Re: [PATCH v14 00/12] FUSE passthrough for file io

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Oct 16, 2023 at 7:09 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> Miklos,
>
> I've shared several POC branches since the posting of v13 back in May
> and played with several API choices. It is time to post v14.
>
> The API we converged to is server managed shared backing files that are
> referenced by backing id plus per-file re-opened backing_file.
>
> This model looks coherent to me. I think that the example server [3]
> demonstrates that this API is simple enough to work with.
>
> There is quite a bit of re-factored code in this version - I've actually
> declared this common code as a new vfs subsystem [stackable filesystems]
> in MAINTAINERS per Christian's request.
>
> The re-factored common code is based on overlayfs-next and Christian's
> vfs.misc branch (for the backing_file changes).
>
> I am not posting performance numbers again. Alessio has already posted
> performance numbers back in v12 and nothing has changed in this regard.
> We are using a variant of v12 patches in production and the performance
> improvement is very noticable.
>
> Bernd and Nikolaus have helped with improving running fstests on fuse
> passthrough examples.
>
> I have ran the -g auto fstests with v14 patches with the example server.
> Compared to the baseline test results with passthrough_hp, the backing
> file passthrough passes several more test, mainly tests related to data
> coherency, such as generic/451.

FWIW, baseline passthrough_hp --nocache does pass generic/451.
Not surprising considering that the test is for
"Test data integrity when mixing buffered reads and asynchronous
 direct writes a file."

>
> The following tests are the only ones that pass on baseline passthtough_hp
> and fail with my backing file passthrough example:
>
>   generic/080 generic/120 generic/193 generic/215 generic/355
>
> Those tests are failing because of missing mtime/atime/ctime updates

Some more detailed analysis:

generic/120 tests -o noatime and fails because atime is
updated (on the backing file).
This is a general FUSE issue and passthrough_hp --nocache fails
the same test (i.e. it only passed because of attribute cache).

generic/080, generic/215 both test for c/mtime updates after mapped writes.
It is not surprising that backing file passthrough fails these tests -
there is no "passthrough getattr" like overlayfs and there is no opportunity
to invalidate the FUSE inode attribute cache.

> in some use cases and failure to strip suid/sgid bits in some cases.

The suid/sgid strip failures (generic/193, generic/355) were just a silly bug.
Forgot to add file_remove_privs() in passthrough write.
I now moved it from overlayfs into the common backing_file helpers.

I don't think that the issue with mapped writes c/mtime update is a
show stopper?

Thanks,
Amir.

>
> Changes from v13 [1]:
> - rebase on 6.6-rc6 (and overlayfs and vfs next branches)
> - server managed shared backing files without auto-close mode
> - open a backing_file per fuse_file with fuse file's path and flags
> - factor out common read/write/splice/mmap helpers from overlayfs
> - factor out ioctl helpers
>
> [1] https://lore.kernel.org/r/20230519125705.598234-1-amir73il@gmail.com/
> [2] https://github.com/amir73il/linux/commits/fuse-backing-fd-v14
> [3] https://github.com/amir73il/libfuse/commits/fuse-backing-fd
>
> Amir Goldstein (12):
>   fs: prepare for stackable filesystems backing file helpers
>   fs: factor out backing_file_{read,write}_iter() helpers
>   fs: factor out backing_file_splice_{read,write}() helpers
>   fs: factor out backing_file_mmap() helper
>   fuse: factor out helper for FUSE_DEV_IOC_CLONE
>   fuse: introduce FUSE_PASSTHROUGH capability
>   fuse: pass optional backing_id in struct fuse_open_out
>   fuse: implement ioctls to manage backing files
>   fuse: implement read/write passthrough
>   fuse: implement splice_{read/write} passthrough
>   fuse: implement passthrough for mmap
>   fuse: implement passthrough for readdir
>
>  MAINTAINERS                  |   9 +
>  fs/Kconfig                   |   4 +
>  fs/Makefile                  |   1 +
>  fs/backing-file.c            | 319 ++++++++++++++++++++++++++++
>  fs/fuse/Kconfig              |  11 +
>  fs/fuse/Makefile             |   1 +
>  fs/fuse/cuse.c               |   3 +-
>  fs/fuse/dev.c                |  98 ++++++---
>  fs/fuse/dir.c                |   2 +-
>  fs/fuse/file.c               |  69 ++++--
>  fs/fuse/fuse_i.h             |  72 ++++++-
>  fs/fuse/inode.c              |  25 +++
>  fs/fuse/ioctl.c              |   3 +-
>  fs/fuse/passthrough.c        | 392 +++++++++++++++++++++++++++++++++++
>  fs/fuse/readdir.c            |  12 +-
>  fs/open.c                    |  38 ----
>  fs/overlayfs/Kconfig         |   1 +
>  fs/overlayfs/file.c          | 246 ++++------------------
>  fs/overlayfs/overlayfs.h     |   8 +-
>  fs/overlayfs/super.c         |  11 +-
>  include/linux/backing-file.h |  42 ++++
>  include/linux/fs.h           |   3 -
>  include/uapi/linux/fuse.h    |  23 +-
>  23 files changed, 1085 insertions(+), 308 deletions(-)
>  create mode 100644 fs/backing-file.c
>  create mode 100644 fs/fuse/passthrough.c
>  create mode 100644 include/linux/backing-file.h
>
> --
> 2.34.1
>