[PATCH V8 0/3] fuse: Add support for passthrough read/write

From: Alessio Balsini
Date: Fri Sep 11 2020 - 12:36:13 EST

Add support for file system passthrough read/write of files when enabled in
userspace through the option FUSE_PASSTHROUGH.

There are file systems based on FUSE that are intended to enforce special
policies or trigger complicated decision makings at the file operations
level. Android, for example, uses FUSE to enforce fine-grained access
policies that also depend on the file contents.
Sometimes it happens that at open or create time a file is identified as
not requiring additional checks for consequent reads/writes, thus FUSE
would simply act as a passive bridge between the process accessing the FUSE
file system and the lower file system. Splicing and caching help reduce the
FUSE overhead, but there are still read/write operations forwarded to the
userspace FUSE daemon that could be avoided.

This series has been inspired by the original patches from Nikhilesh Reddy,
the idea and code of which has been elaborated and improved thanks to the
community support.

When the FUSE_PASSTHROUGH capability is enabled, the FUSE daemon may decide
while handling the open/create operations, if the given file can be
accessed in passthrough mode. This means that all the further read and
write operations would be forwarded by the kernel directly to the lower
file system rather than to the FUSE daemon. All requests that are not reads
or writes are still handled by the userspace code.
This allows for improved performance on reads and writes, especially in the
case of reads at random offsets, for which no (readahead) caching mechanism
would help.
Benchmarks show improved performance that is close to native file system
access when doing massive manipulations on a single opened file, especially
in the case of random reads, for which the bandwidth increased by almost 2X
or sequential writes for which the improvement is close to 3X.

The creation of this direct connection (passthrough) between FUSE file
objects and file objects in the lower file system happens in a way that
reminds of passing file descriptors via sockets:
- a process requests the opening of a file handled by FUSE, so the kernel
forwards the request to the FUSE daemon;
- the FUSE daemon opens the target file in the lower file system, getting
its file descriptor;
- the FUSE daemon also decides according to its internal policies if
passthrough can be enabled for that file, and, if so, can perform a
FUSE_DEV_IOC_PASSTHROUGH_OPEN ioctl() on /dev/fuse, passing the file
descriptor obtained at the previous step and the fuse_req unique
- the kernel translates the file descriptor to the file pointer navigating
through the opened files of the "current" process and temporarily stores
it in the associated open/create fuse_req's passthrough_filp;
- when the FUSE daemon has done with the request and it's time for the
kernel to close it, it checks if the passthrough_filp is available and in
case updates the additional field in the fuse_file owned by the process
accessing the FUSE file system.
>From now on, all the read/write operations performed by that process will
be redirected to the file pointer pointing at the lower file system's file.
Asynchronous IO is supported as well, handled by creating separate AIO
requests for the lower file system that will be internally tracked by FUSE,
that intercepts and propagates their completion through an internal
ki_completed callback similar to the current implementation of overlayfs.
Being the userspace FUSE daemon unaware of the read/write operations
happening in passthrough mode, the stats of the FUSE file must be
continuously updated with the lower file system's file to avoid
inconsistencies when stats caching is enabled.
The ioctl() has been designed taking as a reference and trying to converge
to the fuse2 implementation. For example, the fuse_passthrough_out data
structure has extra fields that will allow for further extensions of the


What follows has been performed with this change [V6] rebased on top of
vanilla v5.8 Linux kernel, using a custom passthrough_hp FUSE daemon that
enables pass-through for each file that is opened during both "open" and
"create". Tests were run on an Intel Xeon E5-2678V3, 32GiB of RAM, with an
ext4-formatted SSD as the lower file system, with no special tuning, e.g.,
all the involved processes are SCHED_OTHER, ondemand is the frequency
governor with no frequency restrictions, and turbo-boost, as well as
p-state, are active. This is because I noticed that, for such high-level
benchmarks, results consistency was minimally affected by these features.
The source code of the updated libfuse library and passthrough_hp is shared
at the following repository:


Two different kinds of benchmarks were done for this change, the first set
of tests evaluates the bandwidth improvements when manipulating a huge
single file, the second set of tests verify that no performance regressions
were introduced when handling many small files.

