[PATCH v2 0/2] fuse: skip LOOKUP before exclusive create

[Jimmy Zuber <jamz@amazon.com>]

mkdir(), mknod(), symlink(), link() and renameat2() with RENAME_NOREPLACE
look the new name up with FUSE_LOOKUP before sending the request. This
series adds an INIT flag, FUSE_EXCL_CREATE, with which the server fails
these requests with -EEXIST for an existing name, so the kernel can skip
the LOOKUP.

This series is independent of "fuse: atomic open" [1], which covers
open(). Both add a 7.48 entry to the uapi changelog.

Tested with the new selftest under UML and KVM.

Changes in v2 (from the Sashiko review of v1 [2]):
- Use INIT bit 46; bit 45 is FUSE_PASSTHROUGH_V2.
- Document that an existing name can fail with the error of a VFS check
  that runs before the request, such as -EROFS.
- Keep the test server's state mapped at teardown, where the server
  thread may still read it, and mount in a private mount namespace.
  Skip without /dev/fuse or kernel support. Test mknod() and link() of
  an existing name, and the dcache after each request.
- Rebase onto fuse for-next.

v1: https://lore.kernel.org/fuse-devel/20261020153432.3812719-1-jamz@amazon.com/

[1] https://lore.kernel.org/fuse-devel/20261029130758.818617-1-jamz@amazon.com/
[2] https://sashiko.dev/#/patchset/20261020153432.3812719-1-jamz@amazon.com

Jimmy Zuber (2):
  fuse: skip LOOKUP before exclusive create
  selftests/fuse: test FUSE_EXCL_CREATE

 fs/fuse/dir.c                                 |  10 +
 fs/fuse/fuse_i.h                              |   6 +
 fs/fuse/inode.c                               |   4 +-
 include/uapi/linux/fuse.h                     |  12 +-
 .../selftests/filesystems/fuse/.gitignore     |   1 +
 .../selftests/filesystems/fuse/Makefile       |   3 +-
 .../filesystems/fuse/excl_create_test.c       | 386 ++++++++++++++++++
 7 files changed, 419 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/filesystems/fuse/excl_create_test.c


base-commit: 9b1da7b4a4e197072e630c3b85d4100a399752ff
-- 
2.50.1