RFC Btrfs: make a source length of 0 imply EOF for dedupe

From: Omar Sandoval <[email protected]>

This is the follow-up to the discussions here [1] and here [2] that makes Btrfs treat a src_length of 0 to dedupe as "until EOF". The implementation is straightforward, but there are a few catches that convinced me to post this as an RFC:

1. We still don't know for sure whether userspace cares about the original Btrfs behavior. Darrick and I both seem to think not, but hopefully someone will speak up if it matters to them.
2. When doing the implicit EOF, XFS returns 0 for bytes_deduped. I copied this in my implementation, but I'm guessing that's a bug rather than a feature.
3. Both XFS and Btrfs cap each dedupe operation to 16MB, but the implicit EOF gets around this in the existing XFS implementation. I copied this for the Btrfs implementation.

So now we have 3 options:

a) Apply these patches as-is.
b) Fix XFS to both return the actual bytes_deduped and cap the length for the EOF case. Do the same for Btrfs.
c) Make XFS consistent with the existing Btrfs behavior.

I'm starting to lean towards option c after writing this cover letter, but I might as well send these out and get a second opinion.

Thanks!

1: http://marc.info/?l=linux-btrfs&m=146828374631829&w=2
2: http://marc.info/?l=linux-btrfs&m=147694962912532&w=2

Omar Sandoval (2):
Btrfs: refactor btrfs_extent_same() slightly
Btrfs: make a source length of 0 imply EOF for dedupe

 fs/btrfs/ioctl.c | 45 ++++++++++++++++++++-------------------------
 1 file changed, 20 insertions(+), 25 deletions(-)

2.10.2