NACK/Cmt: [J/L][PATCH 0/2] loop: fix regression from max_loop default value change

[Roxana Nicolescu]

On 25/07/2023 20:27, Mauricio Faria de Oliveira wrote:
> BugLink: https://bugs.launchpad.net/bugs/2015400
>
> [ Impact ]
>
>   * Regression in the loop block driver in Jammy kernel
>     between 5.15.0-67 to 5.15.0-68 (in v5.15.86 stable),
>     due to a change in the default behavior (value) of
>     kernel parameter `max_loop` (from 0 to 8) in commit:
>     `loop: Fix the max_loop commandline argument treatment
>     when it is set to 0` (comment #6).
>
>   * Users of loop devices (major 7) with minor >= 8 now
>     fail to `open()` a loop device created with `mknod()`.
>
>   * This is a corner case, as most people use `losetup`
>     with usual /dev/loopNUMBER (or `--find`) which are
>     not affected as it uses a different code path.
>     (`losetup` for `/dev/loopNOT-A-NUMBER` is affected.)
>
>   * Workaround: kernel parameter `max_loop=0`.
>
> [ Test Steps ]
>
>   * Run the test cases (losetup and test-loop.c in comment #6):
>     - max_loop not set (default)
>     - max_loop=0
>     - max_loop=8
>
>   * Verify the default behavior (max_loop not set) is restored.
>
>   * Verify the modified behavior (max_loop is set) is unchanged.
>
>   * Patches verified with jammy/lunar-proposed.
>   
> [ Regression Potential ]
>
>   * Regressions would be limited to the loop block driver,
>     more specifically its default behavior (but it's that now)
>     or specific usage of max_loop parameter (tested; looks OK).
>
> [ Other Info ]
>
>   * The fix on Jammy is just a Revert, since it had not been
>     released with the offending patch.
>
>   * The fix on Lunar is the recently accepted 2 patches [1, 2]
>     as it was released with the offending patch, so let's keep
>     that patch's improvement/behavior for the max_loop=0 case,
>     and "fix"/restore the historical no-limit default behavior.
>
>   * The fix on Mantic is the recently accepted 2 patches too,
>     now in v6.5-rc3, which should be automatically incorporated
>     as Mantic apparently will release with the 6.5 kernel [3]
>
>   * Patch 1 [1] is not quite a fix, but adds CONFIG guards that
>     Patch 2 [2] depends on. (Alternatively, a Patch 2 backport
>     with that could be done, but Patch 1 seems trivial enough.)
>
>     [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=23881aec85f3219e8462e87c708815ee2cd82358
>     [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bb5faa99f0ce40756ab7bbbce4f16c01ca5ebd5a
>     [3] https://discourse.ubuntu.com/t/introducing-kernel-6-5-for-the-23-10-mantic-minotaur-release
>
> Mauricio Faria de Oliveira (1):
>    UBUNTU: SAUCE: Revert "loop: Fix the max_loop commandline argument
>      treatment when it is set to 0"
>
>   drivers/block/loop.c | 28 ++++++++++++++++------------
>   1 file changed, 16 insertions(+), 12 deletions(-)
>
>
I agree with Stefan here. I think jammy should include the proper fix 
too (commit bb5faa99f0ce40756ab7bbbce4f16c01ca5ebd5a) even though you 
will also need fbdee71bb5d8d054e1bdb5af4c540f2cb86fe296: "block: 
deprecate autoloading based on dev_t".

Roxana