ioctl: add note regarding CLONE_RANGE(len=0) behaviour

A BTRFS_IOC_CLONE_RANGE request with a src_length value of zero has the
effect of cloning all data from src_offset through to end-of-file.

Document this behaviour in the header file for those who (like me)
incorrectly assume that no data is cloned in such a case.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/ioctl.h b/ioctl.h
index 2cf9c73e..7f7e1d67 100644
--- a/ioctl.h
+++ b/ioctl.h
@@ -506,6 +506,7 @@ static inline char *btrfs_err_str(enum btrfs_err_code err_code)
 #define BTRFS_IOC_SCAN_DEV _IOW(BTRFS_IOCTL_MAGIC, 4, \
 				   struct btrfs_ioctl_vol_args)
 
+/* With a @src_length of zero, the range from @src_offset->EOF is cloned! */
 struct btrfs_ioctl_clone_range_args {
 	__s64 src_fd;
 	__u64 src_offset, src_length;