Re: [PATCH] fuse: allow skipping abort interface for virtiofs

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jun 08, 2022 at 09:57:51PM +0800, Yongji Xie wrote:
> On Wed, Jun 8, 2022 at 8:44 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Wed, Jun 08, 2022 at 04:42:46PM +0800, Yongji Xie wrote:
> > > On Wed, Jun 8, 2022 at 3:34 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > >
> > > > On Tue, Jun 07, 2022 at 07:05:04PM +0800, Xie Yongji wrote:
> > > > > The commit 15c8e72e88e0 ("fuse: allow skipping control
> > > > > interface and forced unmount") tries to remove the control
> > > > > interface for virtio-fs since it does not support aborting
> > > > > requests which are being processed. But it doesn't work now.
> > > >
> > > > Aha.., so "no_control" basically has no effect? I was looking at
> > > > the code and did not find anybody using "no_control" and I was
> > > > wondering who is making use of "no_control" variable.
> > > >
> > > > I mounted virtiofs and noticed a directory named "40" showed up
> > > > under /sys/fs/fuse/connections/. That must be belonging to
> > > > virtiofs instance, I am assuming.
> > > >
> > >
> > > I think so.
> > >
> > > > BTW, if there are multiple fuse connections, how will one figure
> > > > out which directory belongs to which instance. Because without knowing
> > > > that, one will be shooting in dark while trying to read/write any
> > > > of the control files.
> > > >
> > >
> > > We can use "stat $mountpoint" to get the device minor ID which is the
> > > name of the corresponding control directory.
> > >
> > > > So I think a separate patch should be sent which just gets rid of
> > > > "no_control" saying nobody uses. it.
> > > >
> > >
> > > OK.
> > >
> > > > >
> > > > > This commit fixes the bug, but only remove the abort interface
> > > > > instead since other interfaces should be useful.
> > > >
> > > > Hmm.., so writing to "abort" file is bad as it ultimately does.
> > > >
> > > > fc->connected = 0;
> > > >
> > >
> > > Another problem is that it might trigger UAF since
> > > virtio_fs_request_complete() doesn't know the requests are aborted.
> > >
> > > > So getting rid of this file till we support aborting the pending
> > > > requests properly, makes sense.
> > > >
> > > > I think this probably should be a separate patch which explains
> > > > why adding "no_abort_control" is a good idea.
> > > >
> > >
> > > OK.
> >
> > BTW, which particular knob you are finding useful in control filesystem
> > for virtiofs. As you mentioned "abort" we will not implement. "waiting"
> > might not have much significance as well because requests are handed
> > over to virtiofs immidiately and if they can be sent to server (because
> > virtqueue is full) these are queued internally and fuse layer will not
> > have an idea.
> >
> 
> Couldn't it be used to check the inflight I/O for virtiofs?

Actually I might be wrong. It probably should work. Looking at
implementation.

fuse_conn_waiting_read() looks at fc->num_waiting to figure out
how many requests are in flight.

And either fuse_get_req()/fuse_simple_request() will bump up the
fc->num_request count and fuse_put_request() will drop that count
once request completes. And this seems to be independent of
virtiofs.

So looks like it should work even with virtiofs. Please give it a try.

> 
> > That leaves us with "congestion_threshold" and "max_background".
> > max_background seems to control how many background requests can be
> > submitted at a time. That probably can be useful if server is overwhelemed
> > and we want to slow down the client a bit.
> >
> > Not sure about congestion threshold.
> >
> > So have you found some knob useful for your use case?
> >
> 
> Since it doesn't do harm to the system, I think it would be better to
> just keep it as it is. Maybe some fuse users can make use of it.

I guess fair enough. I don't mind creating "control" file system for
virtiofs. Either we don't create "abort" file or may be somehow
writing to file returns error. I guess both the solutions should
work. 

Thanks
Vivek