Re: [PATCH 4/4] ovl: alllow remote upper

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Jan 31, 2020 at 5:38 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Fri, Jan 31, 2020 at 4:30 PM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Fri, Jan 31, 2020 at 1:51 PM Miklos Szeredi <mszeredi@redhat.com> wrote:
> > >
> > > No reason to prevent upper layer being a remote filesystem.  Do the
> > > revalidation in that case, just as we already do for lower layers.
> > >
> >
> > No reason to prevent upper layer from being a remote filesystem, but
> > the !remote criteria for upper fs kept away a lot of filesystems from
> > upper. Those filesystems have never been tested as upper and many
> > of them are probably not fit for upper.
> >
> > The goal is to lift the !remote limitation, not to allow for lots of new
> > types of upper fs's.
> >
> > What can we do to minimize damages?
> >
> > We can assert that is upper is remote, it must qualify for a more strict
> > criteria as upper fs, that is:
> > - support d_type
> > - support xattr
> > - support RENAME_EXCHANGE|RENAME_WHITEOUT
> >
> > I have a patch on branch ovl-strict which implements those restrictions.
>
> Sounds good.  Not sure how much this is this going to be a
> compatibility headache.  If it does, then we can conditionally enable
> this with a config/module option.
>

No headache at all:
- For now, do not change criteria for !remote fs
- Only remote fs needs to meet the most strict criteria
- We can add the 'strict' config later if we want impose
  same criteria also for local fs

> >
> > Now I know fuse doesn't support RENAME_WHITEOUT, but it does
> > support RENAME_EXCHANGE, which already proves to be a very narrow
> > filter for remote fs: afs, fuse, gfs2.
> > Did not check if afs, gfs2 qualify for the rest of the criteria.
> >

I checked - afs has d_automount and gfs2 is d_hash.
They do not qualify as any layer.

> > Is it simple to implement RENAME_WHITEOUT for fuse/virtiofs?
>
> Trivial.
>

So that leaves only fuse after implementing RENAME_WHITEOUT.
We are back in control ;-)

Thanks,
Amir.