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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 6 Mar 2023 20:18:11 +0100
Message-ID: <CAJfpeguYO9J=np5vxH+HjCSAxn=8fcQRhh_-BVadTt86zWfkpQ@mail.gmail.com>
Subject: Re: [RFC PATCH 0/9] fuse: API for Checkpoint/Restore
To:     Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Cc:     mszeredi@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
        Amir Goldstein <amir73il@gmail.com>,
        =?UTF-8?Q?St=C3=A9phane_Graber?= <stgraber@ubuntu.com>,
        Seth Forshee <sforshee@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        criu@openvz.org, flyingpeng@tencent.com
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On Mon, 6 Mar 2023 at 17:44, Aleksandr Mikhalitsyn
<aleksandr.mikhalitsyn@canonical.com> wrote:
>
> On Mon, Mar 6, 2023 at 5:15=E2=80=AFPM Miklos Szeredi <miklos@szeredi.hu>=
 wrote:

> > Apparently all of the added mechanisms (REINIT, BM_REVAL, conn_gen)
> > are crash recovery related, and not useful for C/R.  Why is this being
> > advertised as a precursor for CRIU support?
>
> It's because I'm doing this with CRIU in mind too, I think it's a good
> way to make a universal interface
> which can address not only the recovery case but also the C/R, cause
> in some sense it's a close problem.

That's what I'm wondering about...

Crash recovery is about restoring (or at least regenerating) state in
the userspace server.

In CRIU restoring the state of the userspace server is a solved
problem, the issue is restoring state in the kernel part of fuse.  In
a sense it's the exact opposite problem that crash recovery is doing.

> But of course, Checkpoint/Restore is a way more trickier. But before
> doing all the work with CRIU PoC,
> I wanted to consult with you and folks if there are any serious
> objections to this interface/feature or, conversely,
> if there is someone else who is interested in it.
>
> Now about interfaces REINIT, BM_REVAL.
>
> I think it will be useful for CRIU case, but probably I need to extend
> it a little bit, as I mentioned earlier in the cover letter:
> > >* "fake" daemon has to reply to FUSE_INIT request from the kernel and =
initialize fuse connection somehow.
> > > This setup can be not consistent with the original daemon (protocol v=
ersion, daemon capabilities/settings
> > > like no_open, no_flush, readahead, and so on).
>
> So, after the "fake" demon has done its job during CRIU restore, we
> need to replace it with the actual demon from
> the dumpee tree and performing REINIT looks like a sanner way.

I don't get it.  How does REINIT help with switching to the real daemon?

Thanks,
Miklos