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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Sep 12, 2019 at 10:14:11AM +0200, Miklos Szeredi wrote:
> On Wed, Sep 11, 2019 at 5:54 PM Stefan Hajnoczi <stefanha@redhat.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...

The issue occurs only on restore.  The core virtio driver code resets
the device so we lose state and cannot resume.

virtio-9p and virtio-gpu do not implement the .freeze() callback but
this is problematic since the system will think freeze succeeded.  It's
safer for virtio-fs to implement .freeze() and return -EOPNOTSUPP.

Can you squash in a trivial return -EOPNOTSUPP .freeze() function?

Thanks,
Stefan

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