Re: [PATCH 02/14] xfs: introduce new file range exchange ioctls

[Christoph Hellwig <hch@infradead.org>]

On Mon, Feb 26, 2024 at 06:21:23PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Introduce a pair of new ioctls to handle exchanging ranges of bytes
> between files.  The goal here is to perform the exchange atomically with
> respect to applications -- either they see the file contents before the
> exchange or they see that A-B is now B-A, even if the kernel crashes.
> 
> The simpler of the two ioctls is XFS_IOC_EXCHANGE_RANGE, which performs
> the exchange unconditionally.  XFS_IOC_COMMIT_RANGE, on the other hand,
> requires the caller to sample the file attributes of one of the files
> participating in the exchange, and aborts the exchange if that file has
> changed in the meantime (presumably by another thread).

So per all the discussions, wouldn't it make sense to separate out
XFS_IOC_COMMIT_RANGE (plus the new start commit one later), and if
discussions are still going on just get XFS_IOC_EXCHANGE_RANGE
done ASAP to go on with online repair, and give XFS_IOC_COMMIT_RANGE
enough time to discuss the finer details?

> +struct xfs_exch_range {
> +	__s64		file1_fd;

I should have noticed this last time, by why are we passing a fd
as a 64-bit value when it actually just is a 32-bit value in syscalls?
(same for commit).

> +	if (((fxr->file1->f_flags | fxr->file2->f_flags) & (__O_SYNC | O_DSYNC)) ||

Nit: overly long line here.

> +	if (fxr->flags & ~(XFS_EXCHRANGE_ALL_FLAGS | __XFS_EXCHRANGE_CHECK_FRESH2))

.. and here

> +	/*
> +	 * The ioctl enforces that src and dest files are on the same mount.
> +	 * However, they only need to be on the same file system.
> +	 */
> +	if (inode1->i_sb != inode2->i_sb)
> +		return -EXDEV;

How about only doing this checks once further up?  As the same sb also
applies the same mount.