From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E28B6406269 for ; Thu, 30 Apr 2026 12:57:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777553824; cv=none; b=VFQBCwMMer8WX670mC2v9PF8Gv/EwW48j+REXqHQPaeoDAPNLBXRoR+xnUmSQLHHaZPDqn5M1aMLO2ind47WQo3qsAxCV97ztQ6AhEe8puT4RT5DNs5wJgvtq48Zi9rQfEDxh03Fu+k5/hyglE8gFOg64WlXIolH+TBKkiI+Feg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777553824; c=relaxed/simple; bh=Cax0EkstDZnIZoxEe/zAVh3M5U+jCPNrC7XUMm+Iszs=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=LamzGAq9GAwmJxBNqGYO+pidx4LiX8CE0NDDnizH2XFDlAvyfM8ZZL5IwzVEbze1o37KGbxiMXSql8q2nSge4dFayAvBXO5Vh+o8nkD7oDma0JdOpHFB0Pko8NfBoczKKdnEN9Riq+Ng+SrPDHubnXJKyyTx48nCzz87KUqjLNA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=T7r3hyU4; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="T7r3hyU4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6C0DCC2BCB9; Thu, 30 Apr 2026 12:57:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777553823; bh=Cax0EkstDZnIZoxEe/zAVh3M5U+jCPNrC7XUMm+Iszs=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=T7r3hyU4hQVtLqZLJrws3ygn0K7OveYbXvk6o3szhNtPhS32eZ0BYO+egR5lxjA1d B4otLlV7Zh+/sgqf3j5rYgDJ4F3IQPhqQzGRASUn0aB86IWRxcqKvQZVV1gL/Mcxtt KhDxh0vegcCUtirv2JnYd5iTqrtir/fqMHmqZClXTCsDqNGJaybpH84ATe2XZm5CF3 y8VGqiWBoosvTn6p2/nKOD/ZdyBnfM3Ev4+kr4kM+ugpo2phoIMhOHTHDR0EXMlAil AODH34r2vlEkxNIYaSprseZzIea6R7H6vcAaMhgeQuO/oL2iEGoOcBnyog/oCZnXLA OsvGwbsUMjH7g== Message-ID: Subject: Re: [PATCH v2 14/14] docs: fuse: add io-uring bufring and zero-copy documentation From: Jeff Layton To: Joanne Koong , miklos@szeredi.hu Cc: bernd@bsbernd.com, axboe@kernel.dk, linux-fsdevel@vger.kernel.org Date: Thu, 30 Apr 2026 13:57:00 +0100 In-Reply-To: <20260402162840.2989717-15-joannelkoong@gmail.com> References: <20260402162840.2989717-1-joannelkoong@gmail.com> <20260402162840.2989717-15-joannelkoong@gmail.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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fuse over io-uring usage of buffer rings and > zero-copy. >=20 > Signed-off-by: Joanne Koong > --- > .../filesystems/fuse/fuse-io-uring.rst | 189 ++++++++++++++++++ > 1 file changed, 189 insertions(+) >=20 > diff --git a/Documentation/filesystems/fuse/fuse-io-uring.rst b/Documenta= tion/filesystems/fuse/fuse-io-uring.rst > index d73dd0dbd238..bc47686c023f 100644 > --- a/Documentation/filesystems/fuse/fuse-io-uring.rst > +++ b/Documentation/filesystems/fuse/fuse-io-uring.rst > @@ -95,5 +95,194 @@ Sending requests with CQEs > | | =20 > +Buffer rings > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > =20 > +Buffer rings have two main advantages: > =20 > +* Reduced memory usage: payload buffers are pooled and selected on deman= d > + rather than dedicated per-entry, allowing fewer buffers than entries. = This > + infrastructure also allows for future optimizations like incremental b= uffer > + consumption where non-overlapping parts of a buffer may be used across > + concurrent requests. > +* Foundation for pinned buffers: contiguous buffer allocations allow the > + kernel to pin and vmap the entire region, avoiding per-request page > + resolution overhead > + > +At a high-level, this is how fuse uses buffer rings: > + > +* The first REGISTER SQE for a queue creates the queue and sets up the > + buffer ring. The server provides two iovecs: one for headers and one f= or > + payload buffers. Each entry gets a fixed ID (sqe->buf_index) that maps > + to a specific header slot. > +* When a client request arrives, the kernel selects a payload buffer fro= m > + the ring (if the request has copyable data), copies headers and payloa= d > + data, and completes the sqe. > +* The buf_id of the selected payload buffer is communicated to the serve= r > + via the fuse_uring_ent_in_out header. The server uses this to locate t= he > + payload data in its buffer. > +* The server processes the request and sends a COMMIT_AND_FETCH SQE with > + the reply. The kernel processes the reply and recycles the buffer. > + > +Visually, this looks like:: > + > + Headers buffer: > + +-----------------------+-----------------------+-----+ > + | fuse_uring_req_header | fuse_uring_req_header | ... | > + | [ent 0] | [ent 1] | | > + +-----------------------+-----------------------+-----+ > + ^ ^ > + | | > + ent 0 header slot ent 1 header slot > + (sqe->buf_index=3D0) (sqe->buf_index=3D1) > + > + Payload buffer pool: > + +-----------+-----------+-----------+-----+ > + | buf 0 | buf 1 | buf 2 | ... | > + | (buf_size)| (buf_size)| (buf_size)| | > + +-----------+-----------+-----------+-----+ > + selected on demand, recycled after each request > + > +Buffer ring request flow > +------------------------:: > + > +| Kernel | FUSE daemon > +| | > +| [client request arrives] | > +| >fuse_uring_send() | > +| [select payload buf from ring] | > +| >fuse_uring_select_buffer() | > +| [copy headers to ent's header slot] | > +| >copy_header_to_ring() | > +| [copy payload to selected buf] | > +| >fuse_uring_copy_to_ring() | > +| [set buf_id in ent_in_out header] | > +| >io_uring_cmd_done() | > +| | [CQE received] > +| | [read headers from header = slot] > +| | [read payload from buf_id] > +| | [process request] > +| | [write reply to header slo= t] > +| | [write reply payload to bu= f] > +| | >io_uring_submit() > +| | COMMIT_AND_FETCH > +| >fuse_uring_commit_fetch() | > +| >fuse_uring_commit() | > +| [copy reply from ring] | > +| >fuse_uring_recycle_buffer() | > +| >fuse_uring_get_next_fuse_req() | > + > +Pinned buffers > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > + > +Servers can optionally pin their header and/or payload buffers by settin= g > +FUSE_URING_PINNED_HEADERS and/or FUSE_URING_PINNED_BUFFERS flags. When > +set, the kernel pins the user pages and vmaps them during queue setup, > +enabling memcpy to/from the kernel virtual address instead of > +copy_to_user/copy_from_user. > + > +This avoids the per-request cost of pinning/unpinning user pages and > +translating virtual addresses. Buffers must be page-aligned. The pinned = pages > +are accounted against RLIMIT_MEMLOCK (bypassable with CAP_IPC_LOCK). > + > +Zero-copy > +=3D=3D=3D=3D=3D=3D=3D=3D=3D > + > +Fuse io-uring zero-copy allows the server to directly read from / write = to > +the client's pages, bypassing any intermediary buffer copies. This requi= res > +the FUSE_URING_ZERO_COPY flag, buffer rings with pinned headers and buff= ers, > +and CAP_SYS_ADMIN. > + > +The kernel registers the client's underlying pages as a sparse buffer at > +the entry's fixed id via io_buffer_register_bvec(). The fuse server can > +then perform io_uring read/write operations directly on these pages. > +Non-page-backed args (eg out headers) go through the payload buffer as > +normal. Pages are unregistered when the request completes. > + > +The request flow for the zero-copy write path (client writes data, serve= r > +reads it) is as follows: > + > +Zero-copy write > +---------------:: > +| Kernel | FUSE server > +| | > +| "write(fd, buf, 1MB)" | > +| | > +| >sys_write() | > +| >fuse_file_write_iter() | > +| >fuse_send_one() | > +| [req->args->in_pages =3D true] | > +| [folios hold client write data] | > +| | > +| >fuse_uring_copy_to_ring() | > +| >copy_header_to_ring(IN_OUT) | > +| [memcpy fuse_in_header to | > +| pinned headers buf via kaddr] | > +| >copy_header_to_ring(OP) | > +| [memcpy write_in header] | > +| | > +| >fuse_uring_args_to_ring() | > +| >setup_fuse_copy_state() | > +| [is_kaddr =3D true] | > +| [skip_folio_copy =3D true] | > +| | > +| >fuse_uring_set_up_zero_copy() | > +| [folio_get for each client folio] | > +| [build bio_vec array from folios] | > +| >io_buffer_register_bvec() | > +| [register pages at ent->id] | > +| [ent->zero_copied =3D true] | > +| | > +| >fuse_copy_args() | > +| [skip_folio_copy =3D> return 0 | > +| for page arg, skip data copy] | > +| | > +| >copy_header_to_ring(RING_ENT) | > +| [memcpy ent_in_out] | > +| >io_uring_cmd_done() | > +| | > +| | [CQE received] > +| | > +| | [issue io_uring READ at > +| | ent->id] > +| | [reads directly from > +| | client's pages (ZERO_COPY)= ] > +| | > +| | [write data to backing > +| | store] > +| | [submit COMMIT AND FETCH] > +| | > +| >fuse_uring_commit_fetch() | > +| >fuse_uring_commit() | > +| >fuse_uring_copy_from_ring() | > +| >fuse_uring_req_end() | > +| >io_buffer_unregister(ent->id) | > +| [unregister sparse buffer] | > +| >fuse_zero_copy_release() | > +| [folio_put for each folio] | > +| [ent->zero_copied =3D false] | > +| >fuse_request_end() | > +| [wake up client] | > + > +The zero-copy read path is analogous. > + > +Some requests may have both page-backed args and non-page-backed args. > +For these requests, the page-backed args are zero-copied while the > +non-page-backed args are copied to the buffer selected from the buffer > +ring: > + zero-copy: pages registered via io_buffer_register_bvec() > + non-page-backed: copied to payload buffer via fuse_copy_args() > + > +For a request whose payload is zero-copied, the registration/unregistrat= ion > +path looks like: > + > +register: fuse_uring_set_up_zero_copy() > + folio_get() for each folio > + io_buffer_register_bvec(ent->id) > + > +[server accesses pages via io_uring fixed buf at ent->id] > + > +unregister: fuse_uring_req_end() > + io_buffer_unregister(ent->id) > + -> fuse_zero_copy_release() callback > + folio_put() for each folio Reviewed-by: Jeff Layton