The first benchmarks were done by running FIO (fio-3.21) with:
- bs=4Ki;
- file size: 50Gi;
- ioengine: sync;
- fsync_on_close: true.
The target file has been chosen large enough to avoid it to be entirely
loaded into the page cache.
Results are presented in the following table:

| Bandwidth | FUSE | FUSE | Bind |
| (KiB/s) | | passthrough | mount |
| read | 468897 | 502085 | 516830 |
| randread | 15773 | 26632 | 21386 |
| write | 58185 | 141272 | 141671 |
| randwrite | 59892 | 75236 | 76486 |

As long as this patch has the primary objective of improving bandwidth,
another set of tests has been performed to see how this behaves on a
totally different scenario that involves accessing many small files. For
this purpose, measuring the build time of the Linux kernel has been chosen
as a well-known workload. The kernel has been built with as many processes
as the number of logical CPUs (-j $(nproc)), that besides being a
reasonable number, is also enough to saturate the processor’s utilization
thanks to the additional FUSE daemon’s threads, making it even harder to
get closer to the native file system performance.
The following table shows the total build times in the different

| | AVG duration | Standard |
| | (sec) | deviation |
| FUSE | 144.566 | 0.697 |
| FUSE passthrough | 133.820 | 0.341 |
| Raw | 109.423 | 0.724 |

Similar performance measurements were performed in the current version of
the patch, and results compatible with what is shown above.
Further testing and performance evaluations are welcome.

Description of the series

The first patch introduces the data structures and definitions required
both for the communication with userspace and for the internal kernel use.
It also adds the basic functionalities to establish the bridge between the
FUSE file and the lower file system file through a ioctl().

The second patch enables the synchronous read and write operations for
those FUSE files for which the passthrough functionality is enabled. Those
operations are directly sent to the lower file system.

The third and last patch extends the read and write operations to also
support asynchronous IO.

Changes in v8:
* aio requests now use kmalloc/kfree, instead of kmem_cache
* Switched to call_{read,write}_iter in AIO
* Revisited attributes copy
* Passthrough can only be enabled via ioctl(), fixing the security issue
spotted by Jann
[Proposed by Jann Horn]
* Use an extensible fuse_passthrough_out data structure

Changes in v7:
* Full handling of aio requests as done in overlayfs (update commit
* s/fget_raw/fget.
* Open fails in case of passthrough errors, emitting warning messages.
[Proposed by Jann Horn]
* Create new local kiocb, getting rid of the previously proposed ki_filp
[Proposed by Jann Horn and Jens Axboe]
* Code polishing.

Changes in v6:
* Port to kernel v5.8:
* fuse_file_{read,write}_iter() changed since the v5 of this patch was
* Simplify fuse_simple_request().
* Merge fuse_passthrough.h into fuse_i.h
* Refactor of passthrough.c:
* Remove BUG_ON()s.
* Simplified error checking and request arguments indexing.
* Use call_{read,write}_iter() utility functions.
* Remove get_file() and fputs() during read/write: handle the extra FUSE
references to the lower file object when the fuse_file is
[Proposed by Jann Horn]

Changes in v5:
* Fix the check when setting the passthrough file.
[Found when testing by Mike Shal]

Changes in v3 and v4:
* Use the fs_stack_depth to prevent further stacking and a minor fix.
[Proposed by Jann Horn]

Changes in v2:
* Changed the feature name to passthrough from stacked_io.
[Proposed by Linus Torvalds]

Alessio Balsini (3):
fuse: Definitions and ioctl() for passthrough
fuse: Introduce synchronous read and write for passthrough
fuse: Handle AIO read and write in passthrough

fs/fuse/Makefile | 1 +
fs/fuse/dev.c | 57 ++++++++++-
fs/fuse/dir.c | 2 +
fs/fuse/file.c | 25 +++--
fs/fuse/fuse_i.h | 16 ++++
fs/fuse/inode.c | 9 +-
fs/fuse/passthrough.c | 196 ++++++++++++++++++++++++++++++++++++++
include/uapi/linux/fuse.h | 12 ++-
8 files changed, 305 insertions(+), 13 deletions(-)
create mode 100644 fs/fuse/passthrough.c