Re: Upcoming: Notifications, FS notifications and fsinfo()

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, Mar 31, 2020 at 10:34 AM Karel Zak <kzak@redhat.com> wrote:
>
> On Tue, Mar 31, 2020 at 07:11:11AM +0200, Miklos Szeredi wrote:
> > On Mon, Mar 30, 2020 at 11:17 PM Christian Brauner
> > <christian.brauner@ubuntu.com> wrote:
> >
> > > Fwiw, putting down my kernel hat and speaking as someone who maintains
> > > two container runtimes and various other low-level bits and pieces in
> > > userspace who'd make heavy use of this stuff I would prefer the fd-based
> > > fsinfo() approach especially in the light of across namespace
> > > operations, querying all properties of a mount atomically all-at-once,
> >
> > fsinfo(2) doesn't meet the atomically all-at-once requirement.
>
> I guess your /proc based idea have exactly the same problem...

Yes, that's exactly what I wanted to demonstrate: there's no
fundamental difference between the two API's in this respect.

> I see two possible ways:
>
> - after open("/mnt", O_PATH) create copy-on-write object in kernel to
>   represent mount node -- kernel will able to modify it, but userspace
>   will get unchanged data from the FD until to close()
>
> - improve fsinfo() to provide set (list) of the attributes by one call

I think we are approaching this from the wrong end.   Let's just
ignore all of the proposed interfaces for now and only concentrate on
what this will be used for.

Start with a set of use cases by all interested parties.  E.g.

 - systemd wants to keep track attached mounts in a namespace, as well
as new detached mounts created by fsmount()

 - systemd need to keep information (such as parent, children, mount
flags, fs options, etc) up to date on any change of topology or
attributes.

 - util linux needs to display the topology and state of mounts in the
system that corresponds to a consistent state that set of mounts

 - etc...

From that we can derive a set of requirements for the API.

Thanks,
Miklos