Add FUSE_DEV_IOC_CLONE support for multi-threaded reading

[ejc3]

Summary

Add support for multi-threaded FUSE request processing using FUSE_DEV_IOC_CLONE ioctl.

This enables multiple threads to read FUSE requests in parallel from cloned file descriptors, improving throughput for high-concurrency workloads.

API

// Primary session handles INIT and runs in main thread
let session = Session::new(fs, mountpoint, &Config::default())?;

// Clone fd for additional reader threads
let cloned_fd = session.clone_fd()?;
let reader_session = Session::from_fd_initialized(fs_clone, cloned_fd, SessionACL::All);
std::thread::spawn(move || reader_session.run());

session.run()?;

Changes

• Add Session::clone_fd() - clones the FUSE fd using FUSE_DEV_IOC_CLONE ioctl (Linux only)
• Add Session::from_fd_initialized() - creates a session from a cloned fd that skips INIT handshake
• Make Session::run() public for external multi-reader use
• Add conditional handshake skip when proto_version is pre-set

Tests

Added two integration tests:

• clone_fd_multi_reader - verifies clone_fd() creates a valid fd
• from_fd_initialized_works - tests concurrent readers processing requests

Platform Support

clone_fd() is only available on Linux (#[cfg(target_os = "linux")]).

[cberner]

Please add a test for this functionality. Also, I don't really like the idea of making channel public. I think we should find a way to hide this functionality behind mount() or something like that

[cberner]

@stepancheg I think you're working on a similar feature. Want to take a look and lemme know how they overlap, if at all?

[stepancheg]

I left a comment in the other thread.

[ejc3]

I left a comment in the other thread.

Hi @stepancheg, can you point me there? I might not be on that issue / pull request. Thanks!

[cberner]

It's this one: #572

[ejc3]

It's this one: #572

I rebased onto main, so those changes are incorporated now.

[cberner]

The approach @stepancheg described seems better to me, because it provides a simpler interface to users. Can you take a look at that and let me know if your use case requires additional functionality or lower level access?

[ejc3]

I rebased again. #627 won’t work as is (as mentioned) because it doesn’t support the clone fd ioctl. Otherwise, if that was added, it should work.

[stepancheg]

#627 won’t work as is (as mentioned) because it doesn’t support the clone fd ioctl

multiple event loops can work with and without ioctl. This is how libfuse works — single thread, multiple threads shared fd, or multiple threads clone fd with ioctl.

I started without ioctl, because I presume we should make version without ioctl work first, and then add ioctl on top.

[ejc3]

Yeah, that makes. I was saying that there isn’t a perf improvement if we don’t do the clone though. I will submit pull request on top of 627 that does the clone. Sent from Yahoo Mail for iPhone On Sunday, February 1, 2026, 6:06 PM, Stepan Koltsov ***@***.***> wrote: stepancheg left a comment (cberner/fuser#421) #627 won’t work as is (as mentioned) because it doesn’t support the clone fd ioctl multiple event loops can work with and without ioctl. This is how libfuse works — single thread, multiple threads shared fd, or multiple threads clone fd with ioctl. I started without ioctl, because I presume we should make version without ioctl work first, and then add ioctl on top. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you authored the thread.Message ID: ***@***.***>

[stepancheg]

I was saying that there isn’t a perf improvement if we don’t do the clone though.

It is perf improvement if filesystem implementation does something non-trivial. But not as big as with ioctl.