Re: [PATCH 0/2] btrfs: allow reflinks into NODATASUM files

[Neal Gompa <neal@gompa.dev>]

On Sun, Jul 12, 2026 at 10:25 AM Daan De Meyer via B4 Relay
<devnull+daan.amutable.com@kernel.org> wrote:
>
> The primary use case for this series is building a NOCOW VM image from
> individual partition images that are COW and checksummed. Today those
> partitions cannot be cloned into the NODATACOW and NODATASUM destination,
> so tools fall back to a full copy. This makes provisioning slower and
> duplicates all of the image data up front.
>
> Allow cloning and deduplication from a checksummed file into a NODATASUM
> file. The VM image can then share extents with the partition images.
> Existing checksummed extents are COWed once when modified, protecting the
> source checksums, while newly allocated extents use the destination's
> normal NOCOW behavior.
>
> The reverse direction remains rejected because the destination would
> expect checksums that do not exist.
>
> Patch 1 prevents swap activation from bypassing the COW protection when a
> NODATASUM file references checksummed extents. Patch 2 relaxes the reflink
> restriction in the safe direction.
>
> The xfstests branch is available at:
> https://github.com/kdave/xfstests/pull/6
>
> Signed-off-by: Daan De Meyer <daan@amutable.com>
> ---
> Daan De Meyer (2):
>       btrfs: reject swapfile activation if any extent has checksums
>       btrfs: allow reflinking from checksummed files into nodatasum files
>
>  fs/btrfs/inode.c   | 29 +++++++++++++++++++++++++++++
>  fs/btrfs/reflink.c | 18 ++++++++++++++----
>  2 files changed, 43 insertions(+), 4 deletions(-)
> ---
> base-commit: cab9e339cfbc1a4e075e53e281dfb00391e1a6bb
> change-id: 20260712-reflink-into-nodatasum-aff962f3794e
>

The patch series looks good to me.

Reviewed-by: Neal Gompa <neal@gompa.dev>

That said, I have a question: Is there no reason we couldn't reflink
into a checksummed area and generate *new* checksums for that purpose?
That would be useful for staging for regular backup.

I can foresee that being useful for archiving OS trees or VM images as
snapshots by creating checksums for the newly created snapshots...



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