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

Stefan
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