linux-kernel - Re: [PATCH v9 1/9] kho: make debugfs interface optional

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Message-ID: <CA+CK2bBEe16x0em1gRxQD3jhfV9t3QA2vx5ifk2pKb_WEoMTeg@mail.gmail.com>
Date: Mon, 10 Nov 2025 10:26:46 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Zhu Yanjun <yanjun.zhu@...ux.dev>, akpm@...ux-foundation.org, brauner@...nel.org, 
	corbet@....net, graf@...zon.com, jgg@...pe.ca, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, masahiroy@...nel.org, 
	ojeda@...nel.org, rdunlap@...radead.org, rppt@...nel.org, tj@...nel.org
Subject: Re: [PATCH v9 1/9] kho: make debugfs interface optional

On Mon, Nov 10, 2025 at 8:16 AM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> On Sun, Nov 09 2025, Zhu Yanjun wrote:
>
> > 在 2025/11/8 10:13, Pasha Tatashin 写道:
> >> On Fri, Nov 7, 2025 at 6:36 PM Yanjun.Zhu <yanjun.zhu@...ux.dev> wrote:
> >>>
> >>> On 11/7/25 4:02 AM, Pasha Tatashin wrote:
> >>>> On Fri, Nov 7, 2025 at 7:00 AM Pasha Tatashin <pasha.tatashin@...een.com> wrote:
> >>>>>> Hi, Pasha
> >>>>>>
> >>>>>> In our previous discussion, we talked about counting the number of times
> >>>>>> the kernel is rebooted via kexec. At that time, you suggested adding a
> >>>>>> variable in debugfs to keep track of this count.
> >>>>>> However, since debugfs is now optional, where would be an appropriate
> >>>>>> place to store this variable?
> >>>>> It is an optional config and can still be enabled if the live update
> >>>>> reboot number value needs to be accessed through debugfs. However,
> >>>>> given that debugfs does not guarantee a stable interface, tooling
> >>>>> should not be built to require these interfaces.
> >>>>>
> >>>>> In the WIP LUO [1] I have, I pr_info() the live update number during
> >>>>> boot and also store it in the incoming LUO FDT tree, which can also be
> >>>>> accessed through this optional debugfs interface.
> >>>>>
> >>>>> The pr_info message appears like this during boot:
> >>>>> [    0.000000] luo: Retrieved live update data, liveupdate number: 17
> >>>>>
> >>>>> Pasha
> >>>> Forgot to add link to WIP LUOv5:
> >>>> [1] https://github.com/soleen/linux/tree/luo/v5rc04
> >>>
> >>> Thanks a lot. I’ve carefully read this commit:
> >>> https://github.com/soleen/linux/commit/60205b9a95c319dc9965f119303a1d83f0ff08fa.
> >>>
> >>> To be honest, I’d like to run some tests with who/luo, including the
> >>> selftest for kho/luo. Could you please share the steps with me?
> >>>
> >>> If the testing steps have already been documented somewhere, could you
> >>> please share the link?
> >> Currently the test performs in-kernel tests for FLB data, it creates a
> >> number of FLB for every registered LUO file-handler, which at the
> >> moment is only memfd.
> >>
> >> It works together with any of the kexec based live update tests. In
> >> v5, I introduce two tests:
> >> luo_kexec_simple
> >> luo_multi_session
> >>
> >> For example, with luo_multi_session:
> >
> > Hi, Pasha
> >
> > I enabled "CONFIG_LIVEUPDATE=y"
> >
> > # ./luo_multi_session
> > 1..0 # SKIP Failed to open /dev/liveupdate. Is the luo module loaded?
> >
> > # ls /dev/liveupdate
> > ls: cannot access '/dev/liveupdate': No such file or directory
> >
> > # grep "LIVEUPDATE" -inrHI /boot/config-`uname -r`
> > /boot/config-next-20251107-luo+:349:CONFIG_LIVEUPDATE=y
> > /boot/config-next-20251107-luo+:11985:CONFIG_LIVEUPDATE_TEST=y
> >
> > I made tests on FC42. But /dev/liveupdate is missing.
>
> You need to add liveupdate=1 to your kernel cmdline to enable LUO and
> get /dev/liveupdate.

+1, kernel parameters require: kho=1 liveupdate=1

>
> Pasha, your LUO series doesn't add the liveupdate parameter to
> kernel-parameters.txt. I think it should be done in the next version to
> this parameter is discoverable.

Yeah, that is missing, I will update that in a standalone patch, or in
a next version.

Thanks,
Pasha