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

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Message-ID: <20190906095428-mutt-send-email-mst@kernel.org>
Date:   Fri, 6 Sep 2019 09:57:27 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        linux-fsdevel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, virtio-fs@...hat.com,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH 00/18] virtiofs: Fix various races and cleanups round 1

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@...hat.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@...hat.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.

-- 
MST