On Wed, Mar 30, 2016 at 05:32:42PM -0700, Liu Bo wrote:

Well, btrfs fallocate doesn't allocate space if it's a shared one
because it thinks the space is already allocated.  So a later overwrite
over this shared extent may hit enospc errors.

And this makes it an incorrect implementation of posix_fallocate,
which glibcs implements using fallocate if available.