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

[Florian Weimer]

> On Tue, Nov 11, 2025 at 08:31:30AM +0000, Hans Holmberg wrote:
>> In stead of returning success in fallocate(2), could we in stead return
>> an distinct error code that would tell the caller that:
>> 
>> The optimized allocation not supported, AND there is no use trying to
>> preallocate data using writes?
>> 
>> EUSELESS would be nice to have, but that is not available.
>> 
>> Then posix_fallocate could fail with -EINVAL (which looks legit according
>> to the man page "the underlying filesystem does not support the operation")
>> or skip the writes and return success (whatever is preferable)
>
> The problem is that both the existing direct callers of fallocate(2)
> including all currently released glibc versions do not expect that
> return value.

That could be covered by putting a flag into the mode argument of
allocate that triggers the new behavior.

Thanks,
Florian