[Virtio-fs] Lack of inotify for virtio-fs backed files/directories

[Stefan Hajnoczi]

On Mon, Jan 25, 2021 at 08:22:32PM +0100, Miklos Szeredi wrote:
> On Fri, Jan 22, 2021 at 5:58 PM Eric Ernst <eric.g.ernst at gmail.com> wrote:
> >
> > Hey ya'll,
> >
> > As highlighted nicely at [1], a common pattern in Kubernetes is to watch for changes to files/directories passed in as Configmaps or secrets. Sidecars typically use inotify to watch for changes and then signal the primary container or perform other changes based on the reconfiguration.
> >
> > In the context of Kata, these files/dirs are passed in utilizing virtio-fs. Since FUSE itself doesn't support inotify, this pattern will fail. I'm looking at ways to work-around this gap, including:
> >
> > 1) documenting this as a compatibility limitation ("don't do this!").
> > 2) suggesting end-users modify their sidecars and utilize poll
> > 3) doing an inotify on the host, and when changes are identified, notify the guest agent who can then modify the files on disk to trigger an inotify
> > 4) Working on contributing to libfuse in order to add inotify support.
> >
> > (4) is the right thing, but will take quite a bit of time before it can be used. I'm curious if other folks have suggestions for workarounds (I don't like any of the ones I suggested), and if anyone has taken a look at (4)?
> 
> Here's a thread on enabling inotify in remote fs:
> 
> https://lore.kernel.org/linux-fsdevel/20190507085707.GD30899@veci.piliscsaba.redhat.com/
> 
> The discussion didn't get too far, it seems there's not a very big
> interest in this, but conceptually it doesn't seem too difficult.

Vivek Goyal previously experimented with adding a notification virtqueue
to the virtio-fs device so that FUSE_NOTIFY_* messages can be supported.
That code can be brought back and merged.

However, due to the number of flags in inotify(7) I think this is a
non-trivial project. Whoever wants to tackle this feature should be
aware that it may take a few months (3?) to fully implement and test.

Stefan
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