should copy_file_range(2) have a non-Linux flag?

Hi,

I just proposed a patch for VOP_COPY_FILE_RANGE(9)
that adds a kernel only flag to specify "return after 1second
with a partial copy". I'd like to use it for the NFSv4.2 server,
so that the RPC replies in a reasonable time frame.
https://reviews.freebsd.org/D31829

The question that came up is...
"should this flag be visible to userland?"

The only argument I can think of against doing this is
that it makes the syscall non-Linux compatible.
(Also, the NFS server requirement seems a bit of an oddball
 and I'm not sure an application would want this capability?)

Do you think this flag should be exposed to userland (ie the syscall)?

rick