Re: [PATCH 2/3] xfs: disallow broken ioctls without compat-32-bit-time

[Christoph Hellwig <hch@infradead.org>]

On Tue, Jan 07, 2020 at 10:16:14AM -0800, Darrick J. Wong wrote:
> Yeah.  Fixing that (and maybe adding an ioctl to set the FS UUID online)
> were on my list for 5.6 but clearly I have to defer everything until 5.7
> because we've just run out of time.
> 
> Uh... I started looking into unifying the ext4 and xfs defrag ioctl, but
> gagged when I realized that the ext4 ioctl also handles the data copy
> inside the kernel.  I think that's the sort of behavior we should /not/
> allow into the new ioctl, though that also means that the required
> changes for ext4/e4defrag will be non-trivial.

Well, we should eventually end up with a common defrag tool (e.g. in
util-linux).  We might as well start of with the xfs_fsr codebase
for that or whatever suits us best.

> The btrfs defrag ioctl also contains thresholding information and
> optional knobs to enable compression, which makes me wonder if we should
> focus narrowly on swapext being "swap these extents but only if the
> source file hasn't changed" and not necessarily defrag?

That sounds like the most useful common API.

> ...in which case I wonder, can people (ab)use this interface for atomic
> file updates?  Create an O_TMPFILE, reflink the source file into the
> temp file, make your updates to the tempfile, and then swapext the
> donor's file data back into the source file, but only if the source file
> hasn't changed?

Sure.