linux-kernel - Re: [PATCH v6 02/20] liveupdate: luo

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Message-ID: <aRyLbB8yoQwUJ3dh@kernel.org>
Date: Tue, 18 Nov 2025 17:06:20 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>, pratyush@...nel.org,
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Subject: Re: [PATCH v6 02/20] liveupdate: luo_core: integrate with KHO

On Tue, Nov 18, 2025 at 10:03:00AM -0400, Jason Gunthorpe wrote:
> On Tue, Nov 18, 2025 at 01:21:34PM +0200, Mike Rapoport wrote:
> > On Mon, Nov 17, 2025 at 11:22:54PM -0500, Pasha Tatashin wrote:
> > > > You can avoid that complexity if you register the device with a different
> > > > fops, but that's technicality.
> > > >
> > > > Your point about treating the incoming FDT as an underlying resource that
> > > > failed to initialize makes sense, but nevertheless userspace needs a
> > > > reliable way to detect it and parsing dmesg is not something we should rely
> > > > on.
> > > 
> > > I see two solutions:
> > > 
> > > 1. LUO fails to retrieve the preserved data, the user gets informed by
> > > not finding /dev/liveupdate, and studying the dmesg for what has
> > > happened (in reality in fleets version mismatches should not be
> > > happening, those should be detected in quals).
> > > 2. Create a zombie device to return some errno on open, and still
> > > study dmesg to understand what really happened.
> > 
> > User should not study dmesg. We need another solution.
> > What's wrong with e.g. ioctl()?
> 
> It seems very dangerous to even boot at all if the next kernel doesn't
> understand the serialization information..
> 
> IMHO I think we should not even be thinking about this, it is up to
> the predecessor environment to prevent it from happening. The ideas to
> use ELF metadata/etc to allow a pre-flight validation are the right
> solution.
> 
> If we get into the next kernel and it receives information it cannot
> process it should just BUG_ON and die, or some broad equivalent. 
> It is a catastrophic orchestration error, and we don't need some fine
> grain recovery or userspace visibility. Crash dump the system and
> reboot it.

I was under impression Pasha wanted to get up to the userspace no matter
what.

panic() in liveupdate_early_init() makes perfect sense to me. Parsing dmesg
does not.
 
> IOW, I would not invest time in this.
> 
> Jason

-- 
Sincerely yours,
Mike.