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

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Message-ID: <CA+CK2bBcgkrM0D0w77+UpDZsF2rDOc15iAcEJ7PMw52HQDipvA@mail.gmail.com>
Date: Mon, 16 Jun 2025 10:57:44 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
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Subject: Re: [RFC v2 09/16] luo: luo_files: implement file systems callbacks

On Mon, Jun 16, 2025 at 6:43 AM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> 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
Yes
> 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.

Right, but in this case the agent would have to basically kill all the
processes the regestred FDs through it prior to 'kexec -e', I am not
sure it is its job. However, we can add some pr_warn_once() when rfcnt
!= 1, I think this is a minor change. Lets do that once we have a more
developed userspace setup. We need to start working on liveupdated
that would through some sort of RPCs calls store and restore FDs.

Pasha