Creating detached dm-integrity hashes for existing disk

Hello all!  (Please keep me in CC, as I'm not subscribed to the mailing list.)


I already have a large disk containing important data (backup data)
and I would like to use `dm-integrity` (with meta-data on a separate
device) to make sure that when reading from that disk I actually get
the original, non-corrupted, data.  (It seems that just the SATA
provided CRC isn't enough sometime with a problematic USB-to-SATA
adapter, and I get successful reads with corrupted data.)

Thus, I've tried using `integritysetup format` with `--no-wipe`,
`--data-device` and `--integrity-bitmap-mode`, and then
`integritysetup open` with `--integrity-recalculate`.  However upon
subsequent `integritysetup open` without the recalculate flag, the
`dump` command still shows the recalculating flag.  Searching on the
internet about this I've found only a few issues about this, but no
actual solution or more insight.

As such my question:  is there a way to actually force `dm-integrity`
to compute the hashes without wiping my original data?




Here are the snippets I've tried (on blank files via loop devices):


# create empty files
truncate --size 4GiB -- /tmp/dm-integrity-tests-data
truncate --size 512MiB -- /tmp/dm-integrity-tests-meta

# fill the data with something to make sure dm-integrity doesn't touch
the original data
dd if=/dev/urandom of=/tmp/dm-integrity-tests-data bs=1M count=128
iflag=fullblock status=progress

# compute a hash of the original data
md5sum -b -- /tmp/dm-integrity-tests-data /tmp/dm-integrity-tests-meta


# the data is loop0, and the meta-data is loop1
losetup --show --sector-size 4096 --find /tmp/dm-integrity-tests-data
losetup --show --sector-size 4096 --find /tmp/dm-integrity-tests-meta


# format, but DO NOT TOUCH the original data
# both the bitmap and hash should use 4096 bytes
integritysetup format \
        --batch-mode \
        --verbose \
        --no-wipe \
        --sector-size 4096 \
        --integrity sha256 \
        --integrity-bitmap-mode \
        --bitmap-sectors-per-bit 8 \
        --data-device /dev/loop0 \
        -- \
        /dev/loop1 \
#


# open, requesting a recalculation of the hashes
integritysetup open \
        --verbose \
        --integrity sha256 \
        --data-device /dev/loop0 \
        --integrity-recalculate \
        -- \
        /dev/loop1 \
        dm-integrity-tests \
#

# read the target, checking if we get our original data
dd if=/dev/mapper/dm-integrity-tests bs=1M iflag=fullblock status=progress \
| md5sum -b

integritysetup close dm-integrity-tests


# open, without the recalculation flag
integritysetup open \
        --verbose \
        --integrity sha256 \
        --data-device /dev/loop0 \
        -- \
        /dev/loop1 \
        dm-integrity-tests \
#

integritysetup close dm-integrity-tests


# recalculation still shows up
integritysetup dump -- /tmp/dm-integrity-tests-meta


Am I missing something?  Shouldn't the recalculation flag disappear?

Thanks,
Ciprian.