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

[Qu Wenruo]

在 2026/7/12 23:55, Daan De Meyer via B4 Relay 写道:
> 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.

No, the change will easily cause btrfs check errors, in both lowmem and 
original modes.

And before you do anything weird, I'd prefer you to address the 
regression caused by your new 32bit compatible ioctl first:

https://lore.kernel.org/linux-btrfs/66470c2e-56b9-49a9-9cd2-aa49fac75bbf@xxxxxxxx/
> 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@xxxxxxxxxxxx>
> ---
> 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
>
> Best regards,
> --
> Daan De Meyer <daan@xxxxxxxxxxxx>