[PATCHSET RFC v4 5/6] fuse2fs: handle timestamps and ACLs correctly when iomap is enabled

Darrick J. Wong
2025-08-21T00:50:25Z

Hi all,

When iomap is enabled for a fuse file, we try to keep as much of the
file IO path in the kernel as we possibly can. That means no calling
out to the fuse server in the IO path when we can avoid it. However,
the existing FUSE architecture defers all file attributes to the fuse
server -- [cm]time updates, ACL metadata management, set[ug]id removal,
and permissions checking thereof, etc.

We'd really rather do all these attribute updates in the kernel, and
only push them to the fuse server when it's actually necessary (e.g.
fsync). Furthermore, the POSIX ACL code has the weird behavior that if
the access ACL can be represented entirely by i_mode bits, it will
change the mode and delete the ACL, which fuse servers generally don't
seem to implement.

IOWs, we want consistent and correct (as defined by fstests) behavior
of file attributes in iomap mode. Let's make the kernel manage all that
and push the results to userspace as needed. This improves performance
even further, since it's sort of like writeback_cache mode but more
aggressive.

If you're going to start using this code, I strongly recommend pulling
from my git trees, which are linked below.

Comments and questions are, as always, welcome.

e2fsprogs git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/e2fsprogs.git/log/?h=fuse2fs-iomap-attrs
---
Commits in this patchset:
* fuse2fs: skip permission checking on utimens when iomap is enabled
* fuse2fs: let the kernel tell us about acl/mode updates
* fuse2fs: better debugging for file mode updates
* fuse2fs: debug timestamp updates
* fuse2fs: use coarse timestamps for iomap mode
* fuse2fs: add tracing for retrieving timestamps
* fuse2fs: enable syncfs
* fuse2fs: skip the gdt write in op_destroy if syncfs is working
---
misc/fuse2fs.c | 237 ++++++++++++++++++++++++++++++++++++++++----------------
misc/fuse4fs.c | 193 ++++++++++++++++++++++++++++++++--------------
2 files changed, 304 insertions(+), 126 deletions(-)