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Fri, 15 May 2026 17:53:12 -0700 (PDT) From: Joanne Koong To: amir73il@gmail.com, miklos@szeredi.hu Cc: fuse-devel@lists.linux.dev, linux-unionfs@vger.kernel.org Subject: [PATCH v2 21/21] docs: fuse: document extended passthrough (FUSE_PASSTHROUGH_INO) Date: Fri, 15 May 2026 17:40:04 -0700 Message-ID: <20260516004004.1455526-22-joannelkoong@gmail.com> X-Mailer: git-send-email 2.52.0 In-Reply-To: <20260516004004.1455526-1-joannelkoong@gmail.com> References: <20260516004004.1455526-1-joannelkoong@gmail.com> Precedence: bulk X-Mailing-List: linux-unionfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Add section about extended passthrough (FUSE_PASSTHROUGH_INO) mode. Signed-off-by: Joanne Koong --- .../filesystems/fuse/fuse-passthrough.rst | 126 ++++++++++++++++++ 1 file changed, 126 insertions(+) diff --git a/Documentation/filesystems/fuse/fuse-passthrough.rst b/Documentation/filesystems/fuse/fuse-passthrough.rst index 2b0e7c2da54a..751d27c6fc5c 100644 --- a/Documentation/filesystems/fuse/fuse-passthrough.rst +++ b/Documentation/filesystems/fuse/fuse-passthrough.rst @@ -25,6 +25,11 @@ operations. Currently, passthrough is supported for operations like ``read(2)``/``write(2)`` (via ``read_iter``/``write_iter``), ``splice(2)``, and ``mmap(2)``. +With the extended ``FUSE_PASSTHROUGH_INO`` mode, passthrough is also supported +for inode operations (getattr, setattr) and directory operations (readdir). +In this mode, a backing file is attached to a fuse inode for its entire +lifetime. + Enabling Passthrough ==================== @@ -46,6 +51,127 @@ To use FUSE passthrough: the ``backing_id`` to release the kernel's reference to the backing file when it's no longer needed for passthrough setups. +Extended Passthrough (FUSE_PASSTHROUGH_INO) +============================================ + +``FUSE_PASSTHROUGH_INO`` is a stricter variant of ``FUSE_PASSTHROUGH`` in +which the backing file inode number must match the fuse inode number, enforcing +a one-to-one mapping. The kernel offers this flag during ``FUSE_INIT`` if +``CONFIG_FUSE_PASSTHROUGH`` is enabled and the architecture has 64-bit +``ino_t``. The daemon accepts by returning it back in the init reply. + +Enabling Extended Passthrough +----------------------------- + +To use extended passthrough: + + 1. Follow steps 1-2 from `Enabling Passthrough`_ above. The daemon must + also negotiate the ``FUSE_PASSTHROUGH_INO`` capability during + ``FUSE_INIT``. + 2. When registering a backing file via ``FUSE_DEV_IOC_BACKING_OPEN``, set + the ``ops_mask`` field in ``struct fuse_backing_map`` to declare which + operations should be passed through. At minimum, + ``FUSE_PASSTHROUGH_OP_GETATTR`` must be set for any inode-level + passthrough. + 3. When handling a ``LOOKUP``, ``CREATE``, ``MKNOD``, ``MKDIR``, + ``SYMLINK``, or ``LINK`` request, the daemon responds with a + ``fuse_entry2_out`` (instead of ``fuse_entry_out``). To enable + passthrough on the inode, set ``backing_id`` to the id returned by + ``FUSE_DEV_IOC_BACKING_OPEN``. Set ``backing_id`` to 0 for inodes + that should not use passthrough. The ``nodeid`` in the response must + be the backing file's inode number (``i_ino``). If they don't match, + the kernel rejects the passthrough setup with ``-EIO``. + 4. When handling an ``OPEN`` request for a FUSE file, the daemon + replies with the ``FOPEN_PASSTHROUGH`` flag set in + ``fuse_open_out::open_flags`` and provides the corresponding ``backing_id`` + in ``fuse_open_out::backing_id`` or leaves ``fuse_open_out::backing_id`` + blank. If the daemon would like to opt out of passthrough when the inode + is already in passthrough mode, it may additionally set + ``FOPEN_DIRECT_IO``, which will forward read/write operations directly to + the daemon. + 5. The FUSE daemon should eventually call ``FUSE_DEV_IOC_BACKING_CLOSE`` with + the ``backing_id`` to release the kernel's reference to the backing file + when it's no longer needed for passthrough setups. + +Passthrough Operations Mask +--------------------------- + +When registering a backing file via ``FUSE_DEV_IOC_BACKING_OPEN``, the daemon +sets ``ops_mask`` in ``struct fuse_backing_map`` to declare which operations +should be passed through:: + + FUSE_PASSTHROUGH_OP_READ + FUSE_PASSTHROUGH_OP_WRITE + FUSE_PASSTHROUGH_OP_READDIR + FUSE_PASSTHROUGH_OP_GETATTR + FUSE_PASSTHROUGH_OP_SETATTR + +Operations fall into two categories, which can be combined: + +**Inode operations** (getattr, setattr): Activated on lookup when the daemon +returns a ``backing_id`` in the ``fuse_entry2_out`` response. The backing +file reference persists for the lifetime of the fuse inode. Getattr is the +minimum required inode operation. + +**File operations** (read, write, readdir): Require the file to be opened with +``FOPEN_PASSTHROUGH`` in the daemon's open response. Read and write can be set +independently for partial passthrough. If only one direction is set, the other +falls back to direct IO and mmap is disabled. + +Extended Entry Reply +-------------------- + +When ``FUSE_PASSTHROUGH_INO`` is negotiated, the kernel uses +``fuse_entry2_out`` instead of ``fuse_entry_out`` for entry responses. +This struct carries a ``backing_id`` and ``fuse_statx`` attributes instead +of ``fuse_attr``. + +When ``backing_id > 0``, the kernel associates the inode with the backing file +for passthrough inode operations. Statx attributes are not cached because +passthrough getattr fetches them directly from the backing inode. + +When ``backing_id == 0`` (no passthrough), the statx attributes from the reply +are cached normally. + +A negative ``backing_id`` is treated as an error. The kernel sends +``FUSE_FORGET`` for the returned nodeid and fails the operation. + +IO Mode State Machine +--------------------- + +The ``iocachectr`` field in ``struct fuse_inode`` prevents conflicting access +modes on the same inode (page-cache I/O and passthrough I/O cannot coexist):: + + iocachectr > 0 Cached mode + iocachectr == 0 Idle. No files open, no passthrough + iocachectr < 0 Uncached/passthrough mode + +Each open file in passthrough mode holds one reference (``iocachectr--``). +The inode-level passthrough setup holds one additional long-lived reference +if the backing has inode ops (getattr/setattr). This long-lived reference is +released on inode eviction. + +Cached mode and passthrough mode are mutually exclusive. Attempting either +while the other is active returns ``-ETXTBSY``. + +For directories, the same mechanism arbitrates between cached readdir +(``FOPEN_CACHE_DIR``) and passthrough readdir. A directory opened without +``FOPEN_CACHE_DIR`` and without ``FOPEN_PASSTHROUGH`` is treated as direct I/O +and does not affect io mode. + +Things to note +-------------- + +- ``FUSE_PASSTHROUGH_INO`` requires 64-bit ``ino_t``. +- Readdirplus does not set up inode passthrough. Inodes created via readdirplus + use normal FUSE operations until a fresh lookup occurs. +- An inode's backing association is set once and cannot be changed. +- Passthrough and cached I/O cannot coexist on the same inode. +- If any inode operations are passed through, this means all opened files need + to set the ``FOPEN_PASSTHROUGH`` flag in the open response, even if reads + and writes are not passed through. If reads and writes are not passed + through, they will go directly to the daemon. + Privilege Requirements ====================== -- 2.52.0