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

[Matthew Wilcox]

On Thu, Nov 06, 2025 at 02:52:12PM +0100, Christoph Hellwig wrote:
> 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?

I believe Florian is thinking of CoW in the sense of "share while read
only, then you have a mutable block allocation", rather than the
WAFL (or SMR) sense of "we always put writes in a new location".