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

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Message-ID: <mafs0qzu69gei.fsf@kernel.org>
Date: Mon, 10 Nov 2025 14:16:37 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Zhu Yanjun <yanjun.zhu@...ux.dev>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>,  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,
  pratyush@...nel.org,  rdunlap@...radead.org,  rppt@...nel.org,
  tj@...nel.org
Subject: Re: [PATCH v9 1/9] kho: make debugfs interface optional

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.

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.

-- 
Regards,
Pratyush Yadav