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

[Xiao Ni]

On Fri, Nov 7, 2025 at 1:06 AM Yu Kuai <yukuai@xxxxxxxxx> wrote:
>
> Hi,
>
> 在 2025/11/6 22:56, Xiao Ni 写道:
> > On Thu, Nov 6, 2025 at 9:31 PM Yu Kuai <yukuai@xxxxxxxxx> wrote:
> >> Hi,
> >>
> >> 在 2025/11/6 21:15, Xiao Ni 写道:
> >>> In patch05, the commit says this:
> >>>
> >>> Future mdadm should support setting LBS via metadata field during RAID
> >>> creation and the new sysfs. Though the kernel allows runtime LBS changes,
> >>> users should avoid modifying it after creating partitions or filesystems
> >>> to prevent compatibility issues.
> >>>
> >>> So it only can specify logical block size when creating an array. In
> >>> the case you mentioned above, in step3, the array will be assembled in
> >>> new kernel and the sb->pad3 will not be set, right?
> >> No, lbs will be set to the value array actually use in metadata, otherwise
> >> data loss problem will not be fixed for the array with different lbs from
> >> underlying disks, this is what we want to fix in the first place.
> > But the case you mentioned is to assemble an existing array in a new
> > kernel. The existing array in the old kernel doesn't set lbs. So the
> > sb->pad3 will be zero when assembling it in the new kernel.
>
> Looks like you misunderstood the patch, lbs in sb->pad3 will be updated to the
> real lbs when array is assembled in the new kernel. Set lbs in metadata is
> necessary to avoid data loss.
>
> And please noted this patch is required to be backported to old kernel to
> make it possible that array with default lbs can be assembled again in old
> kernel.

Thanks for the explanation. The patch looks good to me.
Reviewed-by: Xiao Ni <xni@xxxxxxxxxx>

Regards
Xiao
>
> >
> > And as planned, we will not support --lbs (for example) for the `mdadm
> > --assemble` command.
> >
> > The original problem should be fixed by specifying lbs when creating
> > an array (https://www.spinics.net/lists/raid/msg80870.html). Maybe we
> > should avoid updating lbs when adding a new disk？
>
> I don't understand, lbs modification should be forbidden once array is
> created, it's only allowed to be updated before the array is running the
> first time.
>
> >
> > Regards
> > Xiao
> >> Thanks,
> >> Kuai
> >>
>