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

[Yongji Xie <xieyongji@bytedance.com>]

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?

> 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.

Thanks,
Yongji