[PATCH 0/2] vfs: report truthful FIDEDUPERANGE progress safely

FIDEDUPERANGE currently reports the requested length even when the
filesystem shortens a destination range and deduplicates fewer bytes. A
previous one-line correction was reverted after generic/517 exposed the old
expectation and reviewers raised the risk that existing consumers could loop
on a successful zero-progress result.

Patch 1 makes a non-zero request shortened to zero fail per destination with
-EINVAL, while preserving explicit zero-length success. Patch 2 then reports
the filesystem's actual positive progress. This ordering keeps every
intermediate kernel safe for callers that advance by bytes_deduped.

The paired fstests update corrects generic/517 and adds raw ioctl coverage for
zero-length and mixed multi-destination results. Both tests pass on Btrfs and
XFS. Installed duperemove exits successfully on the measured corpus. Installed
rmlint does not hang or silently over-report; it exits 1 after the final
unaligned tail receives -EINVAL, which is recorded explicitly for review.

A current-source consumer audit supports that ABI choice: duperemove completes
the request on a non-zero status; rmlint, bees and jdupes surface -EINVAL as
failure without retrying; dduper and xfs_io stop but can still report command
success. None retries, hangs or risks data corruption. Thus -EINVAL is the
only truthful result that also avoids exposing successful zero progress to
deployed duperemove binaries.

Matthias Goergens (2):
  vfs: fail dedupe requests that cannot make progress
  vfs: report the amount of bytes actually deduplicated

 fs/remap_range.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

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2.55.0