  - transport layer: /dev/fuse, io-uring, viriofs
> 
> I've noticed that each thread in the libfuse uring backend collects a
> pile of CQEs and processes them linearly.  So if it receives 5 CQEs and
> the first request takes 30 seconds, the other four just get stuck in
> line...?

I'm certainly open for suggestions and patches :)
At DDN the queues are polled from reactors (co-routine line), that
additional libfuse API will never go public, but I definitely want to
finish and if possible implement a new API before I leave (less than 2
months left). We had a bit of discussion with Stefan Hajnoczi about that
around last March, but I never came even close that task the whole year.

> 
>>  - filesystem layer: local fs, distributed fs
> 
> <nod>
> 
>> Introduce new version of cleaned up API?
>>
>>  - remove async INIT
>>
>>  - no fixed ROOT_ID
> 
> Can we just merge this?
> https://lore.kernel.org/linux-fsdevel/176169811231.1426070.12996939158894110793.stgit@frogsfrogsfrogs/

Could you create a libfuse PR please?


Thanks,
Bernd