[RFC] xfs: fake fallocate success for always CoW inodes

[Christoph Hellwig]

On Thu, Nov 06, 2025 at 02:48:12PM +0100, Florian Weimer wrote:
> * Hans Holmberg:
> 
> > We don't support preallocations for CoW inodes and we currently fail
> > with -EOPNOTSUPP, but this causes an issue for users of glibc's
> > posix_fallocate[1]. If fallocate fails, posix_fallocate falls back on
> > writing actual data into the range to try to allocate blocks that way.
> > That does not actually gurantee anything for CoW inodes however as we
> > write out of place.
> 
> Why doesn't fallocate trigger the copy instead?  Isn't this what the
> user is requesting?

What copy?