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@xxxxxxxxxx> 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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