About reflink len = 0 behavior

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Hi Darrick, xfs guys and btrfs guys.

Although such question is quite late as reflink generic tests are in 
fstests for a long time, I'm still not sure what's the correct behavior 
for reflink len = 0.

Test case generic/182 is causing different output between btrfs and xfs.

For btrfs, dedupe will just return 0 and check nothing, while for xfs 
len == 0 means to check the whole file length.

Both makes sense for me, for btrfs len = 0 behavior, it just follows 
read/write functions.
And I assume xfs follows reflink behavior, when len is not specified, 
then reflink the length of src file.

But since it's a generic test, we need to unify the behavior.

So, which one is the standard and which document should we follow for 
such behavior definition?

Thanks,
Qu