[PATCH RESEND V11 0/7] fuse: Add support for passthrough read/write

Alessio Balsini
2021-01-18T19:36:07Z

This is the 11th version of the series, rebased on top of v5.11-rc4.
Please find the changelog at the bottom of this cover letter.

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 using the VFS layer rather than to the FUSE daemon.
All the requests other than reads or writes are still handled by the
userspace FUSE daemon.
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, random writes and sequential
writes. Detailed benchmarking results are presented below.

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
identifier;
- 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 corresponding lower file system file by
creating new VFS requests.
Since the read/write operation to the lower file system is executed with
the current process's credentials, it might happen that it does not have
enough privileges to succeed. For this reason, the process temporarily
receives the same credentials as the FUSE daemon, that are reverted as
soon as the read/write operation completes, emulating the behavior of
the request to be performed by the FUSE daemon itself. This solution has
been inspired by the way overlayfs handles read/write operations.
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.
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 feature.

Performance on RAM block device

What follows has been performed using a custom passthrough_hp FUSE
daemon that enables pass-through for each file that is opened during
both "open" and "create". Benchmarks were run on an Intel Xeon W-2135,
64 GiB of RAM workstation, with a RAM block device used as storage
target. More specifically, out of the system's 64 GiB of RAM, 40 GiB
were reserved for /dev/ram0, formatted as ext4. For the FUSE and FUSE
passthrough benchmarks, the FUSE file system was mounted on top of the
mounted /dev/ram0 device.
That file system has been completely filled and then cleaned up before
running the benchmarks: this to ensure that all the /dev/ram0 space was
reserved and not usable as page cache.

The rationale for using a RAM block device is that SSDs may experience
performance fluctuations, especially when dealing with accessing data
random offsets.
Getting rid of the discrete storage device also removes a huge component
of slowness, highlighting the performance difference of the software
parts (and probably the goodness of CPU caching and its coherence
mechanisms).

No special tuning has been performed, 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:

https://github.com/balsini/libfuse/tree/fuse-passthrough-v11-v5.11-rc4

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

All the caches were dropped before running every benchmark with

echo 3 > /proc/sys/vm/drop_caches

All the benchmarks were run 10 times, with 1 minute cool down between
each run.

The first benchmarks were done by running FIO (fio-3.24) with:
- bs=4Ki;
- file size: 35Gi;
- ioengine: sync;
- fsync_on_close=1;
- randseed=0.
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:

+-----------+------------+-------------+-------------+
| MiB/s | fuse | passthrough | native |
+-----------+------------+-------------+-------------+
| read | 471(±1.3%) | 1791(±1.0%) | 1839(±1.8%) |
| write | 95(±.6%) | 1068(±.9%) | 1322(±.8%) |
| randread | 25(±1.7%) | 860(±.8%) | 1135(±.5%) |
| randwrite | 76(±3.0%) | 813(±1.0%) | 1005(±.7%) |
+-----------+------------+-------------+-------------+

This table shows that FUSE, except for the sequential reads, is far
behind FUSE passthrough and native in terms of performance. The
extremely good FUSE performance for sequential reads is the result of a
great read-ahead mechanism. I was able to verify that setting
read_ahead_kb to 0 causes a terrible performance drop.
All the results are stable, as shown by the standard deviations.
Moreover, these numbers show the reasonable gap between passthrough and
native, introduced by the extra traversal through the VFS layer.

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 an appropriate, 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 parallelization value, 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
configurations:

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

Further testing and performance evaluations are welcome.

Description of the series

Patch 1 generalizes the function which converts iocb flags to rw flags
from overlayfs, so that can be used in this patch set.

Patch 2 enables the 32-bit compatibility for the /dev/fuse ioctl.

Patch 3 introduces the data structures, function signatures and ioctl()
required both for the communication with userspace and for the
internal kernel use.

Patch 4 introduces initialization and release functions for FUSE
passthrough.

Patch 5 enables the synchronous read and write operations for those FUSE
files for which the passthrough functionality is enabled.

Patch 6 extends the read and write operations to also support
asynchronous IO.

Patch 7 allows FUSE passthrough to target files for which the requesting
process would not have direct access to, by temporarily performing a
credentials switch to the credentials of the FUSE daemon that issued
the FUSE passthrough ioctl().

Changelog

Changes in v11:
* Fix the FILESYSTEM_MAX_STACK_DEPTH check to allow other file systems
to be stacked
* Moved file system stacking depth check at ioctl() time
* Update cover letter with correct libfuse repository to test the change
[Requested by Peng Tao]
* Fix the file reference counter leak introduced in v10
[Requested by yanwu]

Changes in v10:
* UAPI updated: ioctl() now returns an ID that will be used at
open/create response time to reference the passthrough file
* Synchronous read/write_iter functions does not return silly errors
(fixed in aio patch)
* FUSE daemon credentials updated at ioctl() time instead of mount time
* Updated benchmark results
[Requested by Miklos Szeredi]

Changes in v9:
* Switched to using VFS instead of direct lower FS file ops
[Attempt to address a request from Jens Axboe, Jann Horn, Amir
Goldstein]
* Removal of useless included aio.h header
[Proposed by Jens Axboe]

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
* Use an extensible fuse_passthrough_out data structure [Attempt to
address a request from Nikolaus Rath, Amir Goldstein and Miklos Szeredi]

Changes in v7:
* Full handling of aio requests as done in overlayfs (update commit
message).
* 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
swapping.
[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
proposed.
* 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
created/deleted.
[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 (7):
fs: Generic function to convert iocb to rw flags
fuse: 32-bit user space ioctl compat for fuse device
fuse: Definitions and ioctl for passthrough
fuse: Passthrough initialization and release
fuse: Introduce synchronous read and write for passthrough
fuse: Handle asynchronous read and write in passthrough
fuse: Use daemon creds in passthrough mode

fs/fuse/Makefile | 1 +
fs/fuse/dev.c | 41 +++++--
fs/fuse/dir.c | 2 +
fs/fuse/file.c | 12 +-
fs/fuse/fuse_i.h | 32 +++++
fs/fuse/inode.c | 22 +++-
fs/fuse/passthrough.c | 239 ++++++++++++++++++++++++++++++++++++++
fs/overlayfs/file.c | 23 +---
include/linux/fs.h | 5 +
include/uapi/linux/fuse.h | 14 ++-
10 files changed, 356 insertions(+), 35 deletions(-)
create mode 100644 fs/fuse/passthrough.c