[PATCHSET RFC v2 00/10] add support for name_to, open_by_handle_at() to io_uring

[Thomas Bertschinger <tahbertschinger@gmail.com>]

This series adds support for name_to_handle_at() and open_by_handle_at()
to io_uring. The idea is for these opcodes to be useful for userspace
NFS servers that want to use io_uring.

The biggest change since [v1] is that this adds support for attempting
non-blocking open_by_handle_at(). When io_uring calls handle_to_path(),
it will set a flag to request that the call return -EAGAIN if it cannot
complete using cached data.

The same holds for the call to do_filp_path_open().

Supporting this for handle_to_path() requires a way to communicate to
the filesystem that it should not block in its fh_to_dentry()
implementation. This is done with a new flag FILEID_CACHED which is set
in the file handle by the VFS. If a filesystem supports this new flag,
it will indicate that with a new flag EXPORT_OP_NONBLOCK so that the VFS
knows not to call into a filesystem with the FILEID_CACHED flag, when
the FS does not know about that flag.

Support for the new FILEID_CACHED flag is added for xfs.

Thanks to Amir Goldstein for the review comments and suggesting this
approach on v1.

Testing
=======

As in v1, a liburing branch with a test is available at
https://github.com/bertschingert/liburing/tree/open_by_handle_at

and is run with

$ ./test/open_by_handle_at.t

v1 -> v2:
- rename some new helper functions:
	- do_name_to_handle_at() -> do_sys_name_to_handle_at()
	- __do_handle_open() -> do_filp_path_open()
- create a union member name_to_handle_flags in struct io_uring_sqe
- add new FILEID_CACHED and EXPORT_OP_NONBLOCK flags
- attempt non-blocking open_by_handle_at() in io_uring first, rather
  than always giving up and running in async context
- add support for FILEID_CACHED in xfs

[v1] https://lore.kernel.org/linux-fsdevel/20250814235431.995876-1-tahbertschinger@gmail.com/

Thomas Bertschinger (10):
  fhandle: create helper for name_to_handle_at(2)
  io_uring: add support for IORING_OP_NAME_TO_HANDLE_AT
  fhandle: helper for allocating, reading struct file_handle
  fhandle: create do_filp_path_open() helper
  fhandle: make do_filp_path_open() take struct open_flags
  exportfs: allow VFS flags in struct file_handle
  exportfs: new FILEID_CACHED flag for non-blocking fh lookup
  io_uring: add __io_open_prep() helper
  io_uring: add support for IORING_OP_OPEN_BY_HANDLE_AT
  xfs: add support for non-blocking fh_to_dentry()

 fs/exportfs/expfs.c           |  15 ++-
 fs/fhandle.c                  | 156 ++++++++++++++++-----------
 fs/internal.h                 |  13 +++
 fs/xfs/xfs_export.c           |  32 ++++--
 fs/xfs/xfs_export.h           |   3 +-
 fs/xfs/xfs_handle.c           |   2 +-
 include/linux/exportfs.h      |  34 +++++-
 include/uapi/linux/io_uring.h |   3 +
 io_uring/opdef.c              |  26 +++++
 io_uring/openclose.c          | 191 ++++++++++++++++++++++++++++++++--
 io_uring/openclose.h          |  13 +++
 11 files changed, 405 insertions(+), 83 deletions(-)


base-commit: 76eeb9b8de9880ca38696b2fb56ac45ac0a25c6c
-- 
2.51.0