bug#48164: cp --reflink=never no longer works as expected

[Pádraig Brady]

On 02/05/2021 18:12, Chris Murphy wrote:
> Hi,
>
> Fedora 34 user 'adrum' reports that 'cp --reflink=never' is performing
> a lightweight copy on Btrfs. And filefrag -v shows the extents are
> shared. Strace shows use of copy_file_range even with --reflink=never
>
> The note for copy_file_range() includes:
>
> copy_file_range() gives filesystems an opportunity to implement
>         "copy acceleration" techniques, such as the use of reflinks
>
>
> Downstream bug
> https://bugzilla.redhat.com/show_bug.cgi?id=1956080
>
> The commit that causes the change in behavior:
> https://src.fedoraproject.org/rpms/coreutils/c/1737e5c81fa82451b510f982499bdedb2a1619d1?branch=rawhide
>
> It references five upstream commits.

Thanks for the detailed report. I've fixed this upstream with:
https://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=v8.32-147-gea9af9923
Ideally copy_file_range() would take CAN_REFLINK as a flag,
but for now the patch above avoids the call with --reflink=never.

Note we've not released an official coreutils release with this change yet,
as we've yet to fully test all variations of this new copying logic.
There are subtle interactions like this that need to be considered.
BTW there has been recent discussions on the non generality of
copy_file_range() at https://lwn.net/Articles/846403/

Marking this as done.

cheers,
Pádraig