RFC fs: add AT_REPLACE flag for linkat()

From: Omar Sandoval <[email protected]>

The goal of this series is to allow for updating a file atomically in-place with an O_TMPFILE like so:

- open temporary file with O_TMPFILE
- write temporary file contents
- fsync temporary file
- atomically replace permanent location with the temporary file

This series implements atomic replace step with a new linkat(..., AT_REPLACE).

Al, I took a look at implementing this usecase with renameat2() after we talked. Like you said, on the dcache side, "replace with tmpfile" looks a lot like rename. However, on the filesystem side, trying to use rename for this is much uglier than using link. We can't give i_op->rename() a meaningful old_dir, and dealing with that new special case gets messy fast.

So, here is the approach of having AT_REPLACE just unhash the replaced dentry.

This is a proof-of-concept patch series implementing an AT_REPLACE flag for linkat(2) which allows us to replace the target. This is a nice primitive on its own, but it's most interesting when combined with O_TMPFILE, as it allows you to do an atomic update of a file with an O_TMPFILE.

Patch 1 implements the VFS support for this flag. The implementation resembles sys_renameat2(), and I took care to preserve all of the original error cases and make the new error cases consistent with rename.

Patch 2 adds support for AT_REPLACE to Btrfs. That's the codebase I'm most familiar with so that's where I started, but it should be straightforward to implement for other filesystems.

Omar Sandoval (2):
fs: add AT_REPLACE flag for linkat() which replaces the target
Btrfs: add support for linkat() AT_REPLACE

 fs/btrfs/inode.c           |  65 +++++++++++++++-
 fs/ecryptfs/inode.c        |   2 +-
 fs/namei.c                 | 181 ++++++++++++++++++++++++++++++++++++---------
 fs/nfsd/vfs.c              |   2 +-
 fs/overlayfs/overlayfs.h   |   2 +-
 include/linux/fs.h         |   3 +-
 include/uapi/linux/fcntl.h |   1 +
 7 files changed, 211 insertions(+), 45 deletions(-)

2.12.0