[Virtio-fs] [PATCH] fuse: refcount FORGET requests

[Vivek Goyal]

On Fri, May 31, 2019 at 10:38:22AM -0700, Liu Bo wrote:
> On Fri, May 31, 2019 at 09:22:28AM -0400, Vivek Goyal wrote:
> > On Fri, May 31, 2019 at 05:24:03PM +0800, Peng Tao wrote:
> > > Right now FORGET requests are not tracked and they might be sent
> > > after DESTROY request.
> > 
> > How does that happen?
> > 
> > > Normally this is OK, since user space should
> > > be able to reject them knowing that the file system is already umounted.
> > > But if the same file system is mounted right again and the file is
> > > opened again, user space can be confused by the refcount decrement
> > > carried by the old FORGET requests.
> > > 
> > > E.g., it can trigger an assertion in virtiofsd when running xfstests
> > > generic/129 and generic/130 together:
> > > 
> > > unique: 23, opcode: FORGET (2), nodeid: 4, insize: 64, pid: 0
> > >   forget 4 1 -2
> > > virtiofsd: contrib/virtiofsd/passthrough_ll.c:1044: unref_inode: Assertion `inode->refcount >= n' failed.
> > > 
> > > To avoid confusing user space in such case, refcount FORGET requests so
> > > that fuse_sb_destroy() waits for all inflight requests before returning.
> > 
> > forgets are optional and destroy is supposed to cleanup any pending
> > state.
> 
> Since we've ensured that DESTROY is the last fuse_request sent to userspace
> daemon, we only need to deal with those FORGET requests sent before DESTROY.
> 
> Given that FORGET is sent via HIGHPRI vq, in order to solve the problem, would
> it make sense to route DESTROY to HIGHPRI vq as well?
> 
> Or, we can let lo_init to clear reqs in HIGHPRI vq?

I am looking at another approach. Given sending forget is optional, I am
writing a patch to not send forgets after virtio_kill_sb() has been
called. And also flush any in flight forgets. Something like.

virtio_kill_sb() {
	disconnect fsvq. Do not send any further forgets.
	flush in flight and pending forgets;
	fuse_kill_sb_anon()
}

This should make sure once daemon seems destroy, after that there are
no more forget requests.

Thanks
Vivek