linux-kernel - Re: [PATCH v5 0/4] virtio-fs: shared file system for virtual machines

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Date:   Thu, 12 Sep 2019 10:14:11 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v5 0/4] virtio-fs: shared file system for virtual machines

On Wed, Sep 11, 2019 at 5:54 PM Stefan Hajnoczi <stefanha@...hat.com> wrote:
>
> On Tue, Sep 10, 2019 at 05:12:02PM +0200, Miklos Szeredi wrote:
> > I've folded the series from Vivek and fixed a couple of TODO comments
> > myself.  AFAICS two issues remain that need to be resolved in the short
> > term, one way or the other: freeze/restore and full virtqueue.
>
> I have researched freeze/restore and come to the conclusion that it
> needs to be a future feature.  It will probably come together with live
> migration support for reasons mentioned below.
>
> Most virtio devices have fairly simply power management freeze/restore
> functions that shut down the device and bring it back to the state held
> in memory, respectively.  virtio-fs, as well as virtio-9p and
> virtio-gpu, are different because they contain session state.  It is not
> easily possible to bring back the state held in memory after the device
> has been reset.
>
> The following areas of the FUSE protocol are stateful and need special
> attention:
>
>  * FUSE_INIT - this is pretty easy, we must re-negotiate the same
>    settings as before.
>
>  * FUSE_LOOKUP -> fuse_inode (inode_map)
>
>    The session contains a set of inode numbers that have been looked up
>    using FUSE_LOOKUP.  They are ephemeral in the current virtiofsd
>    implementation and vary across device reset.  Therefore we are unable
>    to restore the same inode numbers upon restore.
>
>    The solution is persistent inode numbers in virtiofsd.  This is also
>    needed to make open_by_handle_at(2) work and probably for live
>    migration.
>
>  * FUSE_OPEN -> fh (fd_map)
>
>    The session contains FUSE file handles for open files.  There is
>    currently no way of re-opening a file so that a specific fh is
>    returned.  A mechanism to do so probably isn't necessary if the
>    driver can update the fh to the new one produced by the device for
>    all open files instead.
>
>  * FUSE_OPENDIR -> fh (dirp_map)
>
>    Same story as for FUSE_OPEN but for open directories.
>
>  * FUSE_GETLK/SETLK/SETLKW -> (inode->posix_locks and fcntl(F_OFD_GET/SETLK))
>
>    The session contains file locks.  The driver must reacquire them upon
>    restore.  It's unclear what to do when locking fails.
>
> Live migration has the same problem since the FUSE session will be moved
> to a new virtio-fs device instance.  It makes sense to tackle both
> features together.  This is something that can be implemented in the
> next year, but it's not a quick fix.

Right.   The question for now is: should the freeze silently succeed
(as it seems to do now) or should it fail instead?

I guess normally freezing should be okay, as long as the virtiofsd
remains connected while the system is frozen.

I tried to test this with "echo -n mem > /sys/power/state", which
indeed resulted in the virtio_fs_freeze() callback being called.
However, I couldn't find a way to wake up the system...

Thanks,
Miklos