Message-ID: <CA+CK2bC_m5GRxCa1szw1v24Ssq8EnCWp4e985RJ5RRCdhztQWg@mail.gmail.com>
Date: Thu, 9 Oct 2025 11:01:25 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Jason Gunthorpe <jgg@...dia.com>
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Subject: Re: [PATCH v4 00/30] Live Update Orchestrator

On Thu, Oct 9, 2025 at 10:48 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Wed, Oct 08, 2025 at 04:26:39PM -0400, Pasha Tatashin wrote:
> > On Wed, Oct 8, 2025 at 3:36 PM Jason Gunthorpe <jgg@...dia.com> wrote:
> > >
> > > On Wed, Oct 08, 2025 at 12:40:34PM -0400, Pasha Tatashin wrote:
> > > > 1. Ordered Un-preservation
> > > > The un-preservation of file descriptors must also be ordered and must
> > > > occur in the reverse order of preservation. For example, if a user
> > > > preserves a memfd first and then an iommufd that depends on it, the
> > > > iommufd must be un-preserved before the memfd when the session is
> > > > closed or the FDs are explicitly un-preserved.
> > >
> > > Why?
> > >
> > > I imagined the first to unpreserve would restore the struct file * -
> > > that would satisfy the order.
> >
> > In my description, "un-preserve" refers to the action of canceling a
> > preservation request in the outgoing kernel, before kexec ever
> > happens. It's the pre-reboot counterpart to the PRESERVE_FD ioctl,
> > used when a user decides not to go through with the live update for a
> > specific FD.
> >
> > The terminology I am using:
> > preserve: Put FD into LUO in the outgoing kernel
> > unpreserve: Remove FD from LUO from the outgoing kernel
> > retrieve: Restore FD and return it to user in the next kernel
>
> Ok
>
> > For the retrieval part, we are going to be using FIFO order, the same
> > as preserve.
>
> This won't work. retrieval is driven by early boot discovery ordering
> and then by userspace. It will be in whatever order it wants. We need
> to be able to do things like make the struct file * at the moment
> something requests it..

I thought we wanted only the user to do "struct file" creation when
the user retrieves FD back. In this case we can enforce strict
ordering during retrieval. If "struct file" can be retrieved by
anything within the kernel, then that could be any kernel process
during boot, meaning that charging is not going to be properly applied
when kernel allocations are performed.

We specifically decided that while "struct file"s are going to be
created only by the user, the other subsystems can have early access
to the preserved file data, if they know how to parse it.

> > > This doesn't seem right, the API should be more like 'luo get
> > > serialization handle for this file *'
> >
> > How about:
> >
> > int liveupdate_find_token(struct liveupdate_session *session,
> >                           struct file *file, u64 *token);
>
> This sort of thing should not be used on the preserve side..
>
> > And if needed:
> > int liveupdate_find_file(struct liveupdate_session *session,
> >                          u64 token, struct file **file);
> >
> > Return: 0 on success, or -ENOENT if the file is not preserved.
>
> I would argue it should always cause a preservation...
>
> But this is still backwards, what we need is something like
>
> liveupdate_preserve_file(session, file, &token);
> my_preserve_blob.file_token = token

We cannot do that, the user should have already preserved that file
and provided us with a token to use, if that file was not preserved by
the user it is a bug. With this proposal, we would have to generate a
token, and it was argued that the kernel should not do that.

> file = liveupdate_retrieve_file(session, my_preserve_blob.file_token);
>
> And these can run in any order, and be called multiple times.
>
> Jason

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