linux-kernel - Re: [PATCH v6 06/20] liveupdate: luo_file: implement file systems callbacks

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Message-ID: <mafs01plv433t.fsf@kernel.org>
Date: Tue, 18 Nov 2025 19:17:42 +0100
From: Pratyush Yadav <pratyush@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Pratyush Yadav <pratyush@...nel.org>,  David Matlack
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Subject: Re: [PATCH v6 06/20] liveupdate: luo_file: implement file systems
 callbacks

On Tue, Nov 18 2025, Pasha Tatashin wrote:

> On Tue, Nov 18, 2025 at 12:43 PM Pratyush Yadav <pratyush@...nel.org> wrote:
>>
>> On Tue, Nov 18 2025, David Matlack wrote:
>>
>> > On 2025-11-15 06:33 PM, Pasha Tatashin wrote:
>> >> This patch implements the core mechanism for managing preserved
>> >> files throughout the live update lifecycle. It provides the logic to
>> >> invoke the file handler callbacks (preserve, unpreserve, freeze,
>> >> unfreeze, retrieve, and finish) at the appropriate stages.
>> >>
>> >> During the reboot phase, luo_file_freeze() serializes the final
>> >> metadata for each file (handler compatible string, token, and data
>> >> handle) into a memory region preserved by KHO. In the new kernel,
>> >> luo_file_deserialize() reconstructs the in-memory file list from this
>> >> data, preparing the session for retrieval.
>> >>
>> >> Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
>> >
>> >> +int liveupdate_register_file_handler(struct liveupdate_file_handler *h);
>> >
>> > Should there be a way to unregister a file handler?
>> >
>> > If VFIO is built as module then I think it  would need to be able to
>> > unregister its file handler when the module is unloaded to avoid leaking
>> > pointers to its text in LUO.
>
> I actually had full unregister functionality in v4 and earlier, but I
> dropped it from this series to minimize the footprint and get the core
> infrastructure landed first.
>
> For now, safety is guaranteed because
> liveupdate_register_file_handler() and liveupdate_register_flb() take
> a module reference. This effectively pins any module that registers
> with LUO, meaning those driver modules cannot be unloaded or upgraded
> dynamically, they can only be updated via Live Update or full reboot.

What if liveupdate_register_flb() fails? It would need to unregister its
file handler too, since the file handler can't really work without its
FLB. Shouldn't happen in practice, but still LUO clients need a way to
handle this failure.

[...]

-- 
Regards,
Pratyush Yadav