Message-ID: <aRSKrxfAb_GG_2Mw@kernel.org>
Date: Wed, 12 Nov 2025 15:25:03 +0200
From: Mike Rapoport <rppt@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [PATCH v5 02/22] liveupdate: luo_core: integrate with KHO

Hi Pasha,

On Tue, Nov 11, 2025 at 03:57:39PM -0500, Pasha Tatashin wrote:
> Hi Mike,
> 
> Thank you for review, my comments below:
> 
> > > This is why this call is placed first in reboot(), before any
> > > irreversible reboot notifiers or shutdown callbacks are performed. If
> > > an allocation problem occurs in KHO, the error is simply reported back
> > > to userspace, and the live update update is safely aborted.

The call to liveupdate_reboot() is just before kernel_kexec(). Why we don't
move it there?

And all the liveupdate_reboot() does if kho_finalize() fails it's massaging
the error value before returning it to userspace. Why kernel_kexec() can't
do the same?

> > This is fine. But what I don't like is that we can't use kho without
> > liveupdate. We are making debugfs optional, we have a way to call
> 
> Yes you can: you can disable liveupdate (i.e. not supply liveupdate=1
> via kernel parameter) and use KHO the old way: drive it from the
> userspace. However, if liveupdate is enabled, liveupdate becomes the
> driver of KHO as unfortunately KHO has these weird states at the
> moment.

The "weird state" is the point where KHO builds its FDT. Replacing the
current memory tracker with one that does not require serialization won't
change it. We still need a way to tell KHO that "there won't be new nodes
in FDT, pack it".
 
> > kho_finalize() on the reboot path and it does not seem an issue to do it
> > even without liveupdate. But then we force kho_finalize() into
> > liveupdate_reboot() allowing weird configurations where kho is there but
> > it's unusable.
> 
> What do you mean KHO is there but unusable, we should not have such a state...

If you compile a kernel with KEXEC_HANDOVER=y, KEXEC_HANDOVER_DEBUGFS=n and
LIVEUPDATE=n and boot with kho=1 there is nothing to trigger
kho_finalize().
 
> > What I'd like to see is that we can finalize KHO on kexec reboot path even
> > when liveupdate is not compiled and until then the patch that makes KHO
> > debugfs optional should not go further IMO.
> >
> > Another thing I didn't check in this series yet is how finalization driven
> > from debugfs interacts with liveupdate internal handling?
> 
> I think what we can do is the following:
> - Remove "Kconfig: make debugfs optional" from this series, and
> instead make that change as part of stateless KHO work.
> - This will ensure that when liveupdate=0 always KHO finalize is fully
> support the old way.
> - When liveupdate=1 always disable KHO debugfs "finalize" API, and
> allow liveupdate to drive it automatically. It would add another
> liveupdate_enable() check to KHO, and is going to be removed as part
> of stateless KHO work.

KHO should not call into liveupdate. That's layering violation.
And "stateless KHO" does not really make it stateless, it only removes the
memory serialization from kho_finalize(), but it's still required to pack
the FDT.

I think we should allow kho finalization in some form from kernel_kexec().

When kho=1 and liveupdate=0, it will actually create the FDT if there was
no previous trigger from debugfs or it will continue with FDT created by
explicit request via debugfs.

When liveupdate=1, liveupdate_reboot() may call a function that actually
finalizes the state to allow safe rollback (although in the current patches
it does not seem necessary). And then kho_finalize() called from
kernel_kexec() will just continue with the state created by
liveupdate_reboot().  If we already finalized the kho state via debugfs,
liveupdate_reboot() can either error out or reset that state.

> Pasha
> 

-- 
Sincerely yours,
Mike.

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