Signed-off-by: Bernd Schubert <bschubert@xxxxxxx> --- Documentation/filesystems/fuse-io-uring.rst | 101 ++++++++++++++++++++++++++++ 1 file changed, 101 insertions(+) diff --git a/Documentation/filesystems/fuse-io-uring.rst b/Documentation/filesystems/fuse-io-uring.rst new file mode 100644 index 0000000000000000000000000000000000000000..6299b65072a8468f08cc4f6978c386546bb9559a --- /dev/null +++ b/Documentation/filesystems/fuse-io-uring.rst @@ -0,0 +1,101 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================================= +FUSE-over-io-uring design documentation +======================================= + +This documentation covers basic details how the fuse +kernel/userspace communication through io-uring is configured +and works. For generic details about FUSE see fuse.rst. + +This document also covers the current interface, which is +still in development and might change. + +Limitations +=========== +As of now not all requests types are supported through io-uring, userspace +is required to also handle requests through /dev/fuse after io-uring setup +is complete. Specifically notifications (initiated from the daemon side) + and interrupts. + +Fuse io-uring configuration +=========================== + +Fuse kernel requests are queued through the classical /dev/fuse +read/write interface - until io-uring setup is complete. + +In order to set up fuse-over-io-uring fuse-server (user-space) +needs to submit SQEs (opcode = IORING_OP_URING_CMD) to the /dev/fuse +connection file descriptor. Initial submit is with the sub command +FUSE_URING_REQ_REGISTER, which will just register entries to be +available in the kernel. + +Once at least one entry per queue is submitted, kernel starts +to enqueue to ring queues. +Note, every CPU core has its own fuse-io-uring queue. +Userspace handles the CQE/fuse-request and submits the result as +subcommand FUSE_URING_REQ_COMMIT_AND_FETCH - kernel completes +the requests and also marks the entry available again. If there are +pending requests waiting the request will be immediately submitted +to the daemon again. + +Initial SQE +----------- + + | | FUSE filesystem daemon + | | + | | >io_uring_submit() + | | IORING_OP_URING_CMD / + | | FUSE_URING_REQ_FETCH + | | [wait cqe] + | | >io_uring_wait_cqe() or + | | >io_uring_submit_and_wait() + | | + | >fuse_uring_cmd() | + | >fuse_uring_fetch() | + | >fuse_uring_ent_release() | + + +Sending requests with CQEs +-------------------------- + + | | FUSE filesystem daemon + | | [waiting for CQEs] + | "rm /mnt/fuse/file" | + | | + | >sys_unlink() | + | >fuse_unlink() | + | [allocate request] | + | >__fuse_request_send() | + | ... | + | >fuse_uring_queue_fuse_req | + | [queue request on fg or | + | bg queue] | + | >fuse_uring_assign_ring_entry() | + | >fuse_uring_send_to_ring() | + | >fuse_uring_copy_to_ring() | + | >io_uring_cmd_done() | + | >request_wait_answer() | + | [sleep on req->waitq] | + | | [receives and handles CQE] + | | [submit result and fetch next] + | | >io_uring_submit() + | | IORING_OP_URING_CMD/ + | | FUSE_URING_REQ_COMMIT_AND_FETCH + | >fuse_uring_cmd() | + | >fuse_uring_commit_and_release() | + | >fuse_uring_copy_from_ring() | + | [ copy the result to the fuse req] | + | >fuse_uring_req_end_and_get_next() | + | >fuse_request_end() | + | [wake up req->waitq] | + | >fuse_uring_ent_release_and_fetch()| + | [wait or handle next req] | + | | + | | + | [req->waitq woken up] | + | <fuse_unlink() | + | <sys_unlink() | + + + -- 2.43.0
