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

[Florian Weimer]

* Christoph Hellwig:

> On Mon, Nov 10, 2025 at 06:27:41AM +0100, Florian Weimer wrote:
>> Sorry, I made the example confusing.
>> 
>> How would the application deal with failure due to lack of fallocate
>> support?  It would have to do a pwrite, like posix_fallocate does to
>> today, or maybe ftruncate.  This is way I think removing the fallback
>> from posix_fallocate completely is mostly pointless.
>
> In general it would ftruncate.  If it thinks it can't work without
> preallocation at all the application will fail, as again the lack
> of posix_fallocate means that space can't be preallocated.

Hmm.  It's not a 1:1 replacement: someone really needs to understand the
code and see what the appropriate way to deal with the situation is.  Of
course the posix_fallocate fallback path (or an application-level
equivalent) has the potential for data loss, too.  It's just a different
trade-off.

Thanks,
Florian