linux-kernel - Re: [regression] Bug 217074 - upgrading to kernel 6.1.12 from 5.15.x can no longer assemble software raid0

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Message-id: <167822843274.8008.1142380266687607670@noble.neil.brown.name>
Date:   Wed, 08 Mar 2023 09:33:52 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Mariusz Tkaczyk" <mariusz.tkaczyk@...ux.intel.com>
Cc:     "Song Liu" <song@...nel.org>,
        "Linux regressions mailing list" <regressions@...ts.linux.dev>,
        Jes.Sorensen@...il.com, "linux-raid" <linux-raid@...r.kernel.org>,
        "LKML" <linux-kernel@...r.kernel.org>,
        "Nikolay Kichukov" <hijacker@...um.net>
Subject: Re: [regression] Bug 217074 - upgrading to kernel 6.1.12 from 5.15.x
 can no longer assemble software raid0

On Tue, 07 Mar 2023, Mariusz Tkaczyk wrote:
> On Mon, 06 Mar 2023 08:21:07 +1100
> "NeilBrown" <neilb@...e.de> wrote:
> 
> > On Sat, 04 Mar 2023, Song Liu wrote:
> > > + Jes.
> > > 
> > > It appeared to me that we can assemble the array if we have any of the
> > > following:
> > > 1. Enable CONFIG_BLOCK_LEGACY_AUTOLOAD;
> > > 2. Have a valid /etc/mdadm.conf;
> > > 3. Update mdadm to handle this case. (I tried some ugly hacks, which worked
> > > but weren't clean).
> > > 
> > > Since we eventually would like to get rid of CONFIG_BLOCK_LEGACY_AUTOLOAD, I
> > > think we need mdadm to handle this properly. But the logistics might
> > > be complicated, as
> > > mdadm are shipped separately.
> > > 
> > > Jes, what do you think about this? AFAICT, we need to update the logic in
> > > mdopen.c:create_mddev().  
> > 
> > mdadm already handles this, but only if 
> >    CREATE names=yes
> > is present in /etc/mdadm.conf
> 
> Hi,
> 
> "CREATE names=yes" enforces creation of /dev/md_name arrays instead of
> /dev/mdXXX. It is a large change for users, too aggressive IMO. It will destroy
> many setups.
> 
> To resolve it, we need is to use create_named_array() but respect old naming
> convention. We already have find_free_devnm(), and we are able to keep
> consistency because we can create 'mdXXX':
> 
> /sys/module/md_mod/parameters # echo md125 > new_array
> 
> /sys/module/md_mod/parameters # ll /sys/block/md125
> lrwxrwxrwx 1 root root 0 Mar  7 10:54 /sys/block/md125 ->
> ../devices/virtual/block/md125
> 
> That will require adjustments in mdadm, but I think that we can keep
> names the same way. I created the test for verification of base creation flows,
> we can use it to avoid regression:
> https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/tree/tests/00createnames
> 
> Thoughts?

Yes, that is probably sensible.

It would be nice to have a way to expose the "names=yes" functionality
more obviously.  I think people would generally prefer it (/proc/mdstat
becomes more meaningful) but I agree that forcing it on people is the
the best approach.

Maybe a super-block flag that enables it for that array, and we start
setting that flag when creating new arrays?

NeilBrown


> 
> BTW. I wanted to get rid of this legacy "create_on_open" from mdadm anyway but
> never had time to. If you agree, I can proceed with fixing it.
> 
> Thanks,
> Mariusz
>