linux-kernel - Re: [RFC v2 09/16] luo: luo_files: implement file systems callbacks

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Message-ID: <mafs0a568vuf5.fsf@kernel.org>
Date: Mon, 16 Jun 2025 12:43:10 +0200
From: Pratyush Yadav <pratyush@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [RFC v2 09/16] luo: luo_files: implement file systems callbacks

On Fri, Jun 13 2025, Pasha Tatashin wrote:

> On Fri, Jun 13, 2025 at 11:18 AM Pratyush Yadav <pratyush@...nel.org> wrote:
>>
>> On Sun, Jun 08 2025, Pasha Tatashin wrote:
>>
>> > On Thu, Jun 5, 2025 at 12:04 PM Pratyush Yadav <pratyush@...nel.org> wrote:
>> >>
>> >> On Thu, May 15 2025, Pasha Tatashin wrote:
>> >>
>> >> > Implements the core logic within luo_files.c to invoke the prepare,
>> >> > reboot, finish, and cancel callbacks for preserved file instances,
>> >> > replacing the previous stub implementations. It also handles
>> >> > the persistence and retrieval of the u64 data payload associated with
>> >> > each file via the LUO FDT.
>> >> >
>> >> > This completes the core mechanism enabling registered filesystem
>> >> > handlers to actively manage file state across the live update
>> >> > transition using the LUO framework.
>> >> >
>> >> > Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
>> >> > ---
>> >> >  drivers/misc/liveupdate/luo_files.c | 105 +++++++++++++++++++++++++++-
>> >> >  1 file changed, 103 insertions(+), 2 deletions(-)
>> >> >
>> >> [...]
>> >> > @@ -305,7 +369,29 @@ int luo_do_files_prepare_calls(void)
>> >> >   */
>> >> >  int luo_do_files_freeze_calls(void)
>> >> >  {
>> >> > -     return 0;
>> >> > +     unsigned long token;
>> >> > +     struct luo_file *h;
>> >> > +     int ret;
>> >> > +
>> >> > +     xa_for_each(&luo_files_xa_out, token, h) {
>> >>
>> >> Should we also ensure at this point that there are no open handles to
>> >> this file? How else would a file system ensure the file is in quiescent
>> >> state to do its final serialization?
>> >
>> > Do you mean check refcnt here? If so, this is a good idea, but first
>> > we need to implement the lifecycle of liveupdate agent correctectly,
>> > where owner of FD must survive through entering into reboot() with
>> > /dev/liveupdate still open.
>>
>> Yes, by this point we should ensure refcnt == 1. IIUC you plan to
>> implement the lifecycle change in the next revision, so this can be
>> added there as well I suppose.
>
> Yes, I am working on that. Current, WIP patch looks like this:
> https://github.com/soleen/linux/commit/fecf912d8b70acd23d24185a8c0504764e43a279
>
> However, I am not sure about refcnt == 1 at freeze() time. We can have
> programs, that never terminated while we were still in userspace (i.e.
> kexec -e -> reboot() -> freeze()), in that case refcnt can be anything
> at the time of freeze, no?

Do you mean the agent that controls the liveupdate session? Then in that
case the agent can keep running with the /dev/liveupdate FD open, but it
must close all of the FDs preserved via LUO before doing kexec -e.

-- 
Regards,
Pratyush Yadav