Re: [PATCH stable/6.18-6.17] md: add check_new_feature module parameter

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Dec 17, 2025 at 10:31:30PM +0500, Roman Mamedov wrote:
> On Thu, 18 Dec 2025 01:11:43 +0800
> "Yu Kuai" <yukuai@fnnas.com> wrote:
> 
> > Hi,
> > 
> > 在 2025/12/17 22:04, Greg KH 写道:
> > > On Wed, Dec 17, 2025 at 09:05:13PM +0800, linan666@huaweicloud.com wrote:
> > >> From: Li Nan <linan122@huawei.com>
> > >>
> > >> commit 9c47127a807da3e36ce80f7c83a1134a291fc021 upstream.
> > >>
> > >> 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.
> > > This is a new feature, why does it need to go to stable kernels?
> > >
> > > And a module parameter?  Ugh, this isn't the 1990's anymore, this is not
> > > good and will be a mess over time (think multiple devices...)
> > 
> > Nan didn't mention the background. We won't backport the new feature to stable
> > kernels(Although this fix a data lost problem in the case array is created
> > with disks in different lbs, anyone is interested can do this). However, this
> > backport is just used to provide a possible solution for user to still assemble
> > arrays after switching to old LTS kernels when they are using the default lbs.
> 
> This is still a bad scenario. Original problem:
> 
> - Boot into a new kernel once, reboot into the old one, the existing array no
>   longer works.
> 
> After this patch:
> 
> - Same. Unless you know how, where and which module parameter to add, to
>   be passed to md module on load. Might be not convenient if the root FS
>   didn't assemble and mount and is inaccessible.
> 
> Not ideal whatsoever.
> 
> Wouldn't it be possible to implement minimal *automatic* recognition (and
> ignoring) of those newly utilized bits instead?

Yes, that should be done instead.

And again, a module parameter does not work for multiple devices in a
system, the upstream change should also be reverted.

thanks,

greg k-h