Re: [PATCH 00/18] virtiofs: Fix various races and cleanups round 1

From: Miklos Szeredi <miklos@szeredi.hu>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	linux-fsdevel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org, virtio-fs@redhat.com,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [PATCH 00/18] virtiofs: Fix various races and cleanups round 1
Date: Fri, 6 Sep 2019 16:11:45 +0200	[thread overview]
Message-ID: <CAJfpeguVvwRCi7+23W2qA+KHeoaYaR7uKsX+JykC3HK00uGSNQ@mail.gmail.com> (raw)
In-Reply-To: <20190906095428-mutt-send-email-mst@kernel.org>

On Fri, Sep 6, 2019 at 3:57 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Fri, Sep 06, 2019 at 08:08:17AM -0400, Vivek Goyal wrote:
> > On Fri, Sep 06, 2019 at 01:52:41PM +0200, Miklos Szeredi wrote:
> > > On Fri, Sep 6, 2019 at 12:36 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > >
> > > > On Fri, Sep 06, 2019 at 10:15:14AM +0200, Miklos Szeredi wrote:
> > > > > On Thu, Sep 5, 2019 at 9:49 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > Michael Tsirkin pointed out issues w.r.t various locking related TODO
> > > > > > items and races w.r.t device removal.
> > > > > >
> > > > > > In this first round of cleanups, I have taken care of most pressing
> > > > > > issues.
> > > > > >
> > > > > > These patches apply on top of following.
> > > > > >
> > > > > > git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git#virtiofs-v4
> > > > > >
> > > > > > I have tested these patches with mount/umount and device removal using
> > > > > > qemu monitor. For example.
> > > > >
> > > > > Is device removal mandatory?  Can't this be made a non-removable
> > > > > device?  Is there a good reason why removing the virtio-fs device
> > > > > makes sense?
> > > >
> > > > Hot plugging and unplugging virtio PCI adapters is common.  I'd very
> > > > much like removal to work from the beginning.
> > >
> > > Can you give an example use case?
> >
> > David Gilbert mentioned this could be useful if daemon stops responding
> > or dies. One could remove device. That will fail all future requests
> > and allow unmounting filesystem.
> >
> > Havind said that, current implementation will help in above situation
> > only if there are no pending requests. If there are pending requests
> > and daemon stops responding, then removal will hang too, as we wait
> > for draining the queues.
> >
> > So at some point of time, we also need some sort of timeout functionality
> > where we end requests with error after a timeout.
> >
> > I feel we should support removing device at some point of time. But its
> > not necessarily a must have feature for first round.
> >
> > Thanks
> > Vivek
>
> Without removal how do we stop guest poking at some files if we want to?
>
> I guess we could invent a special event to block accesses,
> but unplug will just do it.
>
> blk and scsi support removal out of box, if this is supposed
> to be a drop in replacement then I think yes, you want this
> support.

This is not a drop in replacement for blk and scsi transports.  More
for virtio-9p.  Does that have anything similar?

If we get a request for this feature, then yes, what you are saying
makes sense.   But that hasn't happened yet, so I think this can wait.

Thanks,
Miklos