Re: [PATCH v9 4/5] md: add check_new_feature module parameter

[Xiao Ni]

On Mon, Nov 3, 2025 at 9:06 PM <linan666@xxxxxxxxxxxxxxx> wrote:
>
> From: Li Nan <linan122@xxxxxxxxxx>
>
> Raid checks if pad3 is zero when loading superblock from disk. Arrays
> created with new features may fail to assemble on old kernels as pad3
> is used.
>
> Add module parameter check_new_feature to bypass this check.
>
> Signed-off-by: Li Nan <linan122@xxxxxxxxxx>
> ---
>  drivers/md/md.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index dffc6a482181..5921fb245bfa 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -339,6 +339,7 @@ static int start_readonly;
>   */
>  static bool create_on_open = true;
>  static bool legacy_async_del_gendisk = true;
> +static bool check_new_feature = true;
>
>  /*
>   * We have a system wide 'event count' that is incremented
> @@ -1850,9 +1851,13 @@ static int super_1_load(struct md_rdev *rdev, struct md_rdev *refdev, int minor_
>         }
>         if (sb->pad0 ||
>             sb->pad3[0] ||
> -           memcmp(sb->pad3, sb->pad3+1, sizeof(sb->pad3) - sizeof(sb->pad3[1])))
> -               /* Some padding is non-zero, might be a new feature */
> -               return -EINVAL;
> +           memcmp(sb->pad3, sb->pad3+1, sizeof(sb->pad3) - sizeof(sb->pad3[1]))) {
> +               pr_warn("Some padding is non-zero on %pg, might be a new feature\n",
> +                       rdev->bdev);
> +               if (check_new_feature)
> +                       return -EINVAL;
> +               pr_warn("check_new_feature is disabled, data corruption possible\n");
> +       }
>
>         rdev->preferred_minor = 0xffff;
>         rdev->data_offset = le64_to_cpu(sb->data_offset);
> @@ -10704,6 +10709,7 @@ module_param(start_dirty_degraded, int, S_IRUGO|S_IWUSR);
>  module_param_call(new_array, add_named_array, NULL, NULL, S_IWUSR);
>  module_param(create_on_open, bool, S_IRUSR|S_IWUSR);
>  module_param(legacy_async_del_gendisk, bool, 0600);
> +module_param(check_new_feature, bool, 0600);
>
>  MODULE_LICENSE("GPL");
>  MODULE_DESCRIPTION("MD RAID framework");
> --
> 2.39.2
>

Hi

Thanks for finding this problem in time. The default of this kernel
module is true. I don't think people can check new kernel modules
after updating to a new kernel. They will find the array can't
assemble and report bugs. You already use pad3, is it good to remove
the check about pad3 directly here?

By the way, have you run the regression tests?

Regards
Xiao