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Bug#1130775: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL)

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Control: tags -1 + upstream
Control: forwarded -1 https://lore.kernel.org/all/b3e941b0-38d1-4809-a386-34659a20415e@gmail.com/

Hi,

On Sun, Mar 15, 2026 at 11:28:53AM +0800, Steven Shiau wrote:
> Package: src:linux
> Version: 6.19.6+deb14-amd64
> Severity: important
> 
> Description:
> We have identified a forward-compatibility regression in the md subsystem
> introduced between Linux Kernel 6.18 and 6.19. When creating a standard RAID
> 1 array using mdadm, the resulting superblock generated under Kernel 6.19 is
> violently rejected by older enterprise Linux kernels (e.g., older LTS
> kernels with version 6.1) with an -EINVAL argument error during assembly.
> 
> A strict A/B test was performed using Debian Live (based on Sid)
> environments. The exact same version of mdadm (4.5-5) and the exact same
> creation command was used in both environments:
> mdadm --create "/dev/md127" --force --run --level=raid1 --raid-devices=2
> --metadata=1.2 --data-offset=34816 --bitmap=internal --assume-clean
> /dev/nvme0n1p2 /dev/nvme1n1p2
> 
> Test 1 (Success):
> * Environment: Kernel 6.18.15+deb14-amd64
> * Result: Array creates successfully. When the machine reboots into the
> older guest OS, the legacy kernel assembles the array normally.
> 
> Test 2 (Failure/Regression):
> * Environment: Kernel 6.19.6+deb14-amd64
> * Result: Array creates successfully in the live environment. However, when
> the machine reboots into the older guest OS, the legacy kernel rejects the
> component drives with:
>   mdadm: failed to add /dev/nvme1n1p2 to /dev/md/0: Invalid argument.
>   mdadm: failed to add /dev/nvme0n1p2 to /dev/md/0: Invalid argument.
>   mdadm: /dev/md0 assembled from 0 drives
> 
> Conclusion:
> Kernel 6.19 appears to be injecting a new feature flag, modifying the bitmap
> format, or altering the superblock structure during creation in a way that
> older md_mod drivers cannot parse, breaking backward compatibility for
> disaster recovery tools restoring older operating systems.

I believe this is the same as reported upstream in
https://lore.kernel.org/all/b3e941b0-38d1-4809-a386-34659a20415e@gmail.com/

In my understanding it was decided that that new created arrays will
not be possible to assemble with older kernels. Future stable series
kernels might get an additional patch to make it possible that array
with default lbs can be still assembled again in old kernels.

Regards,
Salvatore

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Bug#1130775: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL) Steven Shiau <steven@spider.nchc.org.tw> - 2026-03-15 05:00 +0100
  Bug#1130775: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL) Salvatore Bonaccorso <carnil@debian.org> - 2026-03-15 09:00 +0100
  Processed: Re: Bug#1130775: Regression: MDRAID 1.2 superblock  created in Kernel 6.19 is rejected by older kernels (-EINVAL) "Debian Bug Tracking System" <owner@bugs.debian.org> - 2026-03-15 09:00 +0100
  Bug#1130775: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL) Steven Shiau <steven.shiau@niar.org.tw> - 2026-03-19 14:20 +0100
  Bug#1130775: Regression: MDRAID 1.2 superblock created in Kernel 6.19 is rejected by older kernels (-EINVAL) Steven Shiau <steven.shiau@niar.org.tw> - 2026-03-19 14:20 +0100

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