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a7040a06e4
[Add several documentation updates I had missed after renaming functions and also fixes 'make htmldocs'.] Signed-off-by: Bernd Schubert <bschubert@ddn.com> Reviewed-by: Luis Henriques <luis@igalia.com> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
100 lines
4.1 KiB
ReStructuredText
100 lines
4.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=======================================
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FUSE-over-io-uring design documentation
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=======================================
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This documentation covers basic details how the fuse
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kernel/userspace communication through io-uring is configured
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and works. For generic details about FUSE see fuse.rst.
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This document also covers the current interface, which is
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still in development and might change.
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Limitations
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===========
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As of now not all requests types are supported through io-uring, userspace
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is required to also handle requests through /dev/fuse after io-uring setup
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is complete. Specifically notifications (initiated from the daemon side)
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and interrupts.
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Fuse io-uring configuration
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===========================
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Fuse kernel requests are queued through the classical /dev/fuse
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read/write interface - until io-uring setup is complete.
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In order to set up fuse-over-io-uring fuse-server (user-space)
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needs to submit SQEs (opcode = IORING_OP_URING_CMD) to the /dev/fuse
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connection file descriptor. Initial submit is with the sub command
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FUSE_URING_REQ_REGISTER, which will just register entries to be
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available in the kernel.
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Once at least one entry per queue is submitted, kernel starts
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to enqueue to ring queues.
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Note, every CPU core has its own fuse-io-uring queue.
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Userspace handles the CQE/fuse-request and submits the result as
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subcommand FUSE_URING_REQ_COMMIT_AND_FETCH - kernel completes
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the requests and also marks the entry available again. If there are
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pending requests waiting the request will be immediately submitted
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to the daemon again.
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Initial SQE
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-----------::
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| | FUSE filesystem daemon
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| | >io_uring_submit()
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| | IORING_OP_URING_CMD /
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| | FUSE_URING_CMD_REGISTER
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| | [wait cqe]
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| | >io_uring_wait_cqe() or
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| | >io_uring_submit_and_wait()
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| >fuse_uring_cmd() |
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| >fuse_uring_register() |
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Sending requests with CQEs
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--------------------------::
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| | FUSE filesystem daemon
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| | [waiting for CQEs]
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| "rm /mnt/fuse/file" |
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| >sys_unlink() |
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| >fuse_unlink() |
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| [allocate request] |
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| >fuse_send_one() |
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| ... |
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| >fuse_uring_queue_fuse_req |
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| [queue request on fg queue] |
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| >fuse_uring_add_req_to_ring_ent() |
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| ... |
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| >fuse_uring_copy_to_ring() |
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| >io_uring_cmd_done() |
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| >request_wait_answer() |
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| [sleep on req->waitq] |
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| | [receives and handles CQE]
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| | [submit result and fetch next]
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| | >io_uring_submit()
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| | IORING_OP_URING_CMD/
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| | FUSE_URING_CMD_COMMIT_AND_FETCH
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| >fuse_uring_cmd() |
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| >fuse_uring_commit_fetch() |
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| >fuse_uring_commit() |
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| >fuse_uring_copy_from_ring() |
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| [ copy the result to the fuse req] |
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| >fuse_uring_req_end() |
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| >fuse_request_end() |
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| [wake up req->waitq] |
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| >fuse_uring_next_fuse_req |
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| [wait or handle next req] |
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| [req->waitq woken up] |
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| <fuse_unlink() |
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| <sys_unlink() |
